Type annotation in the for loop causes Unknown return values on method calls

[gaborbernat]

Having a type annotation in a for loop causes method calls on it to be threated as possibly unknown.

Code or Screenshots

❯ uv venv -p 3.13
❯ uv pip install pyright mypy types-boto3[sts] boto3

Resolved 17 packages in 3.89s
Installed 17 packages in 196ms
 + boto3==1.36.13
 + botocore==1.36.13
 + botocore-stubs==1.36.12
 + jmespath==1.0.1
 + mypy==1.15.0
 + mypy-extensions==1.0.0
 + nodeenv==1.9.1
 + pyright==1.1.393
 + python-dateutil==2.9.0.post0
 + s3transfer==0.11.2
 + six==1.17.0
 + types-awscrt==0.23.9
 + types-boto3==1.36.13
 + types-boto3-sts==1.36.0
 + types-s3transfer==0.11.2
 + typing-extensions==4.12.2
 + urllib3==2.3.0

from boto3 import client
from types_boto3_sts import STSClient

aws_access_key_id: str = ""
for role in ["r"]:
    sts_client: STSClient = client("sts", aws_access_key_id=aws_access_key_id)
    try:
        resp = sts_client.assume_role(RoleArn=role, RoleSessionName="S")
    except ValueError:
        continue
    else:
        aws_access_key_id = resp["Credentials"]["AccessKeyId"]

❯ .venv/bin/pyright --pythonpath=.venv/bin/python magic.py
magic.py:8:61 - error: Argument type is partially unknown
    Argument corresponds to parameter "aws_access_key_id" in function "client"
    Argument type is "str | Unknown" (reportUnknownArgumentType)
 magic.py:11:21 - information: Type of "resp" is "AssumeRoleResponseTypeDef | Unknown"
1 error, 0 warnings, 1 information

Passes the type checker on mypy. Removing the for loop also makes the type checker pass.

The type annotations from types-boto3[sts]:

    def assume_role(
        self, **kwargs: Unpack[AssumeRoleRequestRequestTypeDef]
    ) -> AssumeRoleResponseTypeDef:

class CredentialsTypeDef(TypedDict):
    AccessKeyId: str
    SecretAccessKey: str
    SessionToken: str
    Expiration: datetime

class AssumeRoleResponseTypeDef(TypedDict):
    Credentials: CredentialsTypeDef
    AssumedRoleUser: AssumedRoleUserTypeDef
    PackedPolicySize: int
    SourceIdentity: str
    ResponseMetadata: ResponseMetadataTypeDef