Changing types

Even without any additional work, running mypy on an existing code-base can reveal locations that might need fixing.

For example it can point out places where the content of a variable changes type. Python accepts this, and in some places this type of flexibility might have advantages, but it can also lead to confusion for the maintainer of this code.

examples/types/simple.py

x = 23
print(x)

x = "Python"
print(x)

x = ["a", "b"]
print(x)

python simple.py works without complaining.

mypy simple.py reports the following:

simple.py:5: error: Incompatible types in assignment (expression has type "str", variable has type "int")
simple.py:8: error: Incompatible types in assignment (expression has type "List[str]", variable has type "int")
Found 2 errors in 1 file (checked 1 source file)