mirror of https://github.com/python/cpython
122 lines
5.8 KiB
ReStructuredText
122 lines
5.8 KiB
ReStructuredText
.. _library-intro:
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************
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Introduction
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************
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The "Python library" contains several different kinds of components.
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It contains data types that would normally be considered part of the "core" of a
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language, such as numbers and lists. For these types, the Python language core
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defines the form of literals and places some constraints on their semantics, but
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does not fully define the semantics. (On the other hand, the language core does
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define syntactic properties like the spelling and priorities of operators.)
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The library also contains built-in functions and exceptions --- objects that can
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be used by all Python code without the need of an :keyword:`import` statement.
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Some of these are defined by the core language, but many are not essential for
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the core semantics and are only described here.
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The bulk of the library, however, consists of a collection of modules. There are
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many ways to dissect this collection. Some modules are written in C and built
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in to the Python interpreter; others are written in Python and imported in
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source form. Some modules provide interfaces that are highly specific to
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Python, like printing a stack trace; some provide interfaces that are specific
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to particular operating systems, such as access to specific hardware; others
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provide interfaces that are specific to a particular application domain, like
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the World Wide Web. Some modules are available in all versions and ports of
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Python; others are only available when the underlying system supports or
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requires them; yet others are available only when a particular configuration
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option was chosen at the time when Python was compiled and installed.
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This manual is organized "from the inside out:" it first describes the built-in
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functions, data types and exceptions, and finally the modules, grouped in
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chapters of related modules.
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This means that if you start reading this manual from the start, and skip to the
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next chapter when you get bored, you will get a reasonable overview of the
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available modules and application areas that are supported by the Python
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library. Of course, you don't *have* to read it like a novel --- you can also
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browse the table of contents (in front of the manual), or look for a specific
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function, module or term in the index (in the back). And finally, if you enjoy
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learning about random subjects, you choose a random page number (see module
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:mod:`random`) and read a section or two. Regardless of the order in which you
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read the sections of this manual, it helps to start with chapter
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:ref:`built-in-funcs`, as the remainder of the manual assumes familiarity with
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this material.
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Let the show begin!
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.. _availability:
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Notes on availability
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=====================
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* An "Availability: Unix" note means that this function is commonly found on
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Unix systems. It does not make any claims about its existence on a specific
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operating system.
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* If not separately noted, all functions that claim "Availability: Unix" are
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supported on macOS, which builds on a Unix core.
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* If an availability note contains both a minimum Kernel version and a minimum
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libc version, then both conditions must hold. For example a feature with note
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*Availability: Linux >= 3.17 with glibc >= 2.27* requires both Linux 3.17 or
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newer and glibc 2.27 or newer.
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.. _wasm-availability:
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WebAssembly platforms
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---------------------
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The `WebAssembly`_ platforms ``wasm32-emscripten`` (`Emscripten`_) and
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``wasm32-wasi`` (`WASI`_) provide a subset of POSIX APIs. WebAssembly runtimes
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and browsers are sandboxed and have limited access to the host and external
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resources. Any Python standard library module that uses processes, threading,
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networking, signals, or other forms of inter-process communication (IPC), is
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either not available or may not work as on other Unix-like systems. File I/O,
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file system, and Unix permission-related functions are restricted, too.
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Emscripten does not permit blocking I/O. Other blocking operations like
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:func:`~time.sleep` block the browser event loop.
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The properties and behavior of Python on WebAssembly platforms depend on the
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`Emscripten`_-SDK or `WASI`_-SDK version, WASM runtimes (browser, NodeJS,
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`wasmtime`_), and Python build time flags. WebAssembly, Emscripten, and WASI
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are evolving standards; some features like networking may be
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supported in the future.
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For Python in the browser, users should consider `Pyodide`_ or `PyScript`_.
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PyScript is built on top of Pyodide, which itself is built on top of
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CPython and Emscripten. Pyodide provides access to browsers' JavaScript and
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DOM APIs as well as limited networking capabilities with JavaScript's
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``XMLHttpRequest`` and ``Fetch`` APIs.
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* Process-related APIs are not available or always fail with an error. That
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includes APIs that spawn new processes (:func:`~os.fork`,
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:func:`~os.execve`), wait for processes (:func:`~os.waitpid`), send signals
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(:func:`~os.kill`), or otherwise interact with processes. The
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:mod:`subprocess` is importable but does not work.
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* The :mod:`socket` module is available, but is limited and behaves
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differently from other platforms. On Emscripten, sockets are always
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non-blocking and require additional JavaScript code and helpers on the
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server to proxy TCP through WebSockets; see `Emscripten Networking`_
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for more information. WASI snapshot preview 1 only permits sockets from an
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existing file descriptor.
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* Some functions are stubs that either don't do anything and always return
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hardcoded values.
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* Functions related to file descriptors, file permissions, file ownership, and
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links are limited and don't support some operations. For example, WASI does
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not permit symlinks with absolute file names.
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.. _WebAssembly: https://webassembly.org/
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.. _Emscripten: https://emscripten.org/
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.. _Emscripten Networking: https://emscripten.org/docs/porting/networking.html
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.. _WASI: https://wasi.dev/
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.. _wasmtime: https://wasmtime.dev/
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.. _Pyodide: https://pyodide.org/
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.. _PyScript: https://pyscript.net/
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