The “Python library” contains several different kinds of components.
It contains data types that would normally be considered part of the “core” of a language, such as numbers and lists. For these types, the Python language core defines the form of literals and places some constraints on their semantics, but does not fully define the semantics. (On the other hand, the language core does define syntactic properties like the spelling and priorities of operators.)
The library also contains built-in functions and exceptions — objects that can
be used by all Python code without the need of an
Some of these are defined by the core language, but many are not essential for
the core semantics and are only described here.
The bulk of the library, however, consists of a collection of modules. There are many ways to dissect this collection. Some modules are written in C and built in to the Python interpreter; others are written in Python and imported in source form. 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 the World Wide Web. 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.
This manual is organized “from the inside out:” it first describes the built-in functions, data types and exceptions, and finally the modules, grouped in chapters of related modules.
This means that 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 number (see module
random) and read a section or two. Regardless of the order in which you
read the sections of this manual, it helps to start with chapter
Built-in Functions, as the remainder of the manual assumes familiarity with
Let the show begin!
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, which builds 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.
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
Process-related APIs are not available or always fail with an error. That includes APIs that spawn new processes (
execve()), wait for processes (
waitpid()), send signals (
kill()), or otherwise interact with processes. The
subprocessis importable but does not work.
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.