What does -> do in python

Question

I saw a python example today and it used -> for example this was what I saw:

spam = None
bacon = 42
def monty_python(a:spam,b:bacon) -> "different:":
    pass

What is that code doing? I'm not quite sure I've never seen code like that I don't really get what

 a:spam,b:bacon

is doing either, can someone explain this for me? I googled, "what does -> do in python" but no good searches came up that I found.

score 29 · Accepted Answer · edited May 23 '17 at 11:48

29

It is function annotation for a return type. annotations do nothing inside the code, they are there to help a user with code completion (in my experience).

Here is the PEP for it.

Let me demonstrate, what I mean by "annotations do nothing inside the code". Here is an example:

def fun(a: str, b: int) -> str:
    return 1

if __name__ == '__main__':
    print(fun(10, 10))

The above code will run without any errors. but as you can see the first parameter should be a string, and the second an int. But, this only is a problem in my IDE, the code runs just fine:

enter image description here

edited May 23 '17 at 11:48

Community

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answered Oct 20 '13 at 16:13

Games Brainiac

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Oh I see that is interesting thanks for clearing that up for me, and by the way what editor are you using? – ruler Oct 20 '13 at 16:23
1

PyCharm 3.0 from Jetbrains. But this kind of functionality is available in other IDEs too. You can try PyCharm for free. PyDev also offers this, I think. – Games Brainiac Oct 20 '13 at 16:25
What's your IDE? Very nice! – G M Oct 20 '13 at 16:58
Look above at my previous comment :) – Games Brainiac Oct 20 '13 at 16:59
Nice, I did not know you could do that in python. I guest python is becoming more and more object oriented. – Oct 20 '13 at 22:35

score 7 · Answer 2 · answered Oct 20 '13 at 16:12

7

They're function annotations. They don't really do anything by themselves, but they can be used for documentation or in combination with metaprogramming.

answered Oct 20 '13 at 16:12

Danica

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Just to make it clear, these are only available in Python3, not in python 2! – oz123 Jun 18 '14 at 10:41

What does -> do in python

2 Answers2