pybind11 Documentation Wenzel Jakob Sep 25, 2021 CONTENTS 1 Changelog 3 2 Upgrade guide 27 3 Installing the library 35 4 First steps 37 5 Object-oriented code 42 6 Build systems 50 7 Functions 59 8 Classes 68 9 Exceptions 88 10 Smart pointers 94 11 Type conversions 97 12 Python C++ interface 117 13 Embedding the interpreter 131 14 Miscellaneous 136 15 Frequently asked questions 142 16 Benchmark 148 17 Limitations 151 18 Reference 153 19 CMake helpers 170 Bibliography 174 Index 175 i pybind11 Documentation pybind11 is a lightweight header-only library that exposes C++ types in Python and vice versa, mainly to create Python bindings of existing C++ code. Its goals and syntax are similar to the excellent Boost.Python library by David Abrahams: to minimize boilerplate code in traditional extension modules by inferring type information using compile-time introspection. The main issue with Boost.Python—and the reason for creating such a similar project—is Boost. Boost is an enor- mously large and complex suite of utility libraries that works with almost every C++ compiler in existence. This compatibility has its cost: arcane template tricks and workarounds are necessary to support the oldest and buggiest of compiler specimens. Now that C++11-compatible compilers are widely available, this heavy machinery has become an excessively large and unnecessary dependency. Think of this library as a tiny self-contained version of Boost.Python with everything stripped away that isn’t relevant for binding generation. Without comments, the core header files only require ~4K lines of code and depend on Python (2.7 or 3.5+, or PyPy) and the C++ standard library. This compact implementation was possible thanks to some of the new C++11 language features (specifically: tuples, lambda functions and variadic templates). Since its creation, this library has grown beyond Boost.Python in many ways, leading to dramatically simpler binding code in many common situations. Tutorial and reference documentation is provided at pybind11.readthedocs.io. A PDF version of the manual is available here. And the source code is always available at github.com/pybind/pybind11. Core features pybind11 can map the following core C++ features to Python: • Functions accepting and returning custom data structures per value, reference, or pointer • Instance methods and static methods • Overloaded functions • Instance attributes and static attributes • Arbitrary exception types • Enumerations • Callbacks • Iterators and ranges • Custom operators • Single and multiple inheritance • STL data structures • Smart pointers with reference counting like std::shared_ptr • Internal references with correct reference counting • C++ classes with virtual (and pure virtual) methods can be extended in Python Goodies In addition to the core functionality, pybind11 provides some extra goodies: • Python 2.7, 3.5+, and PyPy/PyPy3 7.3 are supported with an implementation-agnostic interface. • It is possible to bind C++11 lambda functions with captured variables. The lambda capture data is stored inside the resulting Python function object. • pybind11 uses C++11 move constructors and move assignment operators whenever possible to efficiently trans- fer custom data types. CONTENTS 1 pybind11 Documentation • It’s easy to expose the internal storage of custom data types through Pythons’ buffer protocols. This is handy e.g.for fast conversion between C++ matrix classes like Eigen and NumPy without expensive copy operations. • pybind11 can automatically vectorize functions so that they are transparently applied to all entries of one or more NumPy array arguments. • Python’s slice-based access and assignment operations can be supported with just a few lines of code. • Everything is contained in just a few header files; there is no need to link against any additional libraries. • Binaries are generally smaller by a factor of at least 2 compared to equivalent bindings generated by Boost.Python. A recent pybind11 conversion of PyRosetta, an enormous Boost.Python binding project, reported a binary size reduction of 5.4x and compile time reduction by 5.8x. • Function signatures are precomputed at compile time (using constexpr), leading to smaller binaries. • With little extra effort, C++ types can be pickled and unpickled similar to regular Python objects. Supported compilers 1. Clang/LLVM 3.3 or newer (for Apple Xcode’s clang, this is 5.0.0 or newer) 2. GCC 4.8 or newer 3. Microsoft Visual Studio 2015 Update 3 or newer 4. Intel classic C++ compiler 18 or newer (ICC 20.2 tested in CI) 5. Cygwin/GCC (previously tested on 2.5.1) 6. NVCC (CUDA 11.0 tested in CI) 7. NVIDIA PGI (20.9 tested in CI) About This project was created by Wenzel Jakob. Significant features and/or improvements to the code were contributed by Jonas Adler, Lori A. Burns, Sylvain Corlay, Eric Cousineau, Aaron Gokaslan, Ralf Grosse-Kunstleve, Trent Houlis- ton, Axel Huebl, @hulucc, Yannick Jadoul, Sergey Lyskov Johan Mabille, Tomasz Mi ˛asko, Dean Moldovan, Ben Pritchard, Jason Rhinelander, Boris Schäling, Pim Schellart, Henry Schreiner, Ivan Smirnov, Boris Staletic, and Patrick Stewart. We thank Google for a generous financial contribution to the continuous integration infrastructure used by this project. Contributing See the contributing guide for information on building and contributing to pybind11. License pybind11 is provided under a BSD-style license that can be found in the LICENSE file. By using, distributing, or contributing to this project, you agree to the terms and conditions of this license. CONTENTS 2 CHAPTER ONE CHANGELOG Starting with version 1.8.0, pybind11 releases use a semantic versioning policy. 1.1 v2.8.0 (WIP) New features: • Added py::raise_from to enable chaining exceptions. #3215 • Allow exception translators to be optionally registered local to a module instead of applying globally across all pybind11 modules. Use register_local_exception_translator(ExceptionTranslator&& translator) instead of register_exception_translator(ExceptionTranslator&& translator) to keep your exception remapping code local to the module. #2650 • Add make_simple_namespace function for instantiating Python SimpleNamespace objects. #2840 • pybind11::scoped_interpreter and initialize_interpreter have new arguments to allow sys.argv initialization. #2341 • Allow Python builtins to be used as callbacks in CPython. #1413 • Added view to view arrays with a different datatype. #987 • Implemented reshape on arrays. #984 • Enable defining custom __new__ methods on classes by fixing bug preventing overriding methods if they have non-pybind11 siblings. #3265 • Add make_value_iterator(), and fix make_key_iterator() to return references instead of copies. #3293 • pybind11::custom_type_setup was added, for customizing the PyHeapTypeObject corresponding to a class, which may be useful for enabling garbage collection support, among other things. #3287 Changes: • Set __file__ constant when running eval_file in an embedded interpreter. #3233 • Python objects and (C++17) std::optional now accepted in py::slice constructor. #1101 • The pybind11 proxy types str, bytes, bytearray, tuple, list now consistently support passing ssize_t values for sizes and indexes. Previously, only size_t was accepted in several interfaces. #3219 • Avoid evaluating PYBIND11_TLS_REPLACE_VALUE arguments more than once. #3290 Fixes: • Bug fix: enum value’s __int__ returning non-int when underlying type is bool or of char type. #1334 3 pybind11 Documentation • Fixes bug in setting error state in Capsule’s pointer methods. #3261 • A long-standing memory leak in py::cpp_function::initialize was fixed. #3229 • Fixes thread safety for some pybind11::type_caster which require lifetime extension, such as for std::string_view. #3237 • Restore compatibility with gcc 4.8.4 as distributed by ubuntu-trusty, linuxmint-17. #3270 Build system improvements: • Fix regression in CMake Python package config: improper use of absolute path. #3144 • Cached Python version information could become stale when CMake was re-run with a different Python version. The build system now detects this and updates this information. #3299 • Specified UTF8-encoding in setup.py calls of open(). #3137 • Fix a harmless warning from CMake 3.21 with the classic Python discovery. #3220 Backend and tidying up: • Reduced thread-local storage required for keeping alive temporary data for type conversion to one key per ABI version, rather than one key per extension module. This makes the total thread-local storage required by pybind11 2 keys per ABI version. #3275 • Optimize NumPy array construction with additional moves. #3183 • Conversion to std::string and std::string_view now avoids making an extra copy of the data on Python >= 3.3. #3257 • Remove const modifier from certain C++ methods on Python collections (list, set, dict) such as (clear(), append(), insert(), etc. ) and annotated them with py-non-const. • Enable readability clang-tidy-const-return and remove useless consts. #3254 #3194 • The clang-tidy google-explicit-constructor option was enabled. #3250 • Mark a pytype move constructor as noexcept (perf). #3236 • Enable clang-tidy check to guard against inheritance slicing. #3210 • Legacy warning suppression pragma were removed from eigen.h. On Unix platforms, please use -isystem for Eigen include directories, to suppress compiler warnings originating from Eigen headers. Note that CMake does this by default. No adjustments are needed for Windows. #3198 • Format pybind11 with isort consistent ordering of imports #3195 • The warnings-suppression