[FIXED] Is there a way to specify a range of valid values for a function argument with type hinting in python?

Issue

I am a big fan of the type hinting in python, however I am curious if there is a way to specify a valid range of values for a given parameter using type hinting.

What I had in mind is something like

from typing import *

def function(
        number: Union[float, int],
        fraction: Float[0.0, 1.0] = 0.5 # give a hint that this should be between 0 and 1,
):
    return fraction * number

I can imagine one can enforce this with an assertion, or perhaps specify what the valid range of values is within the docstring, but it feels like having something like Float[0.0, 1.0] would look more elegant.

Solution

Python 3.9 introduced typing.Annotated:

In [75]: from typing import *

In [76]: from dataclasses import dataclass

In [77]: @dataclass
    ...: class ValueRange:
    ...:     min: float
    ...:     max: float
    ...:

In [78]: def function(
    ...:         number: Union[float, int],
    ...:         fraction: Annotated[float, ValueRange(0.0, 1.0)] = 0.5
    ...: ):
    ...:     return fraction * number
    ...:

Like any other type hint it does not perform any runtime checks:

In [79]: function(1, 2)
Out[79]: 2

However you can implement your own runtime checks. The code below is just an example, it does not cover all cases and probably an overkill for your simple function:

In [88]: import inspect

In [89]: @dataclass
    ...: class ValueRange:
    ...:     min: float
    ...:     max: float
    ...:
    ...:     def validate_value(self, x):
    ...:         if not (self.min <= x <= self.max):
    ...:             raise ValueError(f'{x} must be in range [{self.min}, {self.max}]')
    ...:

In [90]: def check_annotated(func):
    ...:     hints = get_type_hints(func, include_extras=True)
    ...:     spec = inspect.getfullargspec(func)
    ...:
    ...:     def wrapper(*args, **kwargs):
    ...:         for idx, arg_name in enumerate(spec[0]):
    ...:             hint = hints.get(arg_name)
    ...:             validators = getattr(hint, '__metadata__', None)
    ...:             if not validators:
    ...:                 continue
    ...:             for validator in validators:
    ...:                 validator.validate_value(args[idx])
    ...:
    ...:         return func(*args, **kwargs)
    ...:     return wrapper
    ...:
    ...:

In [91]: @check_annotated
    ...: def function_2(
    ...:         number: Union[float, int],
    ...:         fraction: Annotated[float, ValueRange(0.0, 1.0)] = 0.5
    ...: ):
    ...:     return fraction * number
    ...:
    ...:

In [92]: function_2(1, 2)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-92-c9345023c025> in <module>
----> 1 function_2(1, 2)

<ipython-input-90-01115cb628ba> in wrapper(*args, **kwargs)
     10                 continue
     11             for validator in validators:
---> 12                 validator.validate_value(args[idx])
     13
     14         return func(*args, **kwargs)

<ipython-input-87-7f4ac07379f9> in validate_value(self, x)
      6     def validate_value(self, x):
      7         if not (self.min <= x <= self.max):
----> 8             raise ValueError(f'{x} must be in range [{self.min}, {self.max}]')
      9

ValueError: 2 must be in range [0.0, 1.0]

In [93]: function_2(1, 1)
Out[93]: 1

Answered By - awesoon