# Introduction

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 `import` statement. 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 this material.

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, 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.