Introduction
************

The Python standard library consists of a collection of modules. There
are many ways to dissect this collection.  Most modules are written in
Python, but some are written in C. All can be imported into your
program to add functionality.  Some modules provide interfaces that
are highly specific to Python, like printing a stack trace; some
provide interfaces that are specific to particular operating systems,
such as access to specific hardware; others provide interfaces that
are specific to a particular application domain, like web development.
Some modules are available in all versions and ports of Python; others
are only available when the underlying system supports or requires
them; yet others are available only when a particular configuration
option was chosen at the time when Python was compiled and installed.

If you start reading this manual from the start, and skip to the next
chapter when you get bored, you will get a reasonable overview of the
available modules and application areas that are supported by the
Python library.  Of course, you don't *have* to read it like a novel
--- you can also browse the table of contents (in front of the
manual), or look for a specific function, module or term in the index
(in the back).  And finally, if you enjoy learning about random
subjects, you choose a random page and read a section or two.
Regardless of the order in which you read the sections of this manual,
it helps to first read Built-in Functions, as the remainder of this
section assumes familiarity with this material.

See also:

  The built-in functions and classes (which can be used without an
  "import" statement) are described in Python built-ins reference.


Notes on availability
=====================

* An "Availability: Unix" note means that this function is commonly
  found on Unix systems.  It does not make any claims about its
  existence on a specific operating system.

* If not separately noted, all functions that claim "Availability:
  Unix" are supported on macOS, iOS and Android, all of which build on
  a Unix core.

* If an availability note contains both a minimum Kernel version and a
  minimum libc version, then both conditions must hold. For example a
  feature with note *Availability: Linux >= 3.17 with glibc >= 2.27*
  requires both Linux 3.17 or newer and glibc 2.27 or newer.


WebAssembly platforms
---------------------

The WebAssembly platforms "wasm32-emscripten" (Emscripten) and
"wasm32-wasi" (WASI) provide a subset of POSIX APIs. WebAssembly
runtimes and browsers are sandboxed and have limited access to the
host and external resources. Any Python standard library module that
uses processes, threading, networking, signals, or other forms of
inter-process communication (IPC), is either not available or may not
work as on other Unix-like systems. File I/O, file system, and Unix
permission-related functions are restricted, too. Emscripten does not
permit blocking I/O. Other blocking operations like "sleep()" block
the browser event loop.

The properties and behavior of Python on WebAssembly platforms depend
on the Emscripten-SDK or WASI-SDK version, WASM runtimes (browser,
NodeJS, wasmtime), and Python build time flags. WebAssembly,
Emscripten, and WASI are evolving standards; some features like
networking may be supported in the future.

For Python in the browser, users should consider Pyodide or PyScript.
PyScript is built on top of Pyodide, which itself is built on top of
CPython and Emscripten. Pyodide provides access to browsers'
JavaScript and DOM APIs as well as limited networking capabilities
with JavaScript's "XMLHttpRequest" and "Fetch" APIs.

* Process-related APIs are not available or always fail with an error.
  That includes APIs that spawn new processes ("fork()", "execve()"),
  wait for processes ("waitpid()"), send signals ("kill()"), or
  otherwise interact with processes. The "subprocess" is importable
  but does not work.

* The "socket" module is available, but is limited and behaves
  differently from other platforms. On Emscripten, sockets are always
  non-blocking and require additional JavaScript code and helpers on
  the server to proxy TCP through WebSockets; see Emscripten
  Networking for more information. WASI snapshot preview 1 only
  permits sockets from an existing file descriptor.

* Some functions are stubs that either don't do anything and always
  return hardcoded values.

* Functions related to file descriptors, file permissions, file
  ownership, and links are limited and don't support some operations.
  For example, WASI does not permit symlinks with absolute file names.


Mobile platforms
----------------

Android and iOS are, in most respects, POSIX operating systems. File
I/O, socket handling, and threading all behave as they would on any
POSIX operating system. However, there are several major differences:

* Mobile platforms can only use Python in "embedded" mode. There is no
  Python REPL, and no ability to use separate executables such as
  **python** or **pip**. To add Python code to your mobile app, you
  must use the Python embedding API. For more details, see Using
  Python on Android and Using Python on iOS.

* Subprocesses:

  * On Android, creating subprocesses is possible but officially
    unsupported. In particular, Android does not support any part of
    the System V IPC API, so "multiprocessing" is not available.

  * An iOS app cannot use any form of subprocessing, multiprocessing,
    or inter-process communication. If an iOS app attempts to create a
    subprocess, the process creating the subprocess will either lock
    up, or crash. An iOS app has no visibility of other applications
    that are running, nor any ability to communicate with other
    running applications, outside of the iOS-specific APIs that exist
    for this purpose.

* Mobile apps have limited access to modify system resources (such as
  the system clock). These resources will often be *readable*, but
  attempts to modify those resources will usually fail.

* Console input and output:

  * On Android, the native "stdout" and "stderr" are not connected to
    anything, so Python installs its own streams which redirect
    messages to the system log. These can be seen under the tags
    "python.stdout" and "python.stderr" respectively.

  * iOS apps have a limited concept of console output. "stdout" and
    "stderr" *exist*, and content written to "stdout" and "stderr"
    will be visible in logs when running in Xcode, but this content
    *won't* be recorded in the system log. If a user who has installed
    your app provides their app logs as a diagnostic aid, they will
    not include any detail written to "stdout" or "stderr".

  * Mobile apps have no usable "stdin" at all. While apps can display
    an on-screen keyboard, this is a software feature, not something
    that is attached to "stdin".

    As a result, Python modules that involve console manipulation
    (such as "curses" and "readline") are not available on mobile
    platforms.
