In recent years, Python has experienced explosive growth in adoption, particularly
among open source projects. While Python's dynamically-typed nature provides developers
with powerful programming abstractions, that same dynamic type system allows for type-related
defects to accumulate in code bases. To aid in the early detection of type-related
defects, type annotations were introduced into the Python ecosystem (i.e., PEP-484)
and static type checkers like mypy have appeared on the market. While applying a type
checker like mypy can in theory help to catch type-related defects before they impact
users, little is known about the real impact of adopting a type checker to reveal
defects in Python projects. In this paper, we study the extent to which Python projects
benefit from such type checking features. For this purpose, we mine the issue tracking
and version control repositories of 210 Python projects on GitHub. Inspired by the
work of Gao et al. on type-related defects in JavaScript, we add type annotations
to test whether detects an error that would have helped developers to avoid real defects.
We observe that 15% of the defects could have been prevented by mypy. Moreover, we
find that there is no significant difference between the experience level of developers
committing type-related defects and the experience of developers committing defects
that are not type-related. In addition, a manual analysis of the anti-patterns that
most commonly lead to type-checking faults reveals that the redefinition of Python
references, dynamic attribute initialization and incorrectly handled Null objects
are the most common causes of type-related faults. Since our study is conducted on
fixed public defects that have gone through code reviews and multiple test cycles,
these results represent a lower bound on the benefits of adopting a type checker.
Therefore, we recommend incorporating a static type checker like mypy into the development
workflow, as not only will it prevent type-related defects but also mitigate certain
anti-patterns during development. IEEE
