1039 lines
48 KiB
ReStructuredText
1039 lines
48 KiB
ReStructuredText
.. _glossary:
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********
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Glossary
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********
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.. if you add new entries, keep the alphabetical sorting!
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.. glossary::
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``>>>``
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The default Python prompt of the interactive shell. Often seen for code
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examples which can be executed interactively in the interpreter.
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``...``
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The default Python prompt of the interactive shell when entering code for
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an indented code block or within a pair of matching left and right
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delimiters (parentheses, square brackets or curly braces).
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2to3
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A tool that tries to convert Python 2.x code to Python 3.x code by
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handling most of the incompatibilities which can be detected by parsing the
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source and traversing the parse tree.
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2to3 is available in the standard library as :mod:`lib2to3`; a standalone
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entry point is provided as :file:`Tools/scripts/2to3`. See
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:ref:`2to3-reference`.
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abstract base class
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Abstract base classes complement :term:`duck-typing` by
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providing a way to define interfaces when other techniques like
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:func:`hasattr` would be clumsy or subtly wrong (for example with
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:ref:`magic methods <special-lookup>`). ABCs introduce virtual
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subclasses, which are classes that don't inherit from a class but are
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still recognized by :func:`isinstance` and :func:`issubclass`; see the
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:mod:`abc` module documentation. Python comes with many built-in ABCs for
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data structures (in the :mod:`collections.abc` module), numbers (in the
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:mod:`numbers` module), streams (in the :mod:`io` module), import finders
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and loaders (in the :mod:`importlib.abc` module). You can create your own
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ABCs with the :mod:`abc` module.
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argument
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A value passed to a :term:`function` (or :term:`method`) when calling the
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function. There are two kinds of argument:
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* :dfn:`keyword argument`: an argument preceded by an identifier (e.g.
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``name=``) in a function call or passed as a value in a dictionary
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preceded by ``**``. For example, ``3`` and ``5`` are both keyword
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arguments in the following calls to :func:`complex`::
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complex(real=3, imag=5)
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complex(**{'real': 3, 'imag': 5})
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* :dfn:`positional argument`: an argument that is not a keyword argument.
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Positional arguments can appear at the beginning of an argument list
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and/or be passed as elements of an :term:`iterable` preceded by ``*``.
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For example, ``3`` and ``5`` are both positional arguments in the
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following calls::
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complex(3, 5)
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complex(*(3, 5))
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Arguments are assigned to the named local variables in a function body.
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See the :ref:`calls` section for the rules governing this assignment.
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Syntactically, any expression can be used to represent an argument; the
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evaluated value is assigned to the local variable.
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See also the :term:`parameter` glossary entry, the FAQ question on
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:ref:`the difference between arguments and parameters
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<faq-argument-vs-parameter>`, and :pep:`362`.
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asynchronous context manager
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An object which controls the environment seen in an
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:keyword:`async with` statement by defining :meth:`__aenter__` and
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:meth:`__aexit__` methods. Introduced by :pep:`492`.
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asynchronous generator
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A function which returns an :term:`asynchronous generator iterator`. It
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looks like a coroutine function defined with :keyword:`async def` except
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that it contains :keyword:`yield` expressions for producing a series of
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values usable in an :keyword:`async for` loop.
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Usually refers to a asynchronous generator function, but may refer to an
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*asynchronous generator iterator* in some contexts. In cases where the
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intended meaning isn't clear, using the full terms avoids ambiguity.
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An asynchronous generator function may contain :keyword:`await`
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expressions as well as :keyword:`async for`, and :keyword:`async with`
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statements.
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asynchronous generator iterator
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An object created by a :term:`asynchronous generator` function.
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This is an :term:`asynchronous iterator` which when called using the
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:meth:`__anext__` method returns an awaitable object which will execute
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that the body of the asynchronous generator function until the
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next :keyword:`yield` expression.
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Each :keyword:`yield` temporarily suspends processing, remembering the
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location execution state (including local variables and pending
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try-statements). When the *asynchronous generator iterator* effectively
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resumes with another awaitable returned by :meth:`__anext__`, it
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picks-up where it left-off. See :pep:`492` and :pep:`525`.
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asynchronous iterable
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An object, that can be used in an :keyword:`async for` statement.
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Must return an :term:`asynchronous iterator` from its
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:meth:`__aiter__` method. Introduced by :pep:`492`.
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asynchronous iterator
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An object that implements :meth:`__aiter__` and :meth:`__anext__`
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methods. ``__anext__`` must return an :term:`awaitable` object.
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:keyword:`async for` resolves awaitable returned from asynchronous
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iterator's :meth:`__anext__` method until it raises
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:exc:`StopAsyncIteration` exception. Introduced by :pep:`492`.
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attribute
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A value associated with an object which is referenced by name using
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dotted expressions. For example, if an object *o* has an attribute
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*a* it would be referenced as *o.a*.
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awaitable
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An object that can be used in an :keyword:`await` expression. Can be
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a :term:`coroutine` or an object with an :meth:`__await__` method.
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See also :pep:`492`.
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BDFL
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Benevolent Dictator For Life, a.k.a. `Guido van Rossum
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<https://www.python.org/~guido/>`_, Python's creator.
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binary file
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A :term:`file object` able to read and write
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:term:`bytes-like objects <bytes-like object>`.
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Examples of binary files are files opened in binary mode (``'rb'``,
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``'wb'`` or ``'rb+'``), :data:`sys.stdin.buffer`,
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:data:`sys.stdout.buffer`, and instances of :class:`io.BytesIO` and
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:class:`gzip.GzipFile`.
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.. seealso::
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A :term:`text file` reads and writes :class:`str` objects.
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bytes-like object
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An object that supports the :ref:`bufferobjects` and can
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export a C-:term:`contiguous` buffer. This includes all :class:`bytes`,
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:class:`bytearray`, and :class:`array.array` objects, as well as many
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common :class:`memoryview` objects. Bytes-like objects can
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be used for various operations that work with binary data; these include
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compression, saving to a binary file, and sending over a socket.
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Some operations need the binary data to be mutable. The documentation
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often refers to these as "read-write bytes-like objects". Example
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mutable buffer objects include :class:`bytearray` and a
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:class:`memoryview` of a :class:`bytearray`.
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Other operations require the binary data to be stored in
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immutable objects ("read-only bytes-like objects"); examples
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of these include :class:`bytes` and a :class:`memoryview`
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of a :class:`bytes` object.
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bytecode
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Python source code is compiled into bytecode, the internal representation
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of a Python program in the CPython interpreter. The bytecode is also
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cached in ``.pyc`` files so that executing the same file is
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faster the second time (recompilation from source to bytecode can be
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avoided). This "intermediate language" is said to run on a
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:term:`virtual machine` that executes the machine code corresponding to
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each bytecode. Do note that bytecodes are not expected to work between
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different Python virtual machines, nor to be stable between Python
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releases.
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A list of bytecode instructions can be found in the documentation for
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:ref:`the dis module <bytecodes>`.
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class
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A template for creating user-defined objects. Class definitions
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normally contain method definitions which operate on instances of the
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class.
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coercion
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The implicit conversion of an instance of one type to another during an
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operation which involves two arguments of the same type. For example,
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``int(3.15)`` converts the floating point number to the integer ``3``, but
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in ``3+4.5``, each argument is of a different type (one int, one float),
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and both must be converted to the same type before they can be added or it
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will raise a ``TypeError``. Without coercion, all arguments of even
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compatible types would have to be normalized to the same value by the
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programmer, e.g., ``float(3)+4.5`` rather than just ``3+4.5``.
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complex number
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An extension of the familiar real number system in which all numbers are
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expressed as a sum of a real part and an imaginary part. Imaginary
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numbers are real multiples of the imaginary unit (the square root of
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``-1``), often written ``i`` in mathematics or ``j`` in
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engineering. Python has built-in support for complex numbers, which are
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written with this latter notation; the imaginary part is written with a
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``j`` suffix, e.g., ``3+1j``. To get access to complex equivalents of the
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:mod:`math` module, use :mod:`cmath`. Use of complex numbers is a fairly
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advanced mathematical feature. If you're not aware of a need for them,
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it's almost certain you can safely ignore them.
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context manager
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An object which controls the environment seen in a :keyword:`with`
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statement by defining :meth:`__enter__` and :meth:`__exit__` methods.
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See :pep:`343`.
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contiguous
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.. index:: C-contiguous, Fortran contiguous
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A buffer is considered contiguous exactly if it is either
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*C-contiguous* or *Fortran contiguous*. Zero-dimensional buffers are
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C and Fortran contiguous. In one-dimensional arrays, the items
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must be laid out in memory next to each other, in order of
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increasing indexes starting from zero. In multidimensional
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C-contiguous arrays, the last index varies the fastest when
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visiting items in order of memory address. However, in
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Fortran contiguous arrays, the first index varies the fastest.
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coroutine
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Coroutines is a more generalized form of subroutines. Subroutines are
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entered at one point and exited at another point. Coroutines can be
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entered, exited, and resumed at many different points. They can be
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implemented with the :keyword:`async def` statement. See also
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:pep:`492`.
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coroutine function
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A function which returns a :term:`coroutine` object. A coroutine
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function may be defined with the :keyword:`async def` statement,
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and may contain :keyword:`await`, :keyword:`async for`, and
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:keyword:`async with` keywords. These were introduced
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by :pep:`492`.
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CPython
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The canonical implementation of the Python programming language, as
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distributed on `python.org <https://www.python.org>`_. The term "CPython"
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is used when necessary to distinguish this implementation from others
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such as Jython or IronPython.
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decorator
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A function returning another function, usually applied as a function
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transformation using the ``@wrapper`` syntax. Common examples for
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decorators are :func:`classmethod` and :func:`staticmethod`.
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The decorator syntax is merely syntactic sugar, the following two
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function definitions are semantically equivalent::
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def f(...):
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...
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f = staticmethod(f)
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@staticmethod
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def f(...):
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...
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The same concept exists for classes, but is less commonly used there. See
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the documentation for :ref:`function definitions <function>` and
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:ref:`class definitions <class>` for more about decorators.
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descriptor
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Any object which defines the methods :meth:`__get__`, :meth:`__set__`, or
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:meth:`__delete__`. When a class attribute is a descriptor, its special
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binding behavior is triggered upon attribute lookup. Normally, using
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*a.b* to get, set or delete an attribute looks up the object named *b* in
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the class dictionary for *a*, but if *b* is a descriptor, the respective
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descriptor method gets called. Understanding descriptors is a key to a
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deep understanding of Python because they are the basis for many features
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including functions, methods, properties, class methods, static methods,
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and reference to super classes.
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For more information about descriptors' methods, see :ref:`descriptors`.
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dictionary
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An associative array, where arbitrary keys are mapped to values. The
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keys can be any object with :meth:`__hash__` and :meth:`__eq__` methods.
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Called a hash in Perl.
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dictionary view
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The objects returned from :meth:`dict.keys`, :meth:`dict.values`, and
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:meth:`dict.items` are called dictionary views. They provide a dynamic
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view on the dictionary’s entries, which means that when the dictionary
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changes, the view reflects these changes. To force the
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dictionary view to become a full list use ``list(dictview)``. See
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:ref:`dict-views`.
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docstring
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A string literal which appears as the first expression in a class,
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function or module. While ignored when the suite is executed, it is
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recognized by the compiler and put into the :attr:`__doc__` attribute
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of the enclosing class, function or module. Since it is available via
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introspection, it is the canonical place for documentation of the
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object.
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duck-typing
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A programming style which does not look at an object's type to determine
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if it has the right interface; instead, the method or attribute is simply
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called or used ("If it looks like a duck and quacks like a duck, it
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must be a duck.") By emphasizing interfaces rather than specific types,
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well-designed code improves its flexibility by allowing polymorphic
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substitution. Duck-typing avoids tests using :func:`type` or
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:func:`isinstance`. (Note, however, that duck-typing can be complemented
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with :term:`abstract base classes <abstract base class>`.) Instead, it
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typically employs :func:`hasattr` tests or :term:`EAFP` programming.
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EAFP
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Easier to ask for forgiveness than permission. This common Python coding
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style assumes the existence of valid keys or attributes and catches
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exceptions if the assumption proves false. This clean and fast style is
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characterized by the presence of many :keyword:`try` and :keyword:`except`
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statements. The technique contrasts with the :term:`LBYL` style
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common to many other languages such as C.
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expression
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A piece of syntax which can be evaluated to some value. In other words,
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an expression is an accumulation of expression elements like literals,
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names, attribute access, operators or function calls which all return a
|
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value. In contrast to many other languages, not all language constructs
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are expressions. There are also :term:`statement`\s which cannot be used
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as expressions, such as :keyword:`if`. Assignments are also statements,
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not expressions.
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extension module
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A module written in C or C++, using Python's C API to interact with the
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core and with user code.
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f-string
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String literals prefixed with ``'f'`` or ``'F'`` are commonly called
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"f-strings" which is short for
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:ref:`formatted string literals <f-strings>`. See also :pep:`498`.
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file object
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An object exposing a file-oriented API (with methods such as
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:meth:`read()` or :meth:`write()`) to an underlying resource. Depending
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on the way it was created, a file object can mediate access to a real
|
||
on-disk file or to another type of storage or communication device
|
||
(for example standard input/output, in-memory buffers, sockets, pipes,
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etc.). File objects are also called :dfn:`file-like objects` or
|
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:dfn:`streams`.
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There are actually three categories of file objects: raw
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:term:`binary files <binary file>`, buffered
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:term:`binary files <binary file>` and :term:`text files <text file>`.
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Their interfaces are defined in the :mod:`io` module. The canonical
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way to create a file object is by using the :func:`open` function.
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file-like object
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A synonym for :term:`file object`.
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finder
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An object that tries to find the :term:`loader` for a module that is
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being imported.
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||
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Since Python 3.3, there are two types of finder: :term:`meta path finders
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||
<meta path finder>` for use with :data:`sys.meta_path`, and :term:`path
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||
entry finders <path entry finder>` for use with :data:`sys.path_hooks`.
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||
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See :pep:`302`, :pep:`420` and :pep:`451` for much more detail.
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floor division
|
||
Mathematical division that rounds down to nearest integer. The floor
|
||
division operator is ``//``. For example, the expression ``11 // 4``
|
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evaluates to ``2`` in contrast to the ``2.75`` returned by float true
|
||
division. Note that ``(-11) // 4`` is ``-3`` because that is ``-2.75``
|
||
rounded *downward*. See :pep:`238`.
|
||
|
||
function
|
||
A series of statements which returns some value to a caller. It can also
|
||
be passed zero or more :term:`arguments <argument>` which may be used in
|
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the execution of the body. See also :term:`parameter`, :term:`method`,
|
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and the :ref:`function` section.
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function annotation
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||
An arbitrary metadata value associated with a function parameter or return
|
||
value. Its syntax is explained in section :ref:`function`. Annotations
|
||
may be accessed via the :attr:`__annotations__` special attribute of a
|
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function object.
|
||
|
||
Python itself does not assign any particular meaning to function
|
||
annotations. They are intended to be interpreted by third-party libraries
|
||
or tools. See :pep:`3107`, which describes some of their potential uses.
|
||
|
||
__future__
|
||
A pseudo-module which programmers can use to enable new language features
|
||
which are not compatible with the current interpreter.
|
||
|
||
By importing the :mod:`__future__` module and evaluating its variables,
|
||
you can see when a new feature was first added to the language and when it
|
||
becomes the default::
|
||
|
||
>>> import __future__
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>>> __future__.division
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_Feature((2, 2, 0, 'alpha', 2), (3, 0, 0, 'alpha', 0), 8192)
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|
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garbage collection
|
||
The process of freeing memory when it is not used anymore. Python
|
||
performs garbage collection via reference counting and a cyclic garbage
|
||
collector that is able to detect and break reference cycles. The
|
||
garbage collector can be controlled using the :mod:`gc` module.
|
||
|
||
.. index:: single: generator
|
||
|
||
generator
|
||
A function which returns a :term:`generator iterator`. It looks like a
|
||
normal function except that it contains :keyword:`yield` expressions
|
||
for producing a series of values usable in a for-loop or that can be
|
||
retrieved one at a time with the :func:`next` function.
|
||
|
||
Usually refers to a generator function, but may refer to a
|
||
*generator iterator* in some contexts. In cases where the intended
|
||
meaning isn't clear, using the full terms avoids ambiguity.
|
||
|
||
generator iterator
|
||
An object created by a :term:`generator` function.
|
||
|
||
Each :keyword:`yield` temporarily suspends processing, remembering the
|
||
location execution state (including local variables and pending
|
||
try-statements). When the *generator iterator* resumes, it picks-up where
|
||
it left-off (in contrast to functions which start fresh on every
|
||
invocation).
|
||
|
||
.. index:: single: generator expression
|
||
|
||
generator expression
|
||
An expression that returns an iterator. It looks like a normal expression
|
||
followed by a :keyword:`for` expression defining a loop variable, range,
|
||
and an optional :keyword:`if` expression. The combined expression
|
||
generates values for an enclosing function::
|
||
|
||
>>> sum(i*i for i in range(10)) # sum of squares 0, 1, 4, ... 81
|
||
285
|
||
|
||
generic function
|
||
A function composed of multiple functions implementing the same operation
|
||
for different types. Which implementation should be used during a call is
|
||
determined by the dispatch algorithm.
|
||
|
||
See also the :term:`single dispatch` glossary entry, the
|
||
:func:`functools.singledispatch` decorator, and :pep:`443`.
|
||
|
||
|
||
GIL
|
||
See :term:`global interpreter lock`.
|
||
|
||
global interpreter lock
|
||
The mechanism used by the :term:`CPython` interpreter to assure that
|
||
only one thread executes Python :term:`bytecode` at a time.
|
||
This simplifies the CPython implementation by making the object model
|
||
(including critical built-in types such as :class:`dict`) implicitly
|
||
safe against concurrent access. Locking the entire interpreter
|
||
makes it easier for the interpreter to be multi-threaded, at the
|
||
expense of much of the parallelism afforded by multi-processor
|
||
machines.
|
||
|
||
However, some extension modules, either standard or third-party,
|
||
are designed so as to release the GIL when doing computationally-intensive
|
||
tasks such as compression or hashing. Also, the GIL is always released
|
||
when doing I/O.
|
||
|
||
Past efforts to create a "free-threaded" interpreter (one which locks
|
||
shared data at a much finer granularity) have not been successful
|
||
because performance suffered in the common single-processor case. It
|
||
is believed that overcoming this performance issue would make the
|
||
implementation much more complicated and therefore costlier to maintain.
|
||
|
||
hashable
|
||
An object is *hashable* if it has a hash value which never changes during
|
||
its lifetime (it needs a :meth:`__hash__` method), and can be compared to
|
||
other objects (it needs an :meth:`__eq__` method). Hashable objects which
|
||
compare equal must have the same hash value.
|
||
|
||
Hashability makes an object usable as a dictionary key and a set member,
|
||
because these data structures use the hash value internally.
|
||
|
||
All of Python's immutable built-in objects are hashable; mutable
|
||
containers (such as lists or dictionaries) are not. Objects which are
|
||
instances of user-defined classes are hashable by default. They all
|
||
compare unequal (except with themselves), and their hash value is derived
|
||
from their :func:`id`.
|
||
|
||
IDLE
|
||
An Integrated Development Environment for Python. IDLE is a basic editor
|
||
and interpreter environment which ships with the standard distribution of
|
||
Python.
|
||
|
||
immutable
|
||
An object with a fixed value. Immutable objects include numbers, strings and
|
||
tuples. Such an object cannot be altered. A new object has to
|
||
be created if a different value has to be stored. They play an important
|
||
role in places where a constant hash value is needed, for example as a key
|
||
in a dictionary.
|
||
|
||
import path
|
||
A list of locations (or :term:`path entries <path entry>`) that are
|
||
searched by the :term:`path based finder` for modules to import. During
|
||
import, this list of locations usually comes from :data:`sys.path`, but
|
||
for subpackages it may also come from the parent package's ``__path__``
|
||
attribute.
|
||
|
||
importing
|
||
The process by which Python code in one module is made available to
|
||
Python code in another module.
|
||
|
||
importer
|
||
An object that both finds and loads a module; both a
|
||
:term:`finder` and :term:`loader` object.
|
||
|
||
interactive
|
||
Python has an interactive interpreter which means you can enter
|
||
statements and expressions at the interpreter prompt, immediately
|
||
execute them and see their results. Just launch ``python`` with no
|
||
arguments (possibly by selecting it from your computer's main
|
||
menu). It is a very powerful way to test out new ideas or inspect
|
||
modules and packages (remember ``help(x)``).
|
||
|
||
interpreted
|
||
Python is an interpreted language, as opposed to a compiled one,
|
||
though the distinction can be blurry because of the presence of the
|
||
bytecode compiler. This means that source files can be run directly
|
||
without explicitly creating an executable which is then run.
|
||
Interpreted languages typically have a shorter development/debug cycle
|
||
than compiled ones, though their programs generally also run more
|
||
slowly. See also :term:`interactive`.
|
||
|
||
interpreter shutdown
|
||
When asked to shut down, the Python interpreter enters a special phase
|
||
where it gradually releases all allocated resources, such as modules
|
||
and various critical internal structures. It also makes several calls
|
||
to the :term:`garbage collector <garbage collection>`. This can trigger
|
||
the execution of code in user-defined destructors or weakref callbacks.
|
||
Code executed during the shutdown phase can encounter various
|
||
exceptions as the resources it relies on may not function anymore
|
||
(common examples are library modules or the warnings machinery).
|
||
|
||
The main reason for interpreter shutdown is that the ``__main__`` module
|
||
or the script being run has finished executing.
|
||
|
||
iterable
|
||
An object capable of returning its members one at a time. Examples of
|
||
iterables include all sequence types (such as :class:`list`, :class:`str`,
|
||
and :class:`tuple`) and some non-sequence types like :class:`dict`,
|
||
:term:`file objects <file object>`, and objects of any classes you define
|
||
with an :meth:`__iter__` method or with a :meth:`__getitem__` method
|
||
that implements :term:`Sequence` semantics.
|
||
|
||
Iterables can be
|
||
used in a :keyword:`for` loop and in many other places where a sequence is
|
||
needed (:func:`zip`, :func:`map`, ...). When an iterable object is passed
|
||
as an argument to the built-in function :func:`iter`, it returns an
|
||
iterator for the object. This iterator is good for one pass over the set
|
||
of values. When using iterables, it is usually not necessary to call
|
||
:func:`iter` or deal with iterator objects yourself. The ``for``
|
||
statement does that automatically for you, creating a temporary unnamed
|
||
variable to hold the iterator for the duration of the loop. See also
|
||
:term:`iterator`, :term:`sequence`, and :term:`generator`.
|
||
|
||
iterator
|
||
An object representing a stream of data. Repeated calls to the iterator's
|
||
:meth:`~iterator.__next__` method (or passing it to the built-in function
|
||
:func:`next`) return successive items in the stream. When no more data
|
||
are available a :exc:`StopIteration` exception is raised instead. At this
|
||
point, the iterator object is exhausted and any further calls to its
|
||
:meth:`__next__` method just raise :exc:`StopIteration` again. Iterators
|
||
are required to have an :meth:`__iter__` method that returns the iterator
|
||
object itself so every iterator is also iterable and may be used in most
|
||
places where other iterables are accepted. One notable exception is code
|
||
which attempts multiple iteration passes. A container object (such as a
|
||
:class:`list`) produces a fresh new iterator each time you pass it to the
|
||
:func:`iter` function or use it in a :keyword:`for` loop. Attempting this
|
||
with an iterator will just return the same exhausted iterator object used
|
||
in the previous iteration pass, making it appear like an empty container.
|
||
|
||
More information can be found in :ref:`typeiter`.
|
||
|
||
key function
|
||
A key function or collation function is a callable that returns a value
|
||
used for sorting or ordering. For example, :func:`locale.strxfrm` is
|
||
used to produce a sort key that is aware of locale specific sort
|
||
conventions.
|
||
|
||
A number of tools in Python accept key functions to control how elements
|
||
are ordered or grouped. They include :func:`min`, :func:`max`,
|
||
:func:`sorted`, :meth:`list.sort`, :func:`heapq.merge`,
|
||
:func:`heapq.nsmallest`, :func:`heapq.nlargest`, and
|
||
:func:`itertools.groupby`.
|
||
|
||
There are several ways to create a key function. For example. the
|
||
:meth:`str.lower` method can serve as a key function for case insensitive
|
||
sorts. Alternatively, a key function can be built from a
|
||
:keyword:`lambda` expression such as ``lambda r: (r[0], r[2])``. Also,
|
||
the :mod:`operator` module provides three key function constructors:
|
||
:func:`~operator.attrgetter`, :func:`~operator.itemgetter`, and
|
||
:func:`~operator.methodcaller`. See the :ref:`Sorting HOW TO
|
||
<sortinghowto>` for examples of how to create and use key functions.
|
||
|
||
keyword argument
|
||
See :term:`argument`.
|
||
|
||
lambda
|
||
An anonymous inline function consisting of a single :term:`expression`
|
||
which is evaluated when the function is called. The syntax to create
|
||
a lambda function is ``lambda [arguments]: expression``
|
||
|
||
LBYL
|
||
Look before you leap. This coding style explicitly tests for
|
||
pre-conditions before making calls or lookups. This style contrasts with
|
||
the :term:`EAFP` approach and is characterized by the presence of many
|
||
:keyword:`if` statements.
|
||
|
||
In a multi-threaded environment, the LBYL approach can risk introducing a
|
||
race condition between "the looking" and "the leaping". For example, the
|
||
code, ``if key in mapping: return mapping[key]`` can fail if another
|
||
thread removes *key* from *mapping* after the test, but before the lookup.
|
||
This issue can be solved with locks or by using the EAFP approach.
|
||
|
||
list
|
||
A built-in Python :term:`sequence`. Despite its name it is more akin
|
||
to an array in other languages than to a linked list since access to
|
||
elements are O(1).
|
||
|
||
list comprehension
|
||
A compact way to process all or part of the elements in a sequence and
|
||
return a list with the results. ``result = ['{:#04x}'.format(x) for x in
|
||
range(256) if x % 2 == 0]`` generates a list of strings containing
|
||
even hex numbers (0x..) in the range from 0 to 255. The :keyword:`if`
|
||
clause is optional. If omitted, all elements in ``range(256)`` are
|
||
processed.
|
||
|
||
loader
|
||
An object that loads a module. It must define a method named
|
||
:meth:`load_module`. A loader is typically returned by a
|
||
:term:`finder`. See :pep:`302` for details and
|
||
:class:`importlib.abc.Loader` for an :term:`abstract base class`.
|
||
|
||
mapping
|
||
A container object that supports arbitrary key lookups and implements the
|
||
methods specified in the :class:`~collections.abc.Mapping` or
|
||
:class:`~collections.abc.MutableMapping`
|
||
:ref:`abstract base classes <collections-abstract-base-classes>`. Examples
|
||
include :class:`dict`, :class:`collections.defaultdict`,
|
||
:class:`collections.OrderedDict` and :class:`collections.Counter`.
|
||
|
||
meta path finder
|
||
A :term:`finder` returned by a search of :data:`sys.meta_path`. Meta path
|
||
finders are related to, but different from :term:`path entry finders
|
||
<path entry finder>`.
|
||
|
||
See :class:`importlib.abc.MetaPathFinder` for the methods that meta path
|
||
finders implement.
|
||
|
||
metaclass
|
||
The class of a class. Class definitions create a class name, a class
|
||
dictionary, and a list of base classes. The metaclass is responsible for
|
||
taking those three arguments and creating the class. Most object oriented
|
||
programming languages provide a default implementation. What makes Python
|
||
special is that it is possible to create custom metaclasses. Most users
|
||
never need this tool, but when the need arises, metaclasses can provide
|
||
powerful, elegant solutions. They have been used for logging attribute
|
||
access, adding thread-safety, tracking object creation, implementing
|
||
singletons, and many other tasks.
|
||
|
||
More information can be found in :ref:`metaclasses`.
|
||
|
||
method
|
||
A function which is defined inside a class body. If called as an attribute
|
||
of an instance of that class, the method will get the instance object as
|
||
its first :term:`argument` (which is usually called ``self``).
|
||
See :term:`function` and :term:`nested scope`.
|
||
|
||
method resolution order
|
||
Method Resolution Order is the order in which base classes are searched
|
||
for a member during lookup. See `The Python 2.3 Method Resolution Order
|
||
<https://www.python.org/download/releases/2.3/mro/>`_ for details of the
|
||
algorithm used by the Python interpreter since the 2.3 release.
|
||
|
||
module
|
||
An object that serves as an organizational unit of Python code. Modules
|
||
have a namespace containing arbitrary Python objects. Modules are loaded
|
||
into Python by the process of :term:`importing`.
|
||
|
||
See also :term:`package`.
|
||
|
||
module spec
|
||
A namespace containing the import-related information used to load a
|
||
module. An instance of :class:`importlib.machinery.ModuleSpec`.
|
||
|
||
MRO
|
||
See :term:`method resolution order`.
|
||
|
||
mutable
|
||
Mutable objects can change their value but keep their :func:`id`. See
|
||
also :term:`immutable`.
|
||
|
||
named tuple
|
||
Any tuple-like class whose indexable elements are also accessible using
|
||
named attributes (for example, :func:`time.localtime` returns a
|
||
tuple-like object where the *year* is accessible either with an
|
||
index such as ``t[0]`` or with a named attribute like ``t.tm_year``).
|
||
|
||
A named tuple can be a built-in type such as :class:`time.struct_time`,
|
||
or it can be created with a regular class definition. A full featured
|
||
named tuple can also be created with the factory function
|
||
:func:`collections.namedtuple`. The latter approach automatically
|
||
provides extra features such as a self-documenting representation like
|
||
``Employee(name='jones', title='programmer')``.
|
||
|
||
namespace
|
||
The place where a variable is stored. Namespaces are implemented as
|
||
dictionaries. There are the local, global and built-in namespaces as well
|
||
as nested namespaces in objects (in methods). Namespaces support
|
||
modularity by preventing naming conflicts. For instance, the functions
|
||
:func:`builtins.open <.open>` and :func:`os.open` are distinguished by
|
||
their namespaces. Namespaces also aid readability and maintainability by
|
||
making it clear which module implements a function. For instance, writing
|
||
:func:`random.seed` or :func:`itertools.islice` makes it clear that those
|
||
functions are implemented by the :mod:`random` and :mod:`itertools`
|
||
modules, respectively.
|
||
|
||
namespace package
|
||
A :pep:`420` :term:`package` which serves only as a container for
|
||
subpackages. Namespace packages may have no physical representation,
|
||
and specifically are not like a :term:`regular package` because they
|
||
have no ``__init__.py`` file.
|
||
|
||
See also :term:`module`.
|
||
|
||
nested scope
|
||
The ability to refer to a variable in an enclosing definition. For
|
||
instance, a function defined inside another function can refer to
|
||
variables in the outer function. Note that nested scopes by default work
|
||
only for reference and not for assignment. Local variables both read and
|
||
write in the innermost scope. Likewise, global variables read and write
|
||
to the global namespace. The :keyword:`nonlocal` allows writing to outer
|
||
scopes.
|
||
|
||
new-style class
|
||
Old name for the flavor of classes now used for all class objects. In
|
||
earlier Python versions, only new-style classes could use Python's newer,
|
||
versatile features like :attr:`~object.__slots__`, descriptors,
|
||
properties, :meth:`__getattribute__`, class methods, and static methods.
|
||
|
||
object
|
||
Any data with state (attributes or value) and defined behavior
|
||
(methods). Also the ultimate base class of any :term:`new-style
|
||
class`.
|
||
|
||
package
|
||
A Python :term:`module` which can contain submodules or recursively,
|
||
subpackages. Technically, a package is a Python module with an
|
||
``__path__`` attribute.
|
||
|
||
See also :term:`regular package` and :term:`namespace package`.
|
||
|
||
parameter
|
||
A named entity in a :term:`function` (or method) definition that
|
||
specifies an :term:`argument` (or in some cases, arguments) that the
|
||
function can accept. There are five kinds of parameter:
|
||
|
||
* :dfn:`positional-or-keyword`: specifies an argument that can be passed
|
||
either :term:`positionally <argument>` or as a :term:`keyword argument
|
||
<argument>`. This is the default kind of parameter, for example *foo*
|
||
and *bar* in the following::
|
||
|
||
def func(foo, bar=None): ...
|
||
|
||
.. _positional-only_parameter:
|
||
|
||
* :dfn:`positional-only`: specifies an argument that can be supplied only
|
||
by position. Python has no syntax for defining positional-only
|
||
parameters. However, some built-in functions have positional-only
|
||
parameters (e.g. :func:`abs`).
|
||
|
||
.. _keyword-only_parameter:
|
||
|
||
* :dfn:`keyword-only`: specifies an argument that can be supplied only
|
||
by keyword. Keyword-only parameters can be defined by including a
|
||
single var-positional parameter or bare ``*`` in the parameter list
|
||
of the function definition before them, for example *kw_only1* and
|
||
*kw_only2* in the following::
|
||
|
||
def func(arg, *, kw_only1, kw_only2): ...
|
||
|
||
* :dfn:`var-positional`: specifies that an arbitrary sequence of
|
||
positional arguments can be provided (in addition to any positional
|
||
arguments already accepted by other parameters). Such a parameter can
|
||
be defined by prepending the parameter name with ``*``, for example
|
||
*args* in the following::
|
||
|
||
def func(*args, **kwargs): ...
|
||
|
||
* :dfn:`var-keyword`: specifies that arbitrarily many keyword arguments
|
||
can be provided (in addition to any keyword arguments already accepted
|
||
by other parameters). Such a parameter can be defined by prepending
|
||
the parameter name with ``**``, for example *kwargs* in the example
|
||
above.
|
||
|
||
Parameters can specify both optional and required arguments, as well as
|
||
default values for some optional arguments.
|
||
|
||
See also the :term:`argument` glossary entry, the FAQ question on
|
||
:ref:`the difference between arguments and parameters
|
||
<faq-argument-vs-parameter>`, the :class:`inspect.Parameter` class, the
|
||
:ref:`function` section, and :pep:`362`.
|
||
|
||
path entry
|
||
A single location on the :term:`import path` which the :term:`path
|
||
based finder` consults to find modules for importing.
|
||
|
||
path entry finder
|
||
A :term:`finder` returned by a callable on :data:`sys.path_hooks`
|
||
(i.e. a :term:`path entry hook`) which knows how to locate modules given
|
||
a :term:`path entry`.
|
||
|
||
See :class:`importlib.abc.PathEntryFinder` for the methods that path entry
|
||
finders implement.
|
||
|
||
path entry hook
|
||
A callable on the :data:`sys.path_hook` list which returns a :term:`path
|
||
entry finder` if it knows how to find modules on a specific :term:`path
|
||
entry`.
|
||
|
||
path based finder
|
||
One of the default :term:`meta path finders <meta path finder>` which
|
||
searches an :term:`import path` for modules.
|
||
|
||
path-like object
|
||
An object representing a file system path. A path-like object is either
|
||
a :class:`str` or :class:`bytes` object representing a path, or an object
|
||
implementing the :class:`os.PathLike` protocol. An object that supports
|
||
the :class:`os.PathLike` protocol can be converted to a :class:`str` or
|
||
:class:`bytes` file system path by calling the :func:`os.fspath` function;
|
||
:func:`os.fsdecode` and :func:`os.fsencode` can be used to guarantee a
|
||
:class:`str` or :class:`bytes` result instead, respectively. Introduced
|
||
by :pep:`519`.
|
||
|
||
portion
|
||
A set of files in a single directory (possibly stored in a zip file)
|
||
that contribute to a namespace package, as defined in :pep:`420`.
|
||
|
||
positional argument
|
||
See :term:`argument`.
|
||
|
||
provisional API
|
||
A provisional API is one which has been deliberately excluded from
|
||
the standard library's backwards compatibility guarantees. While major
|
||
changes to such interfaces are not expected, as long as they are marked
|
||
provisional, backwards incompatible changes (up to and including removal
|
||
of the interface) may occur if deemed necessary by core developers. Such
|
||
changes will not be made gratuitously -- they will occur only if serious
|
||
fundamental flaws are uncovered that were missed prior to the inclusion
|
||
of the API.
|
||
|
||
Even for provisional APIs, backwards incompatible changes are seen as
|
||
a "solution of last resort" - every attempt will still be made to find
|
||
a backwards compatible resolution to any identified problems.
|
||
|
||
This process allows the standard library to continue to evolve over
|
||
time, without locking in problematic design errors for extended periods
|
||
of time. See :pep:`411` for more details.
|
||
|
||
provisional package
|
||
See :term:`provisional API`.
|
||
|
||
Python 3000
|
||
Nickname for the Python 3.x release line (coined long ago when the
|
||
release of version 3 was something in the distant future.) This is also
|
||
abbreviated "Py3k".
|
||
|
||
Pythonic
|
||
An idea or piece of code which closely follows the most common idioms
|
||
of the Python language, rather than implementing code using concepts
|
||
common to other languages. For example, a common idiom in Python is
|
||
to loop over all elements of an iterable using a :keyword:`for`
|
||
statement. Many other languages don't have this type of construct, so
|
||
people unfamiliar with Python sometimes use a numerical counter instead::
|
||
|
||
for i in range(len(food)):
|
||
print(food[i])
|
||
|
||
As opposed to the cleaner, Pythonic method::
|
||
|
||
for piece in food:
|
||
print(piece)
|
||
|
||
qualified name
|
||
A dotted name showing the "path" from a module's global scope to a
|
||
class, function or method defined in that module, as defined in
|
||
:pep:`3155`. For top-level functions and classes, the qualified name
|
||
is the same as the object's name::
|
||
|
||
>>> class C:
|
||
... class D:
|
||
... def meth(self):
|
||
... pass
|
||
...
|
||
>>> C.__qualname__
|
||
'C'
|
||
>>> C.D.__qualname__
|
||
'C.D'
|
||
>>> C.D.meth.__qualname__
|
||
'C.D.meth'
|
||
|
||
When used to refer to modules, the *fully qualified name* means the
|
||
entire dotted path to the module, including any parent packages,
|
||
e.g. ``email.mime.text``::
|
||
|
||
>>> import email.mime.text
|
||
>>> email.mime.text.__name__
|
||
'email.mime.text'
|
||
|
||
reference count
|
||
The number of references to an object. When the reference count of an
|
||
object drops to zero, it is deallocated. Reference counting is
|
||
generally not visible to Python code, but it is a key element of the
|
||
:term:`CPython` implementation. The :mod:`sys` module defines a
|
||
:func:`~sys.getrefcount` function that programmers can call to return the
|
||
reference count for a particular object.
|
||
|
||
regular package
|
||
A traditional :term:`package`, such as a directory containing an
|
||
``__init__.py`` file.
|
||
|
||
See also :term:`namespace package`.
|
||
|
||
__slots__
|
||
A declaration inside a class that saves memory by pre-declaring space for
|
||
instance attributes and eliminating instance dictionaries. Though
|
||
popular, the technique is somewhat tricky to get right and is best
|
||
reserved for rare cases where there are large numbers of instances in a
|
||
memory-critical application.
|
||
|
||
sequence
|
||
An :term:`iterable` which supports efficient element access using integer
|
||
indices via the :meth:`__getitem__` special method and defines a
|
||
:meth:`__len__` method that returns the length of the sequence.
|
||
Some built-in sequence types are :class:`list`, :class:`str`,
|
||
:class:`tuple`, and :class:`bytes`. Note that :class:`dict` also
|
||
supports :meth:`__getitem__` and :meth:`__len__`, but is considered a
|
||
mapping rather than a sequence because the lookups use arbitrary
|
||
:term:`immutable` keys rather than integers.
|
||
|
||
The :class:`collections.abc.Sequence` abstract base class
|
||
defines a much richer interface that goes beyond just
|
||
:meth:`__getitem__` and :meth:`__len__`, adding :meth:`count`,
|
||
:meth:`index`, :meth:`__contains__`, and
|
||
:meth:`__reversed__`. Types that implement this expanded
|
||
interface can be registered explicitly using
|
||
:func:`~abc.register`.
|
||
|
||
single dispatch
|
||
A form of :term:`generic function` dispatch where the implementation is
|
||
chosen based on the type of a single argument.
|
||
|
||
slice
|
||
An object usually containing a portion of a :term:`sequence`. A slice is
|
||
created using the subscript notation, ``[]`` with colons between numbers
|
||
when several are given, such as in ``variable_name[1:3:5]``. The bracket
|
||
(subscript) notation uses :class:`slice` objects internally.
|
||
|
||
special method
|
||
A method that is called implicitly by Python to execute a certain
|
||
operation on a type, such as addition. Such methods have names starting
|
||
and ending with double underscores. Special methods are documented in
|
||
:ref:`specialnames`.
|
||
|
||
statement
|
||
A statement is part of a suite (a "block" of code). A statement is either
|
||
an :term:`expression` or one of several constructs with a keyword, such
|
||
as :keyword:`if`, :keyword:`while` or :keyword:`for`.
|
||
|
||
struct sequence
|
||
A tuple with named elements. Struct sequences expose an interface similar
|
||
to :term:`named tuple` in that elements can either be accessed either by
|
||
index or as an attribute. However, they do not have any of the named tuple
|
||
methods like :meth:`~collections.somenamedtuple._make` or
|
||
:meth:`~collections.somenamedtuple._asdict`. Examples of struct sequences
|
||
include :data:`sys.float_info` and the return value of :func:`os.stat`.
|
||
|
||
text encoding
|
||
A codec which encodes Unicode strings to bytes.
|
||
|
||
text file
|
||
A :term:`file object` able to read and write :class:`str` objects.
|
||
Often, a text file actually accesses a byte-oriented datastream
|
||
and handles the :term:`text encoding` automatically.
|
||
Examples of text files are files opened in text mode (``'r'`` or ``'w'``),
|
||
:data:`sys.stdin`, :data:`sys.stdout`, and instances of
|
||
:class:`io.StringIO`.
|
||
|
||
.. seealso::
|
||
A :term:`binary file` reads and write :class:`bytes` objects.
|
||
|
||
triple-quoted string
|
||
A string which is bound by three instances of either a quotation mark
|
||
(") or an apostrophe ('). While they don't provide any functionality
|
||
not available with single-quoted strings, they are useful for a number
|
||
of reasons. They allow you to include unescaped single and double
|
||
quotes within a string and they can span multiple lines without the
|
||
use of the continuation character, making them especially useful when
|
||
writing docstrings.
|
||
|
||
type
|
||
The type of a Python object determines what kind of object it is; every
|
||
object has a type. An object's type is accessible as its
|
||
:attr:`~instance.__class__` attribute or can be retrieved with
|
||
``type(obj)``.
|
||
|
||
universal newlines
|
||
A manner of interpreting text streams in which all of the following are
|
||
recognized as ending a line: the Unix end-of-line convention ``'\n'``,
|
||
the Windows convention ``'\r\n'``, and the old Macintosh convention
|
||
``'\r'``. See :pep:`278` and :pep:`3116`, as well as
|
||
:func:`bytes.splitlines` for an additional use.
|
||
|
||
variable annotation
|
||
A type metadata value associated with a module global variable or
|
||
a class attribute. Its syntax is explained in section :ref:`annassign`.
|
||
Annotations are stored in the :attr:`__annotations__` special
|
||
attribute of a class or module object and can be accessed using
|
||
:func:`typing.get_type_hints`.
|
||
|
||
Python itself does not assign any particular meaning to variable
|
||
annotations. They are intended to be interpreted by third-party libraries
|
||
or type checking tools. See :pep:`526`, :pep:`484` which describe
|
||
some of their potential uses.
|
||
|
||
virtual environment
|
||
A cooperatively isolated runtime environment that allows Python users
|
||
and applications to install and upgrade Python distribution packages
|
||
without interfering with the behaviour of other Python applications
|
||
running on the same system.
|
||
|
||
See also :mod:`venv`.
|
||
|
||
virtual machine
|
||
A computer defined entirely in software. Python's virtual machine
|
||
executes the :term:`bytecode` emitted by the bytecode compiler.
|
||
|
||
Zen of Python
|
||
Listing of Python design principles and philosophies that are helpful in
|
||
understanding and using the language. The listing can be found by typing
|
||
"``import this``" at the interactive prompt.
|