type hint returns NameError: name 'datetime' not defined

Question

I have this function below;

def time_in_range(start, end, x):
    """Return true if x is in the range [start, end]"""
    if start <= end:
        return start <= x <= end
    else:
        return start <= x or x <= end

The function parameters are all datetime type. I want to add typing hint to the function. This is what I did;

def time_in_range(start: datetime, end: datetime, x: datetime) -> bool:
    """Return true if x is in the range [start, end]"""
    if start <= end:
        return start <= x <= end
    else:
        return start <= x or x <= end

I get the error NameError: name 'datetime' is not defined. What is the proper way to add typing for this function?

I am using python v3.7

Asked By: user3848207

||

Source

Answer 1

You need to import datetime, or use a string (remember, it is just an hint).

>>> def f(x: datetime):
...     pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'datetime' is not defined
>>> def f(x: 'datetime'):
...     pass
...
>>>
>>> from datetime import datetime
>>> def f(x: datetime):
...     pass
...
>>>

Python 3.7.4

—

UPDATE. For a slightly different version of this issue import __future__.annotations

from __future__ import annotations

def f(dto: Dto):
    pass

class Dto:
    pass

This would otherwise fail in the function reference to the Dto class. Python4 will have this as the default behaviour. For now, the import must be the first statement of the file.

Tested in Python 3.8.10.

See: https://peps.python.org/pep-0563/

Answered By: Pedro Rodrigues

Answer 2

Either import datetime and use datetime.datetime as hint, or from datetime import datetime and use datetime as hint.

Answered By: blue_note

type hint returns NameError: name 'datetime' not defined

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