Using Installed Packages

PEP 561 specifies how to mark a package as supporting type checking. Below is a summary of how to create PEP 561 compatible packages and have mypy use them in type checking.

Using PEP 561 compatible packages with mypy

Generally, you do not need to do anything to use installed packages that support typing for the Python executable used to run mypy. Note that most packages do not support typing. Packages that do support typing should be automatically picked up by mypy and used for type checking.

By default, mypy searches for packages installed for the Python executable running mypy. It is highly unlikely you want this situation if you have installed typed packages in another Python’s package directory.

Generally, you can use the --python-version flag and mypy will try to find the correct package directory. If that fails, you can use the --python-executable flag to point to the exact executable, and mypy will find packages installed for that Python executable.

Note that mypy does not support some more advanced import features, such as zip imports, namespace packages, and custom import hooks.

If you do not want to use typed packages, use the --no-site-packages flag to disable searching.

Making PEP 561 compatible packages

PEP 561 notes three main ways to distribute type information. The first is a package that has only inline type annotations in the code itself. The second is a package that ships stub files with type information alongside the runtime code. The third method, also known as a “stub only package” is a package that ships type information for a package separately as stub files.

If you would like to publish a library package to a package repository (e.g. PyPI) for either internal or external use in type checking, packages that supply type information via type comments or annotations in the code should put a py.typed in their package directory. For example, with a directory structure as follows:

setup.py
package_a/
    __init__.py
    lib.py
    py.typed

the setup.py might look like:

from distutils.core import setup

setup(
    name="SuperPackageA",
    author="Me",
    version="0.1",
    package_data={"package_a": ["py.typed"]},
    packages=["package_a"]
)

Some packages have a mix of stub files and runtime files. These packages also require a py.typed file. An example can be seen below:

setup.py
package_b/
    __init__.py
    lib.py
    lib.pyi
    py.typed

the setup.py might look like:

from distutils.core import setup

setup(
    name="SuperPackageB",
    author="Me",
    version="0.1",
    package_data={"package_b": ["py.typed", "lib.pyi"]},
    packages=["package_b"]
)

In this example, both lib.py and lib.pyi exist. At runtime, the Python interpeter will use lib.py, but mypy will use lib.pyi instead.

If the package is stub-only (not imported at runtime), the package should have a prefix of the runtime package name and a suffix of -stubs. A py.typed file is not needed for stub-only packages. For example, if we had stubs for package_c, we might do the following:

setup.py
package_c-stubs/
    __init__.pyi
    lib.pyi

the setup.py might look like:

from distutils.core import setup

setup(
    name="SuperPackageC",
    author="Me",
    version="0.1",
    package_data={"package_c-stubs": ["__init__.pyi", "lib.pyi"]},
    packages=["package_c-stubs"]
)