What does mean two points and a class after args: def func(value:str)

Question

I don't understand what the arg: str means

def hello(arg: str):
    print(type(arg))
    print('do stuff!', arg)


hello('something')

I thought it was specifing a type, but I tried to call hello, and I haven't got any exception. I've tried to call hello with an int, no error.

I've done a few test, but I haven't come up with anything...

Any idea about what this is doing?

Matt

@ŁukaszRogalski It might be a different question because it's asking about argument type-hinting specifically. — Tankobot, Oct 25 '16 at 06:28

score 4 · Accepted Answer · answered Oct 25 '16 at 06:22

4

You are correct def hello(arg: str): does specify a type for the argument. But in Python, it only provides type-hinting. This means that you can theoretically pass any type of instance to the function. Type-hinting is useful for development and for IDEs because it can help the program notify you if you're passing unexpected arguments to a function.

Extra note, these type-hints are stored in the function under __annotations__:

In [1]: def f(arg: int):
   ...:     return arg
   ...: 

In [2]: f.__annotations__
Out[2]: {'arg': int}

answered Oct 25 '16 at 06:22

Tankobot

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So, it's only to help the dev, python doesn't give a damn if it's not the right type... – math2001 Oct 25 '16 at 06:43
@math2001 Precisely. Your programs will execute with or without type-hinting. – Tankobot Oct 25 '16 at 06:51

What does mean two points and a class after args: def func(value:str)

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