What does variable:expression mean in Python?

Question

When I type aaa: print(1) in Python 3.6, it will print 1 without any error.

I want to know what variable:expression means in Python.

I Googled and cannot find any documentation related to this.

Dimitris Fasarakis Hilliard · Accepted Answer · 2017-07-18 05:05:14Z

2

It's a variable annotation, as described in PEP 526. By running that expression, you've annotated the type of a to None, the return value of the print call, which doesn't make much sense.

You can see this by printing the __annotations__, a dictionary that holds the relation between names-types for a module (in your case, the module will probably be __main__):

print(__annotations__)
{'aaa': None}

Python doesn't do anything with these, it simply executed the print(1) (resulting in the output of 1 you see) expression and uses the return value of that call to annotate the name a. It's up to type-checkers, like mypy, to use them for their own purposes.

edited Jul 18, 2017 at 5:05

answered Jul 18, 2017 at 5:01

Dimitris Fasarakis Hilliard

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2 Comments

Ricky Han Over a year ago

Is it a python 3.6 thing? I don't see it in 3.5.2

Dimitris Fasarakis Hilliard Over a year ago

@RickyHan yup. You should get a SyntaxError in 3.5.2 (and __annotations__ for a module don't exist, either)

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