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Speed up make_simplified_union #14370

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Merged
merged 1 commit into from
Dec 29, 2022
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Speed up make_simplified_union #14370

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11 changes: 8 additions & 3 deletions mypy/typeops.py
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Original file line number Diff line number Diff line change
Expand Up @@ -441,6 +441,7 @@ def make_simplified_union(
* [int, int] -> int
* [int, Any] -> Union[int, Any] (Any types are not simplified away!)
* [Any, Any] -> Any
* [int, Union[bytes, str]] -> Union[int, bytes, str]

Note: This must NOT be used during semantic analysis, since TypeInfos may not
be fully initialized.
Expand All @@ -455,10 +456,14 @@ def make_simplified_union(
# Step 1: expand all nested unions
items = flatten_nested_unions(items)

# Step 2: remove redundant unions
# Step 2: fast path for single item
if len(items) == 1:
return get_proper_type(items[0])

# Step 3: remove redundant unions
simplified_set: Sequence[Type] = _remove_redundant_union_items(items, keep_erased)

# Step 3: If more than one literal exists in the union, try to simplify
# Step 4: If more than one literal exists in the union, try to simplify
if (
contract_literals
and sum(isinstance(get_proper_type(item), LiteralType) for item in simplified_set) > 1
Expand All @@ -471,7 +476,7 @@ def make_simplified_union(
if nitems > 1 and (
nitems > 2 or not (type(items[0]) is NoneType or type(items[1]) is NoneType)
):
# Step 4: At last, we erase any (inconsistent) extra attributes on instances.
# Step 5: At last, we erase any (inconsistent) extra attributes on instances.

# Initialize with None instead of an empty set as a micro-optimization. The set
# is needed very rarely, so we try to avoid constructing it.
Expand Down