Today I learnt about Python type hints 🐍

Up until today I knew type hints existed in Python and had read code which used them but hadn’t written any. I wanted to decide if this is a useful technique for me. Let’s say I have the following function:

def product(x, y):
    return x * y

I like the idea that I could automatically detect if someone calls it with parameters which aren’t numbers. I wrote a few different calls to test how the function behaves.

product(2, 4)
# Returns 8

product(2, -4)
# Returns -8

product(2.1, 4)
# Returns 8.4

product(2, True)
# Returns 2 a bit odd

product('a', 2)
# Returns aa which is unexpected

product('a', '2')
# Throws TypeError: can't multiply sequence by non-int of type 'str'

product('a', 'b')
# Throws TypeError: can't multiply sequence by non-int of type 'str'

Here’s a type hint version of the same code:

def product(x: int, y: int) -> int:
    return x * y

I was hoping the Python interpreter would throw an exception if I call:

product('a', 2)

It doesn’t, so whilst type hints are useful for documenting what type of parameters should be used, and the return type, on their own type hints won’t help catch errors.


To get the behaviour I wanted I needed to install Mypy:

pip install MyPy

Then by running:


I got the following output: error: Argument 1 to "product" has incompatible type "float"; expected "int" error: Argument 1 to "product" has incompatible type "str"; expected "int"
Found 2 errors in 1 file (checked 1 source file)

Very good! However interestingly Mypy is ok with:

product(2, True)

So it’s happy with treating True as 1 which surprised me.

Python Number Type

My function calculates the product of two numbers, so I’d expect the following to be a valid call:

product(2.1, 4)

However I specified the parameter types to be int so this isn’t valid and Mypy tells me off! There’s no “number type” in Python but PEP 484 states if we set the type to be float then an argument of type int is acceptable. So our function now looks like:

def product(x: float, y: float) -> float:
    return x * y


On the one hand I like Python type hints as they are adding extra detail to the code conveying how it should be used. It also feels similar to typing in Swift which I’m a fan of. On the other hand the fact the Python interpreter ignores the type hints and you’re required to run checks separately with MyPy makes this feel a bit more of a faff, also I found Mypy a bit slow to run so maybe this would work best as a CI step.