Examples of callable type vs union confusion

callable-type-vs-union.md

1. We can't naively union a callable type with another type:

union_of_callables: (int) -> str | (bool) -> str  # Syntax error!

# Needs extra parens; Not intuitive or user-friendly.
union_of_callable2: ((int) -> str) | ((bool) -> str)

# Simpler and can be easily split into multiple lines, as Никита mentioned.
union_of_callables3: Union[(int) -> str, (bool) -> str, (list[int]) -> str]

Likewise, int | (str) -> bool is a syntax error.

2. Another huge gotcha is with Optional callables if we naively use <new-syntax> | None:

old: Optional[Callable[[int], str]]

# This works fine
with_pipe: Callable[[int], str] | None

# Incorrect! This specifies a non-optional function with return type `str | None`
naive_translation_to_new_syntax: (int) -> str | None   

# Trying to use order of types to fix a precedence issue won't work
naive_attempt_to_fix_above_error: None | (int) -> str  # Syntax error!

# The solution requires parentheses if we're using `<type> | None`
intended: ((int) -> str) | None

# Expressing it with Optional (If we don't deprecate optional!) is clean:
unambiguous_and_simple: Optional[(int) -> str]