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Contributing to PyTorch

If you are interested in contributing to PyTorch, your contributions will fall into two categories:

  1. You want to propose a new feature and implement it.
    • Post about your intended feature, and we shall discuss the design and implementation. Once we agree that the plan looks good, go ahead and implement it.
  2. You want to implement a feature or bug-fix for an outstanding issue.
    • Search for your issue here:
    • Pick an issue and comment on the task that you want to work on this feature.
    • If you need more context on a particular issue, please ask and we shall provide.

Once you finish implementing a feature or bug-fix, please send a Pull Request to

If you are not familiar with creating a Pull Request, here are some guides:

Developing PyTorch

To develop PyTorch on your machine, here are some tips:

  1. Uninstall all existing PyTorch installs:
conda uninstall pytorch
pip uninstall torch
pip uninstall torch # run this command twice
  1. Clone a copy of PyTorch from source:
git clone
cd pytorch
  1. Install PyTorch in develop mode:

A full set of instructions on installing PyTorch from source is here:

The change you have to make is to replace

python install


python develop

This mode will symlink the Python files from the current local source tree into the Python install. Hence, if you modify a Python file, you do not need to reinstall PyTorch again and again. This is especially useful if you are only changing Python files.

For example:

  • Install local PyTorch in develop mode
  • modify your Python file torch/ (for example)
  • test functionality
  • modify your Python file torch/
  • test functionality
  • modify your Python file torch/
  • test functionality

You do not need to repeatedly install after modifying Python files.

In case you want to reinstall, make sure that you uninstall PyTorch first by running pip uninstall torch and python clean. Then you can install in develop mode again.

Codebase structure

  • c10 - Core library files that work everywhere, both server and mobile. We are slowly moving pieces from ATen/core here. This library is intended only to contain essential functionality, and appropriate to use in settings where binary size matters. (But you'll have a lot of missing functionality if you try to use it directly.)
  • aten - C++ tensor library for PyTorch (no autograd support)
    • src
      • TH THC THNN THCUNN - Legacy library code from the original Torch. Try not to add things here; we're slowly porting these to native.
        • generic - Contains actual implementations of operators, parametrized over scalar_t. Files here get compiled N times per supported scalar type in PyTorch.
      • ATen
        • core - Core functionality of ATen. This is migrating to top-level c10 folder.
        • native - Modern implementations of operators. If you want to write a new operator, here is where it should go. Most CPU operators go in the top level directory, except for operators which need to be compiled specially; see cpu below.
          • cpu - Not actually CPU implementations of operators, but specifically implementations which are compiled with processor-specific instructions, like AVX. See the README for more details.
          • cuda - CUDA implementations of operators.
          • sparse - CPU and CUDA implementations of COO sparse tensor operations
          • mkl mkldnn miopen cudnn
            • implementations of operators which simply bind to some backend library.
  • torch - The actual PyTorch library. Everything that is not in csrc is a Python module, following the PyTorch Python frontend module structure.
    • csrc - C++ files composing the PyTorch library. Files in this directory tree are a mix of Python binding code, and C++ heavy lifting. Consult for the canonical list of Python binding files; conventionally, they are often prefixed with python_.
      • jit - Compiler and frontend for TorchScript JIT frontend.
      • autograd - Implementation of reverse-mode automatic differentiation.
      • api - The PyTorch C++ frontend.
      • distributed - Distributed training support for PyTorch.
  • tools - Code generation scripts for the PyTorch library. See README of this directory for more details.
  • test - Python unit tests for PyTorch Python frontend.
    • - Basic tests for PyTorch functionality.
    • - Tests for non-NN automatic differentiation support.
    • - Tests for NN operators and their automatic differentiation.
    • - Tests for the JIT compiler and TorchScript.
    • ...
    • cpp - C++ unit tests for PyTorch C++ frontend.
    • expect - Automatically generated "expect" files which are used to compare against expected output.
    • onnx - Tests for ONNX export functionality, using both PyTorch and Caffe2.
  • caffe2 - The Caffe2 library.
    • core - Core files of Caffe2, e.g., tensor, workspace, blobs, etc.
    • operators - Operators of Caffe2.
    • python - Python bindings to Caffe2.
    • ...

Unit testing

PyTorch's testing is located under test/. Run the entire test suite with

python test/

or run individual test files, like python test/, for individual test suites.

Better local unit tests with pytest

We don't officially support pytest, but it works well with our unittest tests and offers a number of useful features for local developing. Install it via pip install pytest.

If you want to just run tests that contain a specific substring, you can use the -k flag:

pytest test/ -k Loss -v

The above is an example of testing a change to Loss functions: this command runs tests such as TestNN.test_BCELoss and TestNN.test_MSELoss and can be useful to save keystrokes.

Writing documentation

PyTorch uses Google style for formatting docstrings. Length of line inside docstrings block must be limited to 80 characters to fit into Jupyter documentation popups.

For C++ documentation (, we use Doxygen and then convert it to Sphinx via Breathe and Exhale. Check the Doxygen reference for more information on the documentation syntax. To build the documentation locally, cd into docs/cpp and then make html.

We run Doxygen in CI (Travis) to verify that you do not use invalid Doxygen commands. To run this check locally, run ./ from inside docs/cpp.

Managing multiple build trees

One downside to using python develop is that your development version of PyTorch will be installed globally on your account (e.g., if you run import torch anywhere else, the development version will be used.

If you want to manage multiple builds of PyTorch, you can make use of conda environments to maintain separate Python package environments, each of which can be tied to a specific build of PyTorch. To set one up:

conda create -n pytorch-myfeature
source activate pytorch-myfeature
# if you run python now, torch will NOT be installed
python develop

C++ Development tips

If you are working on the C++ code, there are a few important things that you will want to keep in mind:

  1. How to rebuild only the code you are working on.
  2. How to make rebuilds in the absence of changes go faster.

Build only what you need.

python build will build everything by default, but sometimes you are only interested in a specific component.

  • Working on a test binary? Run (cd build && ninja bin/test_binary_name) to rebuild only that test binary (without rerunning cmake). (Replace ninja with make if you don't have ninja installed).
  • Don't need Caffe2? Pass BUILD_CAFFE2_OPS=0 to disable build of Caffe2 operators.

On the initial build, you can also speed things up with the environment variables DEBUG and NO_CUDA.

  • DEBUG=1 will enable debug builds (-g -O0)
  • REL_WITH_DEB_INFO=1 will enable debug symbols with optimizations (-g -O3)
  • NO_CUDA=1 will disable compiling CUDA (in case you are developing on something not CUDA related), to save compile time.

For example:

NO_CUDA=1 DEBUG=1 python develop

Make sure you continue to pass these flags on subsequent builds.

Code completion and IDE support

When using python develop, PyTorch will generate a compile_commands.json file that can be used by many editors to provide command completion and error highlighting for PyTorch's C++ code. You need to pip install ninja to generate accurate information for the code in torch/csrc. More information at:

Make no-op build fast.

Use Ninja

By default, cmake will use its Makefile generator to generate your build system. You can get faster builds if you install the ninja build system with pip install ninja. If PyTorch was already built, you will need to run python clean once after installing ninja for builds to succeed.

Use CCache

Even when dependencies are tracked with file modification, there are many situations where files get rebuilt when a previous compilation was exactly the same.

Using ccache in a situation like this is a real time-saver. The ccache manual describes two ways to use ccache. In the PyTorch project, currently only the latter method of masquerading as the compiler via symlinks works for CUDA compilation.

Here are the instructions for installing ccache from source (tested at commit 7abac8f of the ccache repo):

# install and export ccache
if ! ls ~/ccache/bin/ccache
    sudo apt-get update
    sudo apt-get install -y automake autoconf
    sudo apt-get install -y asciidoc
    mkdir -p ~/ccache
    pushd /tmp
    rm -rf ccache
    git clone
    pushd ccache
    make install prefix=~/ccache

    mkdir -p ~/ccache/lib
    mkdir -p ~/ccache/cuda
    ln -s ~/ccache/bin/ccache ~/ccache/lib/cc
    ln -s ~/ccache/bin/ccache ~/ccache/lib/c++
    ln -s ~/ccache/bin/ccache ~/ccache/lib/gcc
    ln -s ~/ccache/bin/ccache ~/ccache/lib/g++
    ln -s ~/ccache/bin/ccache ~/ccache/cuda/nvcc

    ~/ccache/bin/ccache -M 25Gi

export PATH=~/ccache/lib:$PATH
export CUDA_NVCC_EXECUTABLE=~/ccache/cuda/nvcc

Alternatively, ccache provided by newer Linux distributions (e.g. Debian/sid) also works, but the nvcc symlink to ccache as described above is still required.

Note that the original nvcc binary (typically at /usr/local/cuda/bin) must be on your PATH, otherwise ccache will emit the following error:

ccache: error: Could not find compiler "nvcc" in PATH

CUDA Development tips

If you are working on the CUDA code, here are some useful CUDA debugging tips:

  1. CUDA_DEVICE_DEBUG=1 will enable CUDA device function debug symbols (-g -G). This will be particularly helpful in debugging device code. However, it will slow down the build process for about 50% (compared to only DEBUG=1), so use wisely.
  2. cuda-gdb and cuda-memcheck are your best CUDA debugging friends. Unlikegdb, cuda-gdb can display actual values in a CUDA tensor (rather than all zeros).

Hope this helps, and thanks for considering to contribute.

Windows development tips

For building from source on Windows, consult our documentation on it.

Occasionally, you will write a patch which works on Linux, but fails CI on Windows. There are a few aspects in which MSVC (the Windows compiler toolchain we use) is stricter than Linux, which are worth keeping in mind when fixing these problems.

  1. Symbols are NOT exported by default on Windows; instead, you have to explicitly mark a symbol as exported/imported in a header file with __declspec(dllexport) / __declspec(dllimport). We have codified this pattern into a set of macros which follow the convention *_API, e.g., CAFFE2_API inside Caffe2 and ATen. (Every separate shared library needs a unique macro name, because symbol visibility is on a per shared library basis. See c10/macros/Macros.h for more details.)

    The upshot is if you see an "unresolved external" error in your Windows build, this is probably because you forgot to mark a function with *_API. However, there is one important counterexample to this principle: if you want a templated function to be instantiated at the call site, do NOT mark it with *_API (if you do mark it, you'll have to explicitly instantiate all of the specializations used by the call sites.)

  2. If you link against a library, this does not make its dependencies transitively visible. You must explicitly specify a link dependency against every library whose symbols you use. (This is different from Linux where in most environments, transitive dependencies can be used to fulfill unresolved symbols.)

  3. If you have a Windows box (we have a few on EC2 which you can request access to) and you want to run the build, the easiest way is to just run .jenkins/pytorch/ If you need to rebuild, run REBUILD=1 .jenkins/pytorch/ (this will avoid blowing away your Conda environment.)

Even if you don't know anything about MSVC, you can use cmake to build simple programs on Windows; this can be helpful if you want to learn more about some peculiar linking behavior by reproducing it on a small example. Here's a simple example cmake file that defines two dynamic libraries, one linking with the other:

project(myproject CXX)
add_library(foo SHARED foo.cpp)
add_library(bar SHARED bar.cpp)
# NB: don't forget to __declspec(dllexport) at least one symbol from foo,
# otherwise foo.lib will not be created.
target_link_libraries(bar PUBLIC foo)

You can build it with:

mkdir build
cd build
cmake ..
cmake --build .

Known MSVC (and MSVC with NVCC) bugs

The PyTorch codebase sometimes likes to use exciting C++ features, and these exciting features lead to exciting bugs in Windows compilers. To add insult to injury, the error messages will often not tell you which line of code actually induced the erroring template instantiation.

We've found the most effective way to debug these problems is to carefully read over diffs, keeping in mind known bugs in MSVC/NVCC. Here are a few well known pitfalls and workarounds:

  • This is not actually a bug per se, but in general, code generated by MSVC is more sensitive to memory errors; you may have written some code that does a use-after-free or stack overflows; on Linux the code might work, but on Windows your program will crash. ASAN may not catch all of these problems: stay vigilant to the possibility that your crash is due to a real memory problem.

  • (NVCC) c10::optional does not work when used from device code. Don't use it from kernels. Upstream issue: and our local issue #10329.

  • constexpr generally works less well on MSVC.

    • The idiom static_assert(f() == f()) to test if f is constexpr does not work; you'll get "error C2131: expression did not evaluate to a constant". Don't use these asserts on Windows. (Example: c10/util/intrusive_ptr.h)
  • (NVCC) Code you access inside a static_assert will eagerly be evaluated as if it were device code, and so you might get an error that the code is "not accessible".

class A {
  static A singleton_;
  static constexpr inline A* singleton() {
    return &singleton_;
static_assert(std::is_same(A*, decltype(A::singleton()))::value, "hmm");
  • The compiler will run out of heap space if you attempt to compile files that are too large. Splitting such files into separate files helps. (Example: THTensorMath, THTensorMoreMath, THTensorEvenMoreMath.)

  • MSVC's preprocessor (but not the standard compiler) has a bug where it incorrectly tokenizes raw string literals, ending when it sees a ". This causes preprocessor tokens inside the literal like an#endif to be incorrectly treated as preprocessor directives. See as an example.

Running Clang-Tidy

Clang-Tidy is a C++ linter and static analysis tool based on the clang compiler. We run clang-tidy in our CI to make sure that new C++ code is safe, sane and efficient. See our .travis.yml file for the simple commands we use for this.

To run clang-tidy locally, follow these steps:

  1. Install clang-tidy. First, check if you already have clang-tidy by simply writing clang-tidy in your terminal. If you don't yet have clang-tidy, you should be able to install it easily with your package manager, e.g. by writing apt-get install clang-tidy on Ubuntu. See for details on how to install the latest version. Note that newer versions of clang-tidy will have more checks than older versions. In our CI, we run clang-tidy-6.0.

  2. Use our driver script to run clang-tidy over any changes relative to some git revision (you may want to replace HEAD~1 with HEAD to pick up uncommitted changes). Changes are picked up based on a git diff with the given revision:

python tools/ -d build -p torch/csrc --diff 'HEAD~1'

Above, it is assumed you are in the PyTorch root folder. path/to/build should be the path to where you built PyTorch from source, e.g. build in the PyTorch root folder if you used build. You can use -c <clang-tidy-binary> to change the clang-tidy this script uses. Make sure you have PyYaml installed, which is in PyTorch's requirements.txt.

Pre-commit Tidy/Linting Hook

We use clang-tidy and flake8 (installed with flake-mypy) to perform additional formatting and semantic checking of code. We provide a pre-commit git hook for performing these checks, before a commit is created:

pip install flake8-mypy
ln -s ../../tools/git-pre-commit .git/hooks/pre-commit

Caffe2 notes

In 2018, we merged Caffe2 into the PyTorch source repository. While the steady state aspiration is that Caffe2 and PyTorch share code freely, in the meantime there will be some separation.

If you submit a PR to only PyTorch or only Caffe2 code, CI will only run for the project you edited. The logic for this is implemented in .jenkins/pytorch/ and .jenkins/caffe2/; you can look at this to see what path prefixes constitute changes. This also means if you ADD a new top-level path, or you start sharing code between projects, you need to modify these files.

There are a few "unusual" directories which, for historical reasons, are Caffe2/PyTorch specific. Here they are:

  • CMakeLists.txt, Makefile, binaries, cmake, conda, modules, scripts are Caffe2-specific. Don't put PyTorch code in them without extra coordination.

  • mypy*, requirements.txt,, test, tools are PyTorch-specific. Don't put Caffe2 code in them without extra coordination.

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