Mypy uses stub files stored in the typeshed repository to determine the types of standard library and third-party library functions, classes, and other definitions. You can also create your own stubs that will be used to type check your code. The basic properties of stubs were introduced back in :ref:`stubs-intro`.
Creating a stub
Here is an overview of how to create a stub file:
Write a stub file for the library (or an arbitrary module) and store it as a
.pyifile in the same directory as the library module.
Alternatively, put your stubs (
.pyifiles) in a directory reserved for stubs (e.g.,
myproject/stubs). In this case you have to set the environment variable
MYPYPATHto refer to the directory. For example:
$ export MYPYPATH=~/work/myproject/stubs
Use the normal Python file name conventions for modules, e.g.
csv. Use a subdirectory with
__init__.pyi for packages. Note
that PEP 561 stub-only packages
must be installed, and may not be pointed at through the
(see :ref:`PEP 561 support <installed-packages>`).
If a directory contains both a
.py and a
.pyi file for the
same module, the
.pyi file takes precedence. This way you can
easily add annotations for a module even if you don't want to modify
the source code. This can be useful, for example, if you use 3rd party
open source libraries in your program (and there are no stubs in
That's it! Now you can access the module in mypy programs and type check code that uses the library. If you write a stub for a library module, consider making it available for other programmers that use mypy by contributing it back to the typeshed repo.
There is more information about creating stubs in the mypy wiki. The following sections explain the kinds of type annotations you can use in your programs and stub files.
You may be tempted to point
MYPYPATH to the standard library or
site-packages directory where your 3rd party packages
are installed. This is almost always a bad idea -- you will likely
get tons of error messages about code you didn't write and that
mypy can't analyze all that well yet, and in the worst case
scenario mypy may crash due to some construct in a 3rd party
package that it didn't expect.
Stub file syntax
Stub files are written in normal Python 3 syntax, but generally leaving out runtime logic like variable initializers, function bodies, and default arguments, or replacing them with ellipses.
In this example, each ellipsis
... is literally written in the
stub file as three dots:
x: int def afunc(code: str) -> int: ... def afunc(a: int, b: int = ...) -> int: ...