573 lines
23 KiB
ReStructuredText
573 lines
23 KiB
ReStructuredText
.. _tut-modules:
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*******
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Modules
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*******
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If you quit from the Python interpreter and enter it again, the definitions you
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have made (functions and variables) are lost. Therefore, if you want to write a
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somewhat longer program, you are better off using a text editor to prepare the
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input for the interpreter and running it with that file as input instead. This
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is known as creating a *script*. As your program gets longer, you may want to
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split it into several files for easier maintenance. You may also want to use a
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handy function that you've written in several programs without copying its
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definition into each program.
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To support this, Python has a way to put definitions in a file and use them in a
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script or in an interactive instance of the interpreter. Such a file is called a
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*module*; definitions from a module can be *imported* into other modules or into
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the *main* module (the collection of variables that you have access to in a
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script executed at the top level and in calculator mode).
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A module is a file containing Python definitions and statements. The file name
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is the module name with the suffix :file:`.py` appended. Within a module, the
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module's name (as a string) is available as the value of the global variable
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``__name__``. For instance, use your favorite text editor to create a file
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called :file:`fibo.py` in the current directory with the following contents::
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# Fibonacci numbers module
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def fib(n): # write Fibonacci series up to n
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a, b = 0, 1
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while a < n:
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print(a, end=' ')
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a, b = b, a+b
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print()
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def fib2(n): # return Fibonacci series up to n
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result = []
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a, b = 0, 1
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while a < n:
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result.append(a)
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a, b = b, a+b
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return result
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Now enter the Python interpreter and import this module with the following
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command::
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>>> import fibo
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This does not enter the names of the functions defined in ``fibo`` directly in
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the current symbol table; it only enters the module name ``fibo`` there. Using
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the module name you can access the functions::
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>>> fibo.fib(1000)
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0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
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>>> fibo.fib2(100)
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[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
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>>> fibo.__name__
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'fibo'
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If you intend to use a function often you can assign it to a local name::
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>>> fib = fibo.fib
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>>> fib(500)
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0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
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.. _tut-moremodules:
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More on Modules
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===============
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A module can contain executable statements as well as function definitions.
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These statements are intended to initialize the module. They are executed only
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the *first* time the module name is encountered in an import statement. [#]_
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(They are also run if the file is executed as a script.)
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Each module has its own private symbol table, which is used as the global symbol
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table by all functions defined in the module. Thus, the author of a module can
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use global variables in the module without worrying about accidental clashes
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with a user's global variables. On the other hand, if you know what you are
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doing you can touch a module's global variables with the same notation used to
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refer to its functions, ``modname.itemname``.
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Modules can import other modules. It is customary but not required to place all
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:keyword:`import` statements at the beginning of a module (or script, for that
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matter). The imported module names are placed in the importing module's global
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symbol table.
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There is a variant of the :keyword:`import` statement that imports names from a
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module directly into the importing module's symbol table. For example::
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>>> from fibo import fib, fib2
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>>> fib(500)
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0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
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This does not introduce the module name from which the imports are taken in the
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local symbol table (so in the example, ``fibo`` is not defined).
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There is even a variant to import all names that a module defines::
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>>> from fibo import *
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>>> fib(500)
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0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
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This imports all names except those beginning with an underscore (``_``).
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In most cases Python programmers do not use this facility since it introduces
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an unknown set of names into the interpreter, possibly hiding some things
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you have already defined.
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Note that in general the practice of importing ``*`` from a module or package is
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frowned upon, since it often causes poorly readable code. However, it is okay to
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use it to save typing in interactive sessions.
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If the module name is followed by :keyword:`as`, then the name
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following :keyword:`as` is bound directly to the imported module.
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::
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>>> import fibo as fib
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>>> fib.fib(500)
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0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
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This is effectively importing the module in the same way that ``import fibo``
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will do, with the only difference of it being available as ``fib``.
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It can also be used when utilising :keyword:`from` with similar effects::
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>>> from fibo import fib as fibonacci
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>>> fibonacci(500)
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0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
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.. note::
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For efficiency reasons, each module is only imported once per interpreter
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session. Therefore, if you change your modules, you must restart the
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interpreter -- or, if it's just one module you want to test interactively,
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use :func:`importlib.reload`, e.g. ``import importlib;
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importlib.reload(modulename)``.
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.. _tut-modulesasscripts:
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Executing modules as scripts
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----------------------------
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When you run a Python module with ::
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python fibo.py <arguments>
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the code in the module will be executed, just as if you imported it, but with
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the ``__name__`` set to ``"__main__"``. That means that by adding this code at
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the end of your module::
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if __name__ == "__main__":
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import sys
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fib(int(sys.argv[1]))
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you can make the file usable as a script as well as an importable module,
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because the code that parses the command line only runs if the module is
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executed as the "main" file:
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.. code-block:: shell-session
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$ python fibo.py 50
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0 1 1 2 3 5 8 13 21 34
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If the module is imported, the code is not run::
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>>> import fibo
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>>>
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This is often used either to provide a convenient user interface to a module, or
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for testing purposes (running the module as a script executes a test suite).
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.. _tut-searchpath:
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The Module Search Path
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----------------------
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.. index:: triple: module; search; path
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When a module named :mod:`spam` is imported, the interpreter first searches for
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a built-in module with that name. If not found, it then searches for a file
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named :file:`spam.py` in a list of directories given by the variable
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:data:`sys.path`. :data:`sys.path` is initialized from these locations:
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* The directory containing the input script (or the current directory when no
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file is specified).
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* :envvar:`PYTHONPATH` (a list of directory names, with the same syntax as the
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shell variable :envvar:`PATH`).
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* The installation-dependent default.
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.. note::
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On file systems which support symlinks, the directory containing the input
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script is calculated after the symlink is followed. In other words the
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directory containing the symlink is **not** added to the module search path.
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After initialization, Python programs can modify :data:`sys.path`. The
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directory containing the script being run is placed at the beginning of the
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search path, ahead of the standard library path. This means that scripts in that
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directory will be loaded instead of modules of the same name in the library
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directory. This is an error unless the replacement is intended. See section
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:ref:`tut-standardmodules` for more information.
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.. %
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Do we need stuff on zip files etc. ? DUBOIS
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"Compiled" Python files
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-----------------------
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To speed up loading modules, Python caches the compiled version of each module
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in the ``__pycache__`` directory under the name :file:`module.{version}.pyc`,
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where the version encodes the format of the compiled file; it generally contains
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the Python version number. For example, in CPython release 3.3 the compiled
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version of spam.py would be cached as ``__pycache__/spam.cpython-33.pyc``. This
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naming convention allows compiled modules from different releases and different
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versions of Python to coexist.
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Python checks the modification date of the source against the compiled version
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to see if it's out of date and needs to be recompiled. This is a completely
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automatic process. Also, the compiled modules are platform-independent, so the
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same library can be shared among systems with different architectures.
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Python does not check the cache in two circumstances. First, it always
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recompiles and does not store the result for the module that's loaded directly
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from the command line. Second, it does not check the cache if there is no
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source module. To support a non-source (compiled only) distribution, the
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compiled module must be in the source directory, and there must not be a source
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module.
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Some tips for experts:
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* You can use the :option:`-O` or :option:`-OO` switches on the Python command
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to reduce the size of a compiled module. The ``-O`` switch removes assert
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statements, the ``-OO`` switch removes both assert statements and __doc__
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strings. Since some programs may rely on having these available, you should
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only use this option if you know what you're doing. "Optimized" modules have
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an ``opt-`` tag and are usually smaller. Future releases may
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change the effects of optimization.
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* A program doesn't run any faster when it is read from a ``.pyc``
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file than when it is read from a ``.py`` file; the only thing that's faster
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about ``.pyc`` files is the speed with which they are loaded.
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* The module :mod:`compileall` can create .pyc files for all modules in a
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directory.
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* There is more detail on this process, including a flow chart of the
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decisions, in :pep:`3147`.
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.. _tut-standardmodules:
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Standard Modules
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================
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.. index:: module: sys
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Python comes with a library of standard modules, described in a separate
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document, the Python Library Reference ("Library Reference" hereafter). Some
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modules are built into the interpreter; these provide access to operations that
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are not part of the core of the language but are nevertheless built in, either
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for efficiency or to provide access to operating system primitives such as
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system calls. The set of such modules is a configuration option which also
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depends on the underlying platform. For example, the :mod:`winreg` module is only
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provided on Windows systems. One particular module deserves some attention:
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:mod:`sys`, which is built into every Python interpreter. The variables
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``sys.ps1`` and ``sys.ps2`` define the strings used as primary and secondary
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prompts::
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>>> import sys
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>>> sys.ps1
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'>>> '
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>>> sys.ps2
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'... '
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>>> sys.ps1 = 'C> '
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C> print('Yuck!')
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Yuck!
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C>
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These two variables are only defined if the interpreter is in interactive mode.
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The variable ``sys.path`` is a list of strings that determines the interpreter's
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search path for modules. It is initialized to a default path taken from the
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environment variable :envvar:`PYTHONPATH`, or from a built-in default if
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:envvar:`PYTHONPATH` is not set. You can modify it using standard list
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operations::
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>>> import sys
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>>> sys.path.append('/ufs/guido/lib/python')
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.. _tut-dir:
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The :func:`dir` Function
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========================
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The built-in function :func:`dir` is used to find out which names a module
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defines. It returns a sorted list of strings::
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>>> import fibo, sys
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>>> dir(fibo)
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['__name__', 'fib', 'fib2']
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>>> dir(sys) # doctest: +NORMALIZE_WHITESPACE
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['__displayhook__', '__doc__', '__excepthook__', '__loader__', '__name__',
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'__package__', '__stderr__', '__stdin__', '__stdout__',
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'_clear_type_cache', '_current_frames', '_debugmallocstats', '_getframe',
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'_home', '_mercurial', '_xoptions', 'abiflags', 'api_version', 'argv',
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'base_exec_prefix', 'base_prefix', 'builtin_module_names', 'byteorder',
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'call_tracing', 'callstats', 'copyright', 'displayhook',
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'dont_write_bytecode', 'exc_info', 'excepthook', 'exec_prefix',
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'executable', 'exit', 'flags', 'float_info', 'float_repr_style',
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'getcheckinterval', 'getdefaultencoding', 'getdlopenflags',
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'getfilesystemencoding', 'getobjects', 'getprofile', 'getrecursionlimit',
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'getrefcount', 'getsizeof', 'getswitchinterval', 'gettotalrefcount',
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'gettrace', 'hash_info', 'hexversion', 'implementation', 'int_info',
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'intern', 'maxsize', 'maxunicode', 'meta_path', 'modules', 'path',
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'path_hooks', 'path_importer_cache', 'platform', 'prefix', 'ps1',
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'setcheckinterval', 'setdlopenflags', 'setprofile', 'setrecursionlimit',
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'setswitchinterval', 'settrace', 'stderr', 'stdin', 'stdout',
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'thread_info', 'version', 'version_info', 'warnoptions']
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Without arguments, :func:`dir` lists the names you have defined currently::
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>>> a = [1, 2, 3, 4, 5]
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>>> import fibo
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>>> fib = fibo.fib
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>>> dir()
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['__builtins__', '__name__', 'a', 'fib', 'fibo', 'sys']
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Note that it lists all types of names: variables, modules, functions, etc.
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.. index:: module: builtins
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:func:`dir` does not list the names of built-in functions and variables. If you
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want a list of those, they are defined in the standard module
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:mod:`builtins`::
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>>> import builtins
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>>> dir(builtins) # doctest: +NORMALIZE_WHITESPACE
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['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException',
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'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning',
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'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError',
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'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning',
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'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False',
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'FileExistsError', 'FileNotFoundError', 'FloatingPointError',
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'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError',
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'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError',
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'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError',
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'MemoryError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented',
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'NotImplementedError', 'OSError', 'OverflowError',
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'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError',
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'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning',
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'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError',
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'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError',
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'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError',
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'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning',
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'ValueError', 'Warning', 'ZeroDivisionError', '_', '__build_class__',
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'__debug__', '__doc__', '__import__', '__name__', '__package__', 'abs',
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'all', 'any', 'ascii', 'bin', 'bool', 'bytearray', 'bytes', 'callable',
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'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits',
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'delattr', 'dict', 'dir', 'divmod', 'enumerate', 'eval', 'exec', 'exit',
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'filter', 'float', 'format', 'frozenset', 'getattr', 'globals', 'hasattr',
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'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass',
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'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview',
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'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property',
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'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr', 'slice',
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'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars',
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'zip']
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.. _tut-packages:
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Packages
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========
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Packages are a way of structuring Python's module namespace by using "dotted
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module names". For example, the module name :mod:`A.B` designates a submodule
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named ``B`` in a package named ``A``. Just like the use of modules saves the
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authors of different modules from having to worry about each other's global
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variable names, the use of dotted module names saves the authors of multi-module
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packages like NumPy or Pillow from having to worry about
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each other's module names.
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Suppose you want to design a collection of modules (a "package") for the uniform
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handling of sound files and sound data. There are many different sound file
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formats (usually recognized by their extension, for example: :file:`.wav`,
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:file:`.aiff`, :file:`.au`), so you may need to create and maintain a growing
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collection of modules for the conversion between the various file formats.
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There are also many different operations you might want to perform on sound data
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(such as mixing, adding echo, applying an equalizer function, creating an
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artificial stereo effect), so in addition you will be writing a never-ending
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stream of modules to perform these operations. Here's a possible structure for
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your package (expressed in terms of a hierarchical filesystem):
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.. code-block:: text
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sound/ Top-level package
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__init__.py Initialize the sound package
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formats/ Subpackage for file format conversions
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__init__.py
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wavread.py
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wavwrite.py
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aiffread.py
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aiffwrite.py
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auread.py
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auwrite.py
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...
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effects/ Subpackage for sound effects
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__init__.py
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echo.py
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surround.py
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reverse.py
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...
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filters/ Subpackage for filters
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__init__.py
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equalizer.py
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vocoder.py
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karaoke.py
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...
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When importing the package, Python searches through the directories on
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``sys.path`` looking for the package subdirectory.
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The :file:`__init__.py` files are required to make Python treat the directories
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as containing packages; this is done to prevent directories with a common name,
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such as ``string``, from unintentionally hiding valid modules that occur later
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on the module search path. In the simplest case, :file:`__init__.py` can just be
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an empty file, but it can also execute initialization code for the package or
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set the ``__all__`` variable, described later.
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Users of the package can import individual modules from the package, for
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example::
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import sound.effects.echo
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This loads the submodule :mod:`sound.effects.echo`. It must be referenced with
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its full name. ::
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sound.effects.echo.echofilter(input, output, delay=0.7, atten=4)
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An alternative way of importing the submodule is::
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from sound.effects import echo
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This also loads the submodule :mod:`echo`, and makes it available without its
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package prefix, so it can be used as follows::
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echo.echofilter(input, output, delay=0.7, atten=4)
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Yet another variation is to import the desired function or variable directly::
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from sound.effects.echo import echofilter
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Again, this loads the submodule :mod:`echo`, but this makes its function
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:func:`echofilter` directly available::
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echofilter(input, output, delay=0.7, atten=4)
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Note that when using ``from package import item``, the item can be either a
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submodule (or subpackage) of the package, or some other name defined in the
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package, like a function, class or variable. The ``import`` statement first
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tests whether the item is defined in the package; if not, it assumes it is a
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module and attempts to load it. If it fails to find it, an :exc:`ImportError`
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exception is raised.
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Contrarily, when using syntax like ``import item.subitem.subsubitem``, each item
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except for the last must be a package; the last item can be a module or a
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package but can't be a class or function or variable defined in the previous
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item.
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.. _tut-pkg-import-star:
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Importing \* From a Package
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---------------------------
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.. index:: single: __all__
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Now what happens when the user writes ``from sound.effects import *``? Ideally,
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one would hope that this somehow goes out to the filesystem, finds which
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submodules are present in the package, and imports them all. This could take a
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long time and importing sub-modules might have unwanted side-effects that should
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only happen when the sub-module is explicitly imported.
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The only solution is for the package author to provide an explicit index of the
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package. The :keyword:`import` statement uses the following convention: if a package's
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:file:`__init__.py` code defines a list named ``__all__``, it is taken to be the
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list of module names that should be imported when ``from package import *`` is
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encountered. It is up to the package author to keep this list up-to-date when a
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new version of the package is released. Package authors may also decide not to
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|
support it, if they don't see a use for importing \* from their package. For
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example, the file :file:`sound/effects/__init__.py` could contain the following
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code::
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__all__ = ["echo", "surround", "reverse"]
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This would mean that ``from sound.effects import *`` would import the three
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named submodules of the :mod:`sound` package.
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If ``__all__`` is not defined, the statement ``from sound.effects import *``
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|
does *not* import all submodules from the package :mod:`sound.effects` into the
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|
current namespace; it only ensures that the package :mod:`sound.effects` has
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|
been imported (possibly running any initialization code in :file:`__init__.py`)
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|
and then imports whatever names are defined in the package. This includes any
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|
names defined (and submodules explicitly loaded) by :file:`__init__.py`. It
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|
also includes any submodules of the package that were explicitly loaded by
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|
previous :keyword:`import` statements. Consider this code::
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import sound.effects.echo
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import sound.effects.surround
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from sound.effects import *
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In this example, the :mod:`echo` and :mod:`surround` modules are imported in the
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current namespace because they are defined in the :mod:`sound.effects` package
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|
when the ``from...import`` statement is executed. (This also works when
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|
``__all__`` is defined.)
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|
Although certain modules are designed to export only names that follow certain
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|
patterns when you use ``import *``, it is still considered bad practice in
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|
production code.
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|
Remember, there is nothing wrong with using ``from Package import
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|
specific_submodule``! In fact, this is the recommended notation unless the
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importing module needs to use submodules with the same name from different
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|
packages.
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Intra-package References
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|
------------------------
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|
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|
When packages are structured into subpackages (as with the :mod:`sound` package
|
|
in the example), you can use absolute imports to refer to submodules of siblings
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|
packages. For example, if the module :mod:`sound.filters.vocoder` needs to use
|
|
the :mod:`echo` module in the :mod:`sound.effects` package, it can use ``from
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|
sound.effects import echo``.
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|
You can also write relative imports, with the ``from module import name`` form
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|
of import statement. These imports use leading dots to indicate the current and
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|
parent packages involved in the relative import. From the :mod:`surround`
|
|
module for example, you might use::
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|
|
from . import echo
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|
from .. import formats
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|
from ..filters import equalizer
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|
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|
Note that relative imports are based on the name of the current module. Since
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|
the name of the main module is always ``"__main__"``, modules intended for use
|
|
as the main module of a Python application must always use absolute imports.
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|
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|
Packages in Multiple Directories
|
|
--------------------------------
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|
|
|
Packages support one more special attribute, :attr:`__path__`. This is
|
|
initialized to be a list containing the name of the directory holding the
|
|
package's :file:`__init__.py` before the code in that file is executed. This
|
|
variable can be modified; doing so affects future searches for modules and
|
|
subpackages contained in the package.
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|
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|
While this feature is not often needed, it can be used to extend the set of
|
|
modules found in a package.
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.. rubric:: Footnotes
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|
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|
.. [#] In fact function definitions are also 'statements' that are 'executed'; the
|
|
execution of a module-level function definition enters the function name in
|
|
the module's global symbol table.
|