python - Use of colon in variable declaration

Question

I was asked recently what this means in python:

>>> char : str

I had no idea. I'd never seen that before. I checked the docs and there isn't anything like that. One person's suggestion was that it is static type declaration, but there is absolutely nothing in the docs about that either.

With the above, if I >>> type(char) it fails

If I >>> char : str = 'abc' it works, and the results of type(char) is <class: str>. It can't be static declaration though, because I can >>> char : str = 4 and type(char) becomes <class: int>.

So I come here to collect the wisdom of the many SO overlords. What does that mean?

Martijn Pieters Martijn Pieters · Accepted Answer · 2018-08-01T17:46:18

You are looking at an annotation for a variable. The hint is moved to the __annotations__ mapping:

>>> char: str
>>> __annotations__
{'char': <class 'str'>}

Variable annotations are there to support third-party tooling, such as type checkers; the syntax is new in Python 3.6.

See PEP 526 -- Syntax for Variable Annotations, and What's new in Python 3.6:

Just as for function annotations, the Python interpreter does not attach any particular meaning to variable annotations and only stores them in the __annotations__ attribute of a class or module.

python - Use of colon in variable declaration

1 Answers