A small exploration with mypy and python type annotations.

mypy-example.py

# Simple script I put together showing mypy in action 

# standard type annotations - we can give arguments a type and expect a return type
def add(x: int, y: int) -> int:
    return x + y 

print(add(1, 2))

# we can let it pick within a list of types
from typing import Union
def square(x: Union[int, float]) -> Union[int, float]:
    return x * x

print(square(4))
print(square(2.5))

# generics - achieved via
# so we can achieve what sum types did but match everything too
from typing import TypeVar
T = TypeVar('T', int, float)
def cube(x: T) -> T:
    return x*x*x

print(cube(4))
print(cube(0.5))
# y: int = cube(0.5) # this throws an error because float has to match with float

# optional - can be None or proper result
from typing import Optional
def factorial(x: int) -> Optional[int]:
    if (x < 0):
        return None
    elif (x == 0 or x == 1):
        return 1
    else:
        return factorial(x - 1)

print(factorial(-1))
print(factorial(10))

Before running it

pip3 install mypy

To type check

mypy mypy-example.py

To run (as normal basically)

python3 mypy-example.py

The output

3
16
6.25
64
0.125
None
1

MIT License