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Make infer_condition_value recognize the whole truth table #1

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cae4615
Make infer_condition_value recognize the whole truth table
sterliakov Apr 20, 2025
a500826
Test that we do not try to handle other operators
sterliakov Apr 21, 2025
648bd84
Remove short-circuiting logic here, it is safe to check truthiness of…
sterliakov Apr 21, 2025
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47 changes: 29 additions & 18 deletions mypy/reachability.py
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Original file line number Diff line number Diff line change
Expand Up @@ -115,31 +115,44 @@ def infer_condition_value(expr: Expression, options: Options) -> int:
MYPY_TRUE if true under mypy and false at runtime, MYPY_FALSE if
false under mypy and true at runtime, else TRUTH_VALUE_UNKNOWN.
"""
if isinstance(expr, UnaryExpr) and expr.op == "not":
positive = infer_condition_value(expr.expr, options)
return inverted_truth_mapping[positive]

pyversion = options.python_version
name = ""
negated = False
alias = expr
if isinstance(alias, UnaryExpr):
if alias.op == "not":
expr = alias.expr
negated = True

result = TRUTH_VALUE_UNKNOWN
if isinstance(expr, NameExpr):
name = expr.name
elif isinstance(expr, MemberExpr):
name = expr.name
elif isinstance(expr, OpExpr) and expr.op in ("and", "or"):
if expr.op not in ("or", "and"):
return TRUTH_VALUE_UNKNOWN
Comment on lines +131 to +132
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medium

This check seems redundant, as the elif condition on line 130 already ensures that expr.op is either "and" or "or". Can this be removed to simplify the code?

Suggested change
if expr.op not in ("or", "and"):
return TRUTH_VALUE_UNKNOWN
left = infer_condition_value(expr.left, options)
right = infer_condition_value(expr.right, options)


left = infer_condition_value(expr.left, options)
if (left in (ALWAYS_TRUE, MYPY_TRUE) and expr.op == "and") or (
left in (ALWAYS_FALSE, MYPY_FALSE) and expr.op == "or"
):
# Either `True and <other>` or `False or <other>`: the result will
# always be the right-hand-side.
return infer_condition_value(expr.right, options)
else:
# The result will always be the left-hand-side (e.g. ALWAYS_* or
# TRUTH_VALUE_UNKNOWN).
return left
right = infer_condition_value(expr.right, options)
results = {left, right}
if expr.op == "or":
if ALWAYS_TRUE in results:
return ALWAYS_TRUE
elif MYPY_TRUE in results:
return MYPY_TRUE
elif left == right == MYPY_FALSE:
return MYPY_FALSE
elif results <= {ALWAYS_FALSE, MYPY_FALSE}:
return ALWAYS_FALSE
elif expr.op == "and":
if ALWAYS_FALSE in results:
return ALWAYS_FALSE
elif MYPY_FALSE in results:
return MYPY_FALSE
elif left == right == ALWAYS_TRUE:
return ALWAYS_TRUE
elif results <= {ALWAYS_TRUE, MYPY_TRUE}:
return MYPY_TRUE
return TRUTH_VALUE_UNKNOWN
Comment on lines +137 to +155
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🛠️ Refactor suggestion

⚠️ Potential issue

Incorrect truth‑table for or when mixing ALWAYS_FALSE and MYPY_FALSE

Example:
cond = ALWAYS_FALSE or MYPY_FALSE

• mypy sees False or False -> False
• runtime sees False or True -> True
→ The result should be MYPY_FALSE, but the current logic returns ALWAYS_FALSE via the
results <= {ALWAYS_FALSE, MYPY_FALSE} branch.

A minimal fix is to short‑circuit on MYPY_FALSE before the subset test:

@@
         if expr.op == "or":
             if ALWAYS_TRUE in results:
                 return ALWAYS_TRUE
             elif MYPY_TRUE in results:
                 return MYPY_TRUE
+            # If at least one side is MYPY_FALSE the overall value is
+            # false for mypy but true at runtime.
+            elif MYPY_FALSE in results:
+                return MYPY_FALSE
-            elif left == right == MYPY_FALSE:
-                return MYPY_FALSE
-            elif results <= {ALWAYS_FALSE, MYPY_FALSE}:
-                return ALWAYS_FALSE
+            elif results == {ALWAYS_FALSE}:
+                return ALWAYS_FALSE

This preserves the existing left == right == MYPY_FALSE optimisation and fixes the mixed case.

(Optional) : consider extracting a small helper (e.g. combine_truth_values(op, left, right)) so the full 5‑state truth table is unit‑tested and easier to reason about.

📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
if expr.op == "or":
if ALWAYS_TRUE in results:
return ALWAYS_TRUE
elif MYPY_TRUE in results:
return MYPY_TRUE
elif left == right == MYPY_FALSE:
return MYPY_FALSE
elif results <= {ALWAYS_FALSE, MYPY_FALSE}:
return ALWAYS_FALSE
elif expr.op == "and":
if ALWAYS_FALSE in results:
return ALWAYS_FALSE
elif MYPY_FALSE in results:
return MYPY_FALSE
elif left == right == ALWAYS_TRUE:
return ALWAYS_TRUE
elif results <= {ALWAYS_TRUE, MYPY_TRUE}:
return MYPY_TRUE
return TRUTH_VALUE_UNKNOWN
if expr.op == "or":
if ALWAYS_TRUE in results:
return ALWAYS_TRUE
elif MYPY_TRUE in results:
return MYPY_TRUE
# If at least one side is MYPY_FALSE the overall value is
# false for mypy but may be true at runtime.
elif MYPY_FALSE in results:
return MYPY_FALSE
elif results == {ALWAYS_FALSE}:
return ALWAYS_FALSE
elif expr.op == "and":
if ALWAYS_FALSE in results:
return ALWAYS_FALSE
elif MYPY_FALSE in results:
return MYPY_FALSE
elif left == right == ALWAYS_TRUE:
return ALWAYS_TRUE
elif results <= {ALWAYS_TRUE, MYPY_TRUE}:
return MYPY_TRUE
return TRUTH_VALUE_UNKNOWN

else:
result = consider_sys_version_info(expr, pyversion)
if result == TRUTH_VALUE_UNKNOWN:
Expand All @@ -155,8 +168,6 @@ def infer_condition_value(expr: Expression, options: Options) -> int:
result = ALWAYS_TRUE
elif name in options.always_false:
result = ALWAYS_FALSE
if negated:
result = inverted_truth_mapping[result]
return result


Expand Down
76 changes: 76 additions & 0 deletions test-data/unit/check-unreachable-code.test
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Original file line number Diff line number Diff line change
Expand Up @@ -500,6 +500,82 @@ reveal_type(h) # N: Revealed type is "builtins.bool"
[builtins fixtures/ops.pyi]
[out]

[case testConditionalValuesBinaryOps]
# flags: --platform linux
import sys

t_and_t = (sys.platform == 'linux' and sys.platform == 'linux') and 's'
t_or_t = (sys.platform == 'linux' or sys.platform == 'linux') and 's'
t_and_f = (sys.platform == 'linux' and sys.platform == 'windows') and 's'
t_or_f = (sys.platform == 'linux' or sys.platform == 'windows') and 's'
f_and_t = (sys.platform == 'windows' and sys.platform == 'linux') and 's'
f_or_t = (sys.platform == 'windows' or sys.platform == 'linux') and 's'
f_and_f = (sys.platform == 'windows' and sys.platform == 'windows') and 's'
f_or_f = (sys.platform == 'windows' or sys.platform == 'windows') and 's'
reveal_type(t_and_t) # N: Revealed type is "Literal['s']"
reveal_type(t_or_t) # N: Revealed type is "Literal['s']"
reveal_type(f_and_t) # N: Revealed type is "builtins.bool"
reveal_type(f_or_t) # N: Revealed type is "Literal['s']"
reveal_type(t_and_f) # N: Revealed type is "builtins.bool"
reveal_type(t_or_f) # N: Revealed type is "Literal['s']"
reveal_type(f_and_f) # N: Revealed type is "builtins.bool"
reveal_type(f_or_f) # N: Revealed type is "builtins.bool"
[builtins fixtures/ops.pyi]

[case testConditionalValuesNegation]
# flags: --platform linux
import sys

not_t = not sys.platform == 'linux' and 's'
not_f = not sys.platform == 'windows' and 's'
not_and_t = not (sys.platform == 'linux' and sys.platform == 'linux') and 's'
not_and_f = not (sys.platform == 'linux' and sys.platform == 'windows') and 's'
not_or_t = not (sys.platform == 'linux' or sys.platform == 'linux') and 's'
not_or_f = not (sys.platform == 'windows' or sys.platform == 'windows') and 's'
reveal_type(not_t) # N: Revealed type is "builtins.bool"
reveal_type(not_f) # N: Revealed type is "Literal['s']"
reveal_type(not_and_t) # N: Revealed type is "builtins.bool"
reveal_type(not_and_f) # N: Revealed type is "Literal['s']"
reveal_type(not_or_t) # N: Revealed type is "builtins.bool"
reveal_type(not_or_f) # N: Revealed type is "Literal['s']"
[builtins fixtures/ops.pyi]

[case testConditionalValuesUnsupportedOps]
# flags: --platform linux
import sys

unary_minus = -(sys.platform == 'linux') and 's'
binary_minus = ((sys.platform == 'linux') - (sys.platform == 'linux')) and 's'
reveal_type(unary_minus) # N: Revealed type is "Union[Literal[0], builtins.str]"
reveal_type(binary_minus) # N: Revealed type is "Union[Literal[0], builtins.str]"
[builtins fixtures/ops.pyi]

[case testMypyFalseValuesInBinaryOps_no_empty]
# flags: --platform linux
import sys
from typing import TYPE_CHECKING

MYPY = 0

if TYPE_CHECKING and sys.platform == 'linux':
def foo1() -> int: ...
if sys.platform == 'linux' and TYPE_CHECKING:
def foo2() -> int: ...
if MYPY and sys.platform == 'linux':
def foo3() -> int: ...
if sys.platform == 'linux' and MYPY:
def foo4() -> int: ...

if TYPE_CHECKING or sys.platform == 'linux':
def bar1() -> int: ... # E: Missing return statement
if sys.platform == 'linux' or TYPE_CHECKING:
def bar2() -> int: ... # E: Missing return statement
if MYPY or sys.platform == 'linux':
def bar3() -> int: ... # E: Missing return statement
if sys.platform == 'linux' or MYPY:
def bar4() -> int: ... # E: Missing return statement
[builtins fixtures/ops.pyi]

[case testShortCircuitAndWithConditionalAssignment]
# flags: --platform linux
import sys
Expand Down