Inconsistent behavior: MyPy rejects isinstance with Any, but accepts cast from Any for ignored imports

[carlos-superior]

Context

When working with the cassandra.cluster module in a Python project, i had to use type: ignore on the import because there are no compatible MyPy stubs available for the module.

Below is a simplified version of how the code is set up.

from cassandra.cluster import Session as CassandraConnection # type: ignore (mypy will treat it just like Any) from typing import TypeAlias, cast, Any

TransactionalConnection: TypeAlias = CassandraConnection

This approach generally works, but it causes issues with type checking in MyPy.

---

Initial Problem

When trying to use isinstance() to check if a variable is an instance of TransactionalConnection, Mypy raises the following error:

Cannot use isinstance() with Any type [misc]

This happens because MyPy is treating CassandraConnection as Any due to the #type: ignore. Here's the code that triggers the error:

if isinstance(db_client, TransactionalConnection): pass -> error occurs here: Cannot use isinstance() with Any type.

---

Paradox

Interestingly, when using cast(Any, CassandraConnection), Mypy accepts the typing without issues. In other words, when i cast Any to CassandraConnection, Mypy does not raise any errors, even though the underlying type is still Any. Here's the example:

TransactionalConnection: TypeAlias = cast(Any, CassandraConnection) -> This works with no MyPy errors.

This behavior is paradoxical because, in both cases: (isinstance and cast), the underlying type is Any, but Mypy handles them differently.

---

Full Scenario Example

# Core
import json
import os
from typing import TypedDict, TypeAlias, cast, Any

# Libraries
from psycopg2.extensions import connection as PostgreSQLConnection
import psycopg2
from psycopg2 import sql as psycopg2_sql
from cassandra.cluster import Session as CassandraConnection    # type: ignore
from cassandra.auth import PlainTextAuthProvider    # type: ignore
from cassandra.cluster import Cluster

# Core functionality
sql = psycopg2_sql
SqlCursor = psycopg2.extensions.cursor

# Defining type aliases
RelationalConnection: TypeAlias = PostgreSQLConnection
TransactionalConnection: TypeAlias = cast(Any, CassandraConnection)

class DbConnections(TypedDict):
    transactional: TransactionalConnection
    relational: RelationalConnection

Here’s the part where the isinstance check causes the error:

if isinstance(db_client, TransactionalConnection):  
    pass

error: Cannot use isinstance() with Any type

But when using cast, MyPy doesn't raise any errors: TransactionalConnection: TypeAlias = cast(Any, CassandraConnection)

---

Conclusion

The inconsistent behavior between using isinstance() and cast when dealing with the Any type seems to be a bug or, at the very least, an inconsistency in Mypy. If cast() accepts Any without issue, isinstance() should behave similarly, as both deal with the Any type. However, Mypy rejects isinstance, but accepts cast.

This inconsistent behavior can confuse developers working with modules that lack proper typing, requiring the use of # type: ignore. I hope this issue can be reviewed, and Mypy can offer consistent behavior between isinstance and cast.

---

Environment

• Python version 3.12.3
• Flags: --strict
• mypy version: 1.11.0
• mypy-extensions version: 1.0.0

[brianschubert]

In other words, when i cast Any to CassandraConnection, Mypy does not raise any errors, even though the underlying type is still Any. Here's the example: [...] cast(Any, CassandraConnection)

I haven't looked at this issue closely, but just in case it was a source of confusion, I wanted to point out that cast(Any, CassandraConnection) is a cast to Any, not from Any. The signature for cast is cast(<assumed type>, <runtime value>). When you write cast(Any, CassandraConnection), you get back the runtime value CassandraConnection, but ask static type checkers to treat it as though it were an Any. Note that this is flipped with respect to the argument order for isinstance.

If cast() accepts Any without issue, isinstance() should behave similarly

There's a big difference between cast and isinstance, which is that cast only affects static type checking whereas isinstance also has runtime behavior. Mypy rejects using Any with isinstance because combing the two at runtime produces a TypeError:

>>> isinstance("foo", Any)
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    isinstance("foo", Any)
    ~~~~~~~~~~^^^^^^^^^^^^
  File "/usr/lib/python3.13/typing.py", line 589, in __instancecheck__
    raise TypeError("typing.Any cannot be used with isinstance()")
TypeError: typing.Any cannot be used with isinstance()