Type checking in a Python class

Question

I am trying to use a static type checking tool to check for wrong assignment to a variable. E.g., assign a string to an int variable.

I tried pytype and mypy. Both didn't give me any warnings.

class A:
    def __init__(self):
        self.x : int = None

if __name__ == '__main__':
    a = A()
    a.x = 'abc'
    print(a.x)

I expect a static type checking tool can give me a warning on the above line:

a.x = 'abc'

Do I need to use some options or other helper tools to detect this kind of assignment statement?

Hey there. Which IDE environment are you using?

PeptideWitch
– PeptideWitch

2019-08-13 04:14:17 +00:00
Commented Aug 13, 2019 at 4:14 — PeptideWitch
– PeptideWitch, Commented Aug 13, 2019 at 4:14

Peter Mortensen · Accepted Answer · 2021-01-13 09:34:16Z

1

So when I copy your code and check it with mypy, I get the following result:

project\scratch.py:7: error: Incompatible types in assignment (expression has type "str", variable has type "int")

I found this by executing mypy path/to/file.py.

Internally, within Visual Studio Code, selecting mypy as the linter underlines the a variable and overlays the mypy error.

So I'm getting the warning error codes displayed correctly; maybe your IDE isn't set up to handle them.

Note: executing python path/to/file.py will not display the mypy error, most likely to keep the typing 'soft' - so that the code will still execute, and typing is more to 'hint', rather than stop the code:

You can always use a Python interpreter to run your statically typed programs, even if they have type errors: $ python3 PROGRAM

From the documentation.

edited Jan 13, 2021 at 9:34

Peter Mortensen

31.4k22 gold badges110 silver badges134 bronze badges

answered Aug 13, 2019 at 4:11

PeptideWitch

2,3591 gold badge19 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Peter Mortensen · Accepted Answer · 2021-01-13 09:35:27Z

0

I can't speak for other IDEs, but for Visual Studio Code (using Python 3.8.5)...

Install pylance (Microsoft's Python language server extension)

Add these two lines to settings.json:

"python.languageServer":"Pylance",
"python.analysis.typeCheckingMode" :"strict"

Note the following problems reported:

(variable) x: None
   Cannot assign member "x" for type "A"
      Expression of type "None" cannot be assigned to member "x" of class "A"
      Type "None" cannot be assigned to type "int"Pylance (reportGeneralTypeIssues) [3, 14]

(variable) x: Literal['abc']
   Cannot assign member "x" for type "A"
      Expression of type "Literal['abc']" cannot be assigned to member "x" of class "A"
      "Literal['abc']" is incompatible with "int"Pylance (reportGeneralTypeIssues) [7, 7]

edited Jan 13, 2021 at 9:35

Peter Mortensen

31.4k22 gold badges110 silver badges134 bronze badges

answered Aug 28, 2020 at 13:06

whecks

791 silver badge3 bronze badges

Collectives™ on Stack Overflow

Type checking in a Python class

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related