# Introduction

SIP is a tool for automatically generating Python bindings for C and C++ libraries. SIP was originally developed in 1998 for PyQt - the Python bindings for the Qt GUI toolkit - but is suitable for generating bindings for any C or C++ library. SIP can also be used write self contained extension modules, i.e. without a library to be wrapped.

This version of SIP generates bindings for Python v3.10 and later.

SIP is hosted at GitHub.

The documentation is hosted at Read the Docs.

## License

SIP is licensed under the BSD 2 clause license.

## Features

SIP, and the bindings it produces, have the following features:

- bindings run under Linux, Windows, macOS, Android and iOS

- bindings can be built to use the PEP 384 stable ABI so that they do not need to be built for each supported version of Python

- an extendable, PEP 517-compliant build system that will build and install your bindings and create sdist and wheel files that you can upload to PyPI

- bindings are fast to load and minimise memory consumption especially when only a small sub-set of a large library is being used

- automatic conversion between standard Python and C/C++ data types

- overloading of functions and methods with different argument signatures

- support for Python’s keyword argument syntax

- support for both explicitly specified and automatically generated docstrings

- access to a C++ class’s protected methods

- the ability to define a Python class that is a sub-class of a C++ class, including abstract C++ classes

- Python sub-classes can implement the `__dtor__()` method which will be called from the C++ class’s virtual destructor

- support for ordinary C++ functions, class methods, static class methods, virtual class methods and abstract class methods

- the ability to re-implement C++ virtual and abstract methods in Python

- support for global and class variables

- support for global and class operators

- support for C++ namespaces

- support for C++ templates

- support for C++ exceptions and wrapping them as Python exceptions

- the automatic generation of complementary rich comparison slots

- support for deprecation warnings

- the ability to define mappings between C++ classes and similar Python data types that are automatically invoked

- the ability to automatically exploit any available run time type information to ensure that the class of a Python instance object matches the class of the corresponding C++ instance

- the ability to change the type and meta-type of the Python object used to wrap a C/C++ data type

- full support of the Python global interpreter lock, including the ability to specify that a C++ function of method may block, therefore allowing the lock to be released and other Python threads to run

- support for the concept of ownership of a C++ instance (i.e. what part of the code is responsible for calling the instance’s destructor) and how the ownership may change during the execution of an application

- the ability to generate bindings for a C++ class library that itself is built on another C++ class library which also has had bindings generated so that the different bindings integrate and share code properly

- a sophisticated versioning system that allows the full lifetime of a C++ class library, including any platform specific or optional features, to be described in a single set of specification files

- support for the automatic generation of PEP 484 type hint stub files

- the ability to include documentation in the specification files which can be extracted and subsequently processed by external tools

- the ability to include copyright notices and licensing information in the specification files that is automatically included in all generated source code.

SIP supports C++ compilers that implement the C++11 standard as a minimum. Older compilers may work depending on what features are used.

## Overview

At its simplest a SIP project contains a specification file (`.sip` file) that describes the API that the generated bindings will wrap, and a `pyproject.toml` file that describes how the bindings will be built. A specification file is very like a C/C++ header file with embedded directives and annotations. The format of a `pyproject.toml` file is described in PEP 518.

A SIP project can either be a standalone project or a package project. A standalone project implements a single set of bindings (i.e. a single extension module) that cannot be extended by another set of bindings. A package project implements one or more sets of mutually dependent bindings (i.e. one set of bindings will import another set of bindings). Such bindings may be defined in the same project or a completely different package project (possibly with a different maintainer). Often the bindings of all related package projects will be installed as part of a single top-level Python package. For example, the whole of PyQt5 is current implemented as 6 separate package projects each containing between 1 and 52 sets of bindings all installed as part of the `PyQt5` top-level package. However there are also 3rd-party packages that extend PyQt5 but are not installed in the `PyQt5` top-level package.

SIP also generates a `sip` module which performs the following functions:

- it implements a private C ABI used by the bindings of package projects that allows them to interact

- it implements a public C API used by bindings authors in hand-written code in situations where SIP’s normal behaviour is insufficient and also when embedding Python in C/C++ applications

- it implements a public Python API used by application authors typically to configure the behaviour of bindings and to aid debugging.

The `sip` module does not use the PEP 384 stable ABI and so must be built for each supported version of Python.

The version number of the `sip` module is the version number of the ABI that the module implements. Like SIP itself, this uses semantic versioning.

When used with standalone projects the `sip` module is not a separate module and is instead embedded in the single set of bindings. When used with package projects the `sip` module is a separate extension module installed somewhere under the top-level package.

PEP 517 describes the concepts of a build frontend and a build backend. SIP implements a compliant backend and provides a number of frontends each performing a specific type of build.

sip-build

  This builds the project but does not install it. This is useful when developing a set of bindings.

sip-install

  This builds and installs a project.

sip-sdist

  This creates an sdist (a source distribution) that can be uploaded to PyPI.

sip-wheel

  This creates a wheel (a binary distribution) that can be uploaded to PyPI.

Collectively the above are SIP’s build tools.

pip can also be used as a build frontend. This has the advantage that the user does not need to explicitly install SIP, pip will do that automatically.

SIP also includes some additional command line tools.

sip-distinfo

  This creates and populates a `.dist-info` directory of an installation or a wheel. It is provided for build systems that extend the SIP build system and need to create the `.dist-info` directory from an external tool such as make.

sip-module

  This builds one or more elements of the `sip` module for a set of package projects:

  - an sdist of the module which can be installed by pip or uploaded to PyPI

  - a `sip.h` header file which defines the module’s ABI. Normally you do not need to worry about this file but this will install a local copy of it if required

  - a `sip.rst` file that documents the Python API of the module for inclusion in your project’s documentation.

## Installation

To install SIP from PyPI, run:

    pip install sip

SIP is also included with all of the major Linux distributions. However, it may be a version or two out of date.

## Support for Old Versions of Python

When a Python version reaches it’s end-of-life, support for it will be removed in the next minor release of SIP. For example, if the current version of SIP is v6.x.y then the support will be removed in v6.x+1.0.