Can't determine the length of a Tuple from a Union type properly

[skytreader]

Bug Report

For a Union type of two possible Tuple lengths, mypy reports index out of range for Tuples of the longer type even with a proper explicit check.

To Reproduce

from typing import Tuple, Union

SpamType = Union[
    Tuple[str, str, str],
    Tuple[str, str]
]

def egg(spam: SpamType) -> str:
    if len(spam) == 3:
        return spam[2]
    else:
        return spam[1]

print(egg(("a", "b")))
print(egg(("a", "b", "c")))

Expected Behavior

The code listing above should be perfectly typed.

Actual Behavior

mypy reports,

mypybug.py:10: error: Tuple index out of range
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used:

mypy==0.782
mypy-extensions==0.4.3

• Mypy command-line flags: --ignore-missing-imports
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.7.0
• Operating system and version: Ubuntu 16.04.7 LTS

[gvanrossum]

Mypy only recognizes certain checks as runtime type checks -- this includes isinstance(x, C) and x is [not] None, but it does not include tuple length checks. So mypy doesn't narrow the union type to a specific union member based on the condition.

I suspect there's probably already another issue open that may have more discussion about this.

[gvanrossum]

Duplicate of #1178.