How are mypy programs different from normal Python?

Since you use a vanilla Python implementation to run mypy programs,mypy programs are also Python programs. The type checker may givewarnings for some valid Python code, but the code is still alwaysrunnable. Also, some Python features and syntax are still notsupported by mypy, but this is gradually improving.

The obvious difference is the availability of static typechecking. The section Common issues and solutions mentions somemodifications to Python code that may be required to make code typecheck without errors. Also, your code must make attributes explicit.

Mypy supports modular, efficient type checking, and this seems torule out type checking some language features, such as arbitrarymonkey patching of methods.