Type is not infered from default value of dict.get()

[qi55wyqu]

Bug Report

When using dict.get(value, default) the returned type is not inferred correctly if the value was initially declared as Optional.

To Reproduce

mapping: Dict[str, str] = {"a": "x"}
val: Optional[str] = None
val = "a"
reveal_type(val) # Revealed type is "builtins.str"
val = mapping[val] if val in mapping else val
reveal_type(val) # Revealed type is "builtins.str"
val = mapping.get(val, val)
reveal_type(val) # Revealed type is "Union[builtins.str, None]"

Expected Behavior

Revealed type is builtins.str.

Actual Behavior

Revealed type is Union[builtins.str, None].

Your Environment

• Mypy version used: mypy 0.910
• Mypy command-line flags: --
• Mypy configuration options from mypy.ini (and other config files): --
• Python version used: 3.8.10
• Operating system and version: Ubuntu 20.04.2

[JelleZijlstra]

Isn't this because val is of type Optional[str]?

[erictraut]

The declared type of val is Optional[str], but at the time it is used in the expression mapping.get(val, val), its type has been narrowed to just str, so I would expect the return type of this call expression to be str in this case. That's the type pyright computes.

[hauntsaninja]

This is another thing that stems from mypy's notion of "type context". If you use a new variable, you get the result you want, i.e.:

new_val = mapping.get(val, val)
reveal_type(new_val)

[qi55wyqu]

mapping[v] if v in mapping else v

and

mapping.get(v, v)

are basically the same thing so I would expect mypy to infer the same type for the return value in both cases.

[AlexWaygood]

Fixed in #14151