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"]
)
Note
If you use setuptools, you must pass the option zip_safe=False
to
setup()
, or mypy will not be able to find the installed package.
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"]
)