Should you put quotes around type annotations in python

user_5434860 0
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What's the difference between these two functions? I've seen people put quotes around type annotations and other times leave them out but I couldn't find why people choose to use one or the other.

def do_something(entity: Entity):
    pass
def do_something(entity: 'Entity'):
    pass

Are there advantages or disadvantages to any of these?

type-hinting static-typing python-3.x python
2 Answers

Putting quotes around type hints is something that makes sense when making a Forward Reference according to PEP 484. In this case putting quotes around a name is used to subdue a NameError that would occur.

In other cases, don't use quotes, it doesn't result in the hint you want: 

>>> def bad_foo(a: 'int'):
...     pass
>>> def good_foo(a: int):
...     pass
>>> bad_foo.__annotations__['a'] == good_foo.__annotations__['a']
False

though for now type checkers (mypy, atleast) don't seem to treat these differently, I wouldn't be sure if that would be the case in the future. Best to be clear and not use quotes when you actually don't need them.

user_49521300

Apparently it can also cause runtime exceptions if you don't quote them in some situations. See this comment. I have no idea why, but it definitely does:

 ~ python3
Python 3.9.2 (default, Mar 26 2021, 23:27:12)
[Clang 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def main() -> OrderedDict[str, str]:
...     x: OrderedDict[str, str] = OrderedDict()
...     print(x)
...     return x
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'OrderedDict' is not defined
>>> def main() -> 'OrderedDict[str, str]':
...     x: OrderedDict[str, str] = OrderedDict()
...     print(x)
...     return x
...
>>>

However you can avoid it by using from __future__ import annotations.

user_2655210
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