Start small

If your codebase is large, pick a subset of your codebase (say, 5,000to 50,000 lines) and run mypy only on this subset at first,without any annotations. This shouldn’t take more than a day or twoto implement, so you start enjoying benefits soon.

You’ll likely need to fix some mypy errors, either by insertingannotations requested by mypy or by adding # type: ignorecomments to silence errors you don’t want to fix now.

In particular, mypy often generates errors about modules that it can’tfind or that don’t have stub files:

core/config.py:7: error: Cannot find implementation or library stub for module named 'frobnicate'
core/model.py:9: error: Cannot find implementation or library stub for module named 'acme'
...

This is normal, and you can easily ignore these errors. For example,here we ignore an error about a third-party module frobnicate thatdoesn’t have stubs using # type: ignore:

import frobnicate  # type: ignore
...
frobnicate.initialize()  # OK (but not checked)

You can also use a mypy configuration file, which is convenient ifthere are a large number of errors to ignore. For example, to disableerrors about importing frobnicate and acme everywhere in yourcodebase, use a config like this:

[mypy-frobnicate.*]
ignore_missing_imports = True
 
[mypy-acme.*]
ignore_missing_imports = True

You can add multiple sections for different modules that should beignored.

If your config file is named mypy.ini, this is how you run mypy:

mypy --config-file mypy.ini mycode/

If you get a large number of errors, you may want to ignore all errorsabout missing imports. This can easily cause problems later on andhide real errors, and it’s only recommended as a last resort.For more details, look here.

Mypy follows imports by default. This can result in a few files passedon the command line causing mypy to process a large number of importedfiles, resulting in lots of errors you don’t want to deal with at themoment. There is a config file option to disable this behavior, butsince this can hide errors, it’s not recommended for most users.