Using mypy to type-check your code

Now that we have mypy set up, let's figure out how to use it for your files and projects.

Running it on a single file is the most straightforward way. Let's make one and test it:

def greet(name):
    print(f"Hello, {name}!")

if __name__ == '__main__':
    name = input("Enter your name: ")
    age = input("Enter your age: ")

    greet(name)
    print(f"You will be {age + 1} years old next year.")

Saving it as a file named code.py. To type check it, just do:

$ mypy code.py
code.py:9: error: Unsupported operand types for + ("str" and "int")
Found 1 error in 1 file (checked 1 source file)

Mypy says there's a problem in line 9, and yeah, we forgot to cast age to an integer. So let's do that:

def greet(name):
    print(f"Hello, {name}!")

if __name__ == '__main__':
    name = input("Enter your name: ")
    age = int(input("Enter your age: "))

    greet(name)
    print(f"You will be {age + 1} years old next year.")

Now let's see what mypy says:

$ mypy code.py
Success: no issues found in 1 source file

You've already seen that mypy has a "strict mode", and a non-strict mode. Mypy by default won't complain about functions that don't have type annotations. To use mypy in strict mode, you just have to pass the --strict flag:

mypy --strict code.py
code.py:1: error: Function is missing a type annotation
code.py:8: error: Call to untyped function "greet" in typed context
Found 2 errors in 1 file (checked 1 source file)

It says accordingly, that we are missing type annotations on def greet(). Let's fix that:

def greet(name):
    print(f"Hello, {name}!")

if __name__ == '__main__':
    name = input("Enter your name: ")
    age = int(input("Enter your age: "))

    greet(name)
    print(f"You will be {age + 1} years old next year.")

And now mypy is happy again.

Using mypy on a project

You can pass an entire folder for mypy to check, and it will find every single python file inside it and check it.

Say you have a folder structure like this:

myproject
├── setup.py
└── src/
    ├── bar.py
    └── foo.py

Using the command mypy <folder name> will check all the 3 files:

$ mypy myproject
Success: no issues found in 3 source files

Notice that it says "3 source files". Similarly, you can also do mypy --strict myproject to run it in strict-mode on all files.

Now that you have mypy set-up and running locally, you can run and type check your code on your own machine as well.