PEP 585 support in 3.7+ using __future__

Describe the bug Vermin reports the minimum version is Python 3.9, even though the code will run correctly in Python 3.7+.

To Reproduce Steps to reproduce the behavior.

Run the following code in Python 3.7, 3.8, 3.9

from __future__ import annotations

def fn(x: list[int]) -> list[int]:
    a: list[int] = []
    return a

fn([])

Output:

Detecting python files..
Analyzing using 12 processes..
!2, 3.9      /Users/tyler.yep/Documents/Github/probs/yo.py
  L1 C5: '__future__' module requires 2.1, 3.0
  L1 C5: '__future__.annotations' member requires !2, 3.7
  L4: annotations requires !2, 3.0
  L4: builtin generic type annotation (list[..]) requires !2, 3.9
  L5: variable annotations requires !2, 3.6
  L5: builtin generic type annotation (list[..]) requires !2, 3.9
  L6: builtin generic type annotation (list[..]) requires !2, 3.9

Minimum required versions: 3.9
Incompatible versions:     2

Expected behavior Minimum version should be Python 3.7

Environment:

• Vermin latest version (1.1.0)

[gousaiyang]

I also discovered this earlier but didn’t have time to organize it and create an issue 😃 Just want to point out some important points to consider for implementation:

• PEP 563 (from __future__ import annotations) makes function and variable annotations stored in string form. They are not evaluated at definition time, but only evaluated when you manually call typing.get_type_hints or eval(obj.__annotations__). The following code snippet:

from __future__ import annotations

def foo() -> list[int]:
    return [0]

foo()

is 3.7+, but this one:

from __future__ import annotations
import typing

def foo() -> list[int]:
    return [0]

typing.get_type_hints(foo)

is 3.9+. Probably there could be a command line option for users to inform vermin whether the annotations in the code will be manually evaluated or not.

• Local variable annotations are never evaluated or stored anywhere. So this:

def foo():
    x: list[int]

is 3.6+ (since variable annotations are 3.6+), even if from __future__ import annotations is not present.

• Even if PEP 563 is enabled, annotations should still be syntactically valid Python expressions. New syntax used should still indicate a higher minimum version. For example,

from __future__ import annotations
x: list[int]  # PEP 585, 3.9+
y: int | str  # PEP 604, 3.10+

is 3.7+, since list[int] and int | str are valid syntax in Python 3.7. While this one:

from __future__ import annotations
def foo():
    x: (a := 1)

is still 3.8+.

• PEP 563 is mandatory in Python 3.10. Let’s assume that int | str were a 3.11 feature (instead of 3.10 in reality). The following code

x: int | str

will still be 3.10+ since the PEP 563 is always enabled (without having to write from __future__ import annotations) in 3.10 and int | str is not new syntax.

[netromdk]

Thanks again, both. Will release version 1.1.1 soon.