はじめに

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.

参考

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

利用可能性について

  • 「利用できる環境 : Unix 」の意味はこの関数が Unix システムにあることが多いということです。このことは特定の OS における存在を主張するものではありません。

  • 特に記述がない場合、「利用できる環境 : Unix 」と書かれている関数は、 Unix をコアにしているmacOS、iOS、Androidでも利用することができます。

  • 利用可能性の注釈に最小カーネルバージョンと最小 libc バージョンの両方が含まれている場合は、両方の条件を満たさなければなりません。例えば、 Availability: Linux >= 3.17 with glibc >= 2.27 という注釈付きの機能には、 Linux 3.17 以上と、 glibc 2.27 以上の両方が必要です。

WebAssembly プラットフォーム

WebAssembly プラットフォームである wasm32-emscripten (Emscripten) と wasm32-wasi (WASI) は、 POSIX API のサブセットを提供します。 WebAssembly ランタイムとブラウザーはサンドボックス化されており、ホストや外部リソースへのアクセスが制限されています。プロセス、スレッド、ネットワーク、シグナル、または他の形のプロセス間通信 (IPC) を使用する標準ライブラリモジュールは、利用不可か、他の Unix ライクなシステムのようには動作しません。ファイル I/O 、ファイルシステム、 Unix の権限関係の機能も制限されています。 Emscripten はブロッキング I/O を許可しません。 sleep() のような他のブロッキング操作は、ブラウザーのイベントループをブロックします。

WebAssembly プラットフォーム上での Python の 性質と振る舞いは、 Emscripten-SDK や WASI-SDK のバージョン、 WASM ランタイム (ブラウザ、 NodeJS 、 wasmtime) 、Python のビルド時フラグによって決まります。 WebAssembly や Emscripten 、 WASI は標準を発展させており、ネットワークなどの機能は将来的にサポートされるかもしれません

ブラウザ上の Python では、ユーザーは Pyodide や PyScript を検討すべきでしょう。 PyScript は、 Pyodide をもとにして構築されており、 Pyodide 自体は CPython や Emscripten 上に構築されています。 Pyodide は、ブラウザの JavaScript と DOM API だけでなく、 JavaScript の XMLHttpRequest や Fetch API による制限されたネットワーク機能へのアクセスを提供します。

  • プロセス関連の API は利用不可能か、常にエラーにより失敗します。これには、新しいプロセスを作成 (fork() や execve()) 、プロセスを待機 (waitpid()) 、シグナルを送信 (kill()) 、もしくはプロセスと通信する API が含まれます。 subprocess はインポート可能ですが、機能しません。

  • 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.

モバイルプラットフォーム

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.

  • サブプロセス:

    • 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.