8000 Distinguish redundant-expr warnings from unreachable warnings (#9125) · python/mypy@f2dfd91 · GitHub
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Distinguish redundant-expr warnings from unreachable warnings (#9125)
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‎docs/source/error_code_list2.rst

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@@ -193,3 +193,24 @@ incorrect control flow or conditional checks that are accidentally always true o
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return
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# Error: Statement is unreachable [unreachable]
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print('unreachable')
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Check that expression is redundant [redundant-expr]
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---------------------------------------------------
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If you use :option:`--enable-error-code redundant-expr <mypy --enable-error-code>`,
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mypy generates an error if it thinks that an expression is redundant.
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.. code-block:: python
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# mypy: enable-error-code redundant-expr
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def example(x: int) -> None:
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# Error: Left operand of 'and' is always true [redundant-expr]
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if isinstance(x, int) and x > 0:
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pass
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# Error: If condition is always true [redundant-expr]
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1 if isinstance(x, int) else 0
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# Error: If condition in comprehension is always true [redundant-expr]
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[i for i in range(x) if isinstance(i, int)]

‎mypy/checkexpr.py

Lines changed: 14 additions & 5 deletions
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@@ -2741,6 +2741,17 @@ def check_boolean_op(self, e: OpExpr, context: Context) -> Type:
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restricted_left_type = true_only(left_type)
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result_is_left = not left_type.can_be_false
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# If left_map is None then we know mypy considers the left expression
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# to be redundant.
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#
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# Note that we perform these checks *before* we take into account
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# the analysis from the semanal phase below. We assume that nodes
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# marked as unreachable during semantic analysis were done so intentionally.
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# So, we shouldn't report an error.
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if codes.REDUNDANT_EXPR in self.chk.options.enabled_error_codes:
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if left_map is None:
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self.msg.redundant_left_operand(e.op, e.left)
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# If right_map is None then we know mypy considers the right branch
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# to be unreachable and therefore any errors found in the right branch
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# should be suppressed.
@@ -2750,10 +2761,8 @@ def check_boolean_op(self, e: OpExpr, context: Context) -> Type:
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# marked as unreachable during semantic analysis were done so intentionally.
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# So, we shouldn't report an error.
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if self.chk.options.warn_unreachable:
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if left_map is None:
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self.msg.redundant_left_operand(e.op, e.left)
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if right_map is None:
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self.msg.redundant_right_operand(e.op, e.right)
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self.msg.unreachable_right_operand(e.op, e.right)
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if e.right_unreachable:
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right_map = None
@@ -3674,7 +3683,7 @@ def check_for_comp(self, e: Union[GeneratorExpr, DictionaryComprehension]) -> No
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for var, type in true_map.items():
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self.chk.binder.put(var, type)
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if self.chk.options.warn_unreachable:
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if codes.REDUNDANT_EXPR in self.chk.options.enabled_error_codes:
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if true_map is None:
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self.msg.redundant_condition_in_comprehension(False, condition)
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elif false_map is None:
@@ -3687,7 +3696,7 @@ def visit_conditional_expr(self, e: ConditionalExpr, allow_none_return: bool = F
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# Gain type information from isinstance if it is there
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# but only for the current expression
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if_map, else_map = self.chk.find_isinstance_check(e.cond)
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if self.chk.options.warn_unreachable:
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if codes.REDUNDANT_EXPR in self.chk.options.enabled_error_codes:
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if if_map is None:
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self.msg.redundant_condition_in_if(False, e.cond)
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elif else_map is None:

‎mypy/errorcodes.py

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@@ -112,6 +112,11 @@ def __str__(self) -> str:
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'General') # type: Final
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UNREACHABLE = ErrorCode(
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'unreachable', "Warn about unreachable statements or expressions", 'General') # type: Final
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REDUNDANT_EXPR = ErrorCode(
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'redundant-expr',
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"Warn about redundant expressions",
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'General',
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default_enabled=False) # type: Final
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# Syntax errors are often blocking.
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SYNTAX = ErrorCode(

‎mypy/main.py

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Original file line numberDiff line numberDiff line change
@@ -574,7 +574,7 @@ def add_invertible_flag(flag: str,
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group=lint_group)
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add_invertible_flag(' A93C --warn-unreachable', default=False, strict_flag=False,
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help="Warn about statements or expressions inferred to be"
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" unreachable or redundant",
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" unreachable",
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group=lint_group)
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# Note: this group is intentionally added here even though we don't add

‎mypy/messages.py

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Original file line numberDiff line numberDiff line change
@@ -1280,7 +1280,7 @@ def redundant_left_operand(self, op_name: str, context: Context) -> None:
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"""
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self.redundant_expr("Left operand of '{}'".format(op_name), op_name == 'and', context)
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def redundant_right_operand(self, op_name: str, context: Context) -> None:
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def unreachable_right_operand(self, op_name: str, context: Context) -> None:
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"""Indicates that the right operand of a boolean expression is redundant:
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it does not change the truth value of the entire condition as a whole.
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'op_name' should either be the string "and" or the string "or".
@@ -1299,7 +1299,7 @@ def redundant_condition_in_assert(self, truthiness: bool, context: Context) -> N
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def redundant_expr(self, description: str, truthiness: bool, context: Context) -> None:
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self.fail("{} is always {}".format(description, str(truthiness).lower()),
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context, code=codes.UNREACHABLE)
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context, code=codes.REDUNDANT_EXPR)
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def impossible_intersection(self,
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formatted_base_class_list: str,

‎test-data/unit/check-errorcodes.test

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@@ -771,3 +771,18 @@ def f(**x: int) -> None:
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f(**1) # type: ignore[arg-type]
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[builtins fixtures/dict.pyi]
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[typing fixtures/typing-typeddict.pyi]
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[case testRedundantExpressions]
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# flags: --enable-error-code redundant-expr
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def foo() -> bool: ...
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lst = [1, 2, 3, 4]
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b = False or foo() # E: Left operand of 'or' is always false [redundant-expr]
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c = True and foo() # E: Left operand of 'and' is always true [redundant-expr]
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g = 3 if True else 4 # E: If condition is always true [redundant-expr]
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h = 3 if False else 4 # E: If condition is always false [redundant-expr]
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i = [x for x in lst if True] # E: If condition in comprehension is always true [redundant-expr]
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j = [x for x in lst if False] # E: If condition in comprehension is always false [redundant-expr]
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k = [x for x in lst if isinstance(x, int) or foo()] # E: If condition in comprehension is always true [redundant-expr]
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[builtins fixtures/isinstancelist.pyi]

‎test-data/unit/check-unreachable-code.test

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Original file line numberDiff line numberDiff line change
@@ -898,18 +898,11 @@ def foo() -> bool: ...
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lst = [1, 2, 3, 4]
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a = True or foo() # E: Right operand of 'or' is never evaluated
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b = False or foo() # E: Left operand of 'or' is always false
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c = True and foo() # E: Left operand of 'and' is always true
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d = False and foo() # E: Right operand of 'and' is never evaluated
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e = True or (True or (True or foo())) # E: Right operand of 'or' is never evaluated
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f = (True or foo()) or (True or foo()) # E: Right operand of 'or' is never evaluated
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g = 3 if True else 4 # E: If condition is always true
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h = 3 if False else 4 # E: If condition is always false
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i = [x for x in lst if True] # E: If condition in comprehension is always true
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j = [x for x in lst if False] # E: If condition in comprehension is always false
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k = [x for x in lst if isinstance(x, int) or foo()] # E: If condition in comprehension is always true \
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# E: Right operand of 'or' is never evaluated
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k = [x for x in lst if isinstance(x, int) or foo()] # E: Right operand of 'or' is never evaluated
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[builtins fixtures/isinstancelist.pyi]
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[case testUnreachableFlagMiscTestCaseMissingMethod]

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