Unexpected error behaviour: Parameterized generics cannot be used with class or instance checks

mypy reports success for

import numpy as np
x = "a"
assert isinstance(x, np.float64)

but fails for

from numpy import float64
x = "a"
assert isinstance(x, float64)

with the error fails.py:3: error: Parameterized generics cannot be used with class or instance checks

I'm using mypy (0.931) and numpy (1.22.2).

Why do I get different results?

Interesting. float64 is defined here: https://github.com/numpy/numpy/blob/51abd9693a78907f2ca7dfacee2017ef44fb9f49/numpy/__init__.pyi#L2973. It's obviously meant as a type alias, but apparently mypy sometimes interprets it as a variable assignment.

I have the same issue with Unions

version:

$  mypy --version
mypy 1.4.1 (compiled: yes)
$ python --version
Python 3.10.9

Config:

[tool.mypy]
namespace_packages=true
explicit_package_bases=true
strict=true
show_error_codes=true

Code

from dataclasses import dataclass


@dataclass
class A:
    val_a: str

@dataclass
class B:
    val_b: str

ABUnion = A|B

obj = A(val_a="hey")

assert isinstance(obj, ABUnion)

Result:

$ mypy check.py 
check.py:16: error: Parameterized generics cannot be used with class or instance checks  [misc]
check.py:16: error: Argument 2 to "isinstance" has incompatible type "<typing special form>"; expected "_ClassInfo"  [arg-type]
Found 2 errors in 1 file (checked 1 source file)

Code works fine

Thank you!

Edit:
One solution is to use get_args on the union

from typing import get_args

assert isinstance(obj, get_args(ABUnion))

This passes both code and check, but does not work as a type guard

Edit 2:
This work as type guard with a TypeGuard, obj is correctly scoped to a shorter set of the union

from dataclasses import dataclass
from typing import get_args, TypeGuard


@dataclass
class A:
    val_a: str
    common: str

@dataclass
class B:
    val_b: str
    common: str

@dataclass
class C:  # no common
    val_c: str

ABUnion = A|B  # has common
ABCUnion = A|B|C

obj: ABCUnion  = A(val_a="hey", common="common")

def is_ab(val: ABCUnion) -> TypeGuard[ABUnion]:
    return type(val) in get_args(ABUnion)

if is_ab(obj):
    print(obj.common)

Any status update?

Note that isinstance is defined as working on unions: "If classinfo is a tuple of type objects (or recursively, other such tuples) or a Union Type of multiple types, return True if object is an instance of any of the types."

I'm also wondering about the status on this.

Should this have the label topic-pep-604? I'm not 100% sure but it seems like it when I look at others open issues that have the label.

Any new development on mitigating this.
I am using cryptography and their aliases are not disected properly and even get_args does not help to mark object as suitable.
Also a Union of child classes is not properly inspected when variable of object is of parent class but the object is of child class.

Fix isinstance with PEP 604 type aliases

Fix isinstance with type aliases to PEP 604 unions (#17371)

@msetina

@msetina cryptography seems to use old-style unions, which appear to still be unsupported #17680

@lindycoder's workaround has been successful as far as I've tried

from numpy import float64
x = "a"
assert isinstance(x, float64)

yields error: Parameterized generics cannot be used with class or instance checks [misc]

while this passes

from typing import get_args
x = "a"
assert isinstance(x, get_args(float64))

with latest release

$ mypy --version
mypy 1.17.0 (compiled: yes)

Relating to cryptography the workaround also seems to do the trick for the general case, can you give a more specific example of the inheritance-related failure?

from cryptography.hazmat.primitives.hashes import SHA256
from cryptography.x509.base import _AllowedHashTypes

assert isinstance(SHA256, _AllowedHashTypes) # fails with [misc, arg-type]
assert isinstance(SHA256, get_args(_AllowedHashTypes)) # passes

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Unexpected error behaviour: Parameterized generics cannot be used with class or instance checks #12155

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Unexpected error behaviour: Parameterized generics cannot be used with class or instance checks #12155

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JelleZijlstra commented on Feb 9, 2022

lindycoder commented on Aug 1, 2023

tibbe commented on Aug 31, 2023

brandon-neth commented on Jan 25, 2024

hunterC90 commented on Feb 22, 2024

msetina commented on Apr 24, 2024

gbhand commented on Jul 23, 2025

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