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This article explains how to build semgrep-cli so that you can make and test changes to the Python wrapper. The semgrep-cli name refers to the project that exposes the actual semgrep command. The README explains the relationship between semgrep-cli and semgrep-core.

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Prerequisite

  • Python >= 3.10 installed in your local machine.
  • pipenv for managing your virtual environment.
    • Install it by following the pipenv documentation.
    • Ensure that pipenv is on your $PATH before proceeding.

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Set up the environment

Most Python development is done inside the cli directory: 
cd cli
Next, initialize and enter the virtual environment. The following command installs developer dependencies, such as pytest, and installs semgrep in editable mode in the virtual environment. From the cli directory, run the following command: 
pipenv shell
By convention, your shell prompt is prepended with (cli) when the virtual environment is active. Next, install the Python dependencies: 
SEMGREP_SKIP_BIN=true pipenv install --dev
INFOSEMGREP_SKIP_BIN tells the installer that you’ll use your own semgrep-core; see below.*
Running which semgrep should return a path within your virtual environment. On macOS, this is likely contained within $HOME/.local/share/virtualenvs/.

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Get the semgrep-core binary

Almost all usages of semgrep-cli require the semgrep-core binary. To get the binary, follow the instructions in Building semgrep-core. It takes approximately 20 minutes.

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Use a precompiled binary

You can use a precompiled binary, but note two downsides:
  • You cannot modify semgrep-core, for example, to fix a parse error.
  • Semgrep scans fail if the interface between semgrep-cli and semgrep-core has changed since the binary was compiled. This has happened roughly every two months historically, but can happen at any time without notice.
If you installed Semgrep using Homebrew (with brew install semgrep), a semgrep-core binary was bundled within that installation. However, it is not made available on your $PATH by default. You can add the bundled binary to your $PATH with this series of commands, provided you have jq installed: 
export SEMGREP_BREW_INSTALLED_VERSION="$(brew info --json semgrep | jq '.[0].installed[0].version' -r)"
export SEMGREP_BREW_INSTALL_PATH="$(brew --cellar semgrep)/${SEMGREP_BREW_INSTALLED_VERSION}"
export SEMGREP_BREW_PYTHON_PACKAGE_PATH="$(${SEMGREP_BREW_INSTALL_PATH}/libexec/bin/python -m pip list -v | grep '^semgrep\b' | awk '{ print $3 }')"
export SEMGREP_BREW_CORE_BINARY_PATH="${SEMGREP_BREW_PYTHON_PACKAGE_PATH}/semgrep/bin"
export PATH="${SEMGREP_BREW_CORE_BINARY_PATH}:${PATH}"

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Run semgrep-cli

Ensure that you are in the cli/ directory, and then issue the following command: 
pipenv run semgrep --help
To try a simple analysis, run: 
echo 'if 1 == 1: pass' | semgrep --lang python --pattern '$X == $X' -
You now have Semgrep running locally.

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Install semgrep

You can always run semgrep from cli/, which will use your latest changes in that directory, but you may also want to install the semgrep binary. To do this, run 
pipenv install --dev
If you encounter difficulties, reach out to the semgrep team on Slack. Now you can run semgrep --help from anywhere. If you have installed semgrep-core from source, there are convenient targets in the root Makefile that let you update all binaries. After you pull, run: 
make rebuild
See the Makefile in cli/

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Add Python packages to semgrep

Semgrep uses mypy to do static type-checking of its Python code. Therefore, when adding a new Python package, you also need to add typing stubs for that package. This can be done in three steps. For example, suppose you are adding the package pyyaml to Semgrep.
1
Install the corresponding package with typing stubs. For this pyyaml example, the corresponding package is types-pyyaml. In the following command, --dev specifies that this package is needed for development but not in production. This command updates cli/Pipfile with the typing stubs package, and adds both the typing stubs and the package itself to your Pipfile.lock. This allows you to import the package in your code (for example, import yaml as pyyaml).
pipenv install --dev types-pyyaml
2
Add the typing stubs package to .pre-commit-config.yaml so that the pre-commit mypy hook can find the package.
      - id: mypy
        additional_dependencies: &mypy-deps
          - ...
          - types-PyYAML
3
Add the original package to cli/setup.py in the install_requires list variable. You can find the version number either in the Pipfile.lock file or by looking up the most recent major version of the package online.
install_requires = [
   ...
   "pyyaml~=6.0",
]
This change makes your package a dependency of published Semgrep. Without this change, if you create a pull request, the CI job called build docker image fails with a ModuleNotFoundError, indicating it cannot find your package.

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Troubleshooting

For a reference build that’s known to work, consult the root Dockerfile to build Semgrep inside a container. You can check that it builds with 
docker build -t semgrep .

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Testing

semgrep-cli uses pytest for testing. To run tests, run the following command: 
pipenv run pytest
There are some much slower tests that run Semgrep on many open source projects. To run these slow tests, run: 
pipenv run pytest tests/qa
If you want to update the tests to match the current output: 
make regenerate-tests
If you want to run a single test file: 
pipenv run pytest path/to/test.py
Or run an individual test function: 
pipenv run pytest path/to/test.py::test_func_name
semgrep-cli also includes pytest-benchmark to allow for basic benchmarking functionality. Run the following command: 
pipenv run pytest --benchmark-only