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reachability.py
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reachability.py
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"""Utilities related to determining the reachability of code (in semantic analysis)."""
from __future__ import annotations
from typing import Final, Tuple, TypeVar
from mypy.literals import literal
from mypy.nodes import (
LITERAL_YES,
AssertStmt,
Block,
CallExpr,
ComparisonExpr,
Expression,
FuncDef,
IfStmt,
Import,
ImportAll,
ImportFrom,
IndexExpr,
IntExpr,
MatchStmt,
MemberExpr,
NameExpr,
OpExpr,
SliceExpr,
StrExpr,
TupleExpr,
UnaryExpr,
)
from mypy.options import Options
from mypy.patterns import AsPattern, OrPattern, Pattern
from mypy.traverser import TraverserVisitor
# Inferred truth value of an expression.
ALWAYS_TRUE: Final = 1
MYPY_TRUE: Final = 2 # True in mypy, False at runtime
ALWAYS_FALSE: Final = 3
MYPY_FALSE: Final = 4 # False in mypy, True at runtime
TRUTH_VALUE_UNKNOWN: Final = 5
inverted_truth_mapping: Final = {
ALWAYS_TRUE: ALWAYS_FALSE,
ALWAYS_FALSE: ALWAYS_TRUE,
TRUTH_VALUE_UNKNOWN: TRUTH_VALUE_UNKNOWN,
MYPY_TRUE: MYPY_FALSE,
MYPY_FALSE: MYPY_TRUE,
}
reverse_op: Final = {"==": "==", "!=": "!=", "<": ">", ">": "<", "<=": ">=", ">=": "<="}
def infer_reachability_of_if_statement(s: IfStmt, options: Options) -> None:
for i in range(len(s.expr)):
result = infer_condition_value(s.expr[i], options)
if result in (ALWAYS_FALSE, MYPY_FALSE):
# The condition is considered always false, so we skip the if/elif body.
mark_block_unreachable(s.body[i])
elif result in (ALWAYS_TRUE, MYPY_TRUE):
# This condition is considered always true, so all of the remaining
# elif/else bodies should not be checked.
if result == MYPY_TRUE:
# This condition is false at runtime; this will affect
# import priorities.
mark_block_mypy_only(s.body[i])
for body in s.body[i + 1 :]:
mark_block_unreachable(body)
# Make sure else body always exists and is marked as
# unreachable so the type checker always knows that
# all control flow paths will flow through the if
# statement body.
if not s.else_body:
s.else_body = Block([])
mark_block_unreachable(s.else_body)
break
def infer_reachability_of_match_statement(s: MatchStmt, options: Options) -> None:
for i, guard in enumerate(s.guards):
pattern_value = infer_pattern_value(s.patterns[i])
if guard is not None:
guard_value = infer_condition_value(guard, options)
else:
guard_value = ALWAYS_TRUE
if pattern_value in (ALWAYS_FALSE, MYPY_FALSE) or guard_value in (
ALWAYS_FALSE,
MYPY_FALSE,
):
# The case is considered always false, so we skip the case body.
mark_block_unreachable(s.bodies[i])
elif pattern_value in (ALWAYS_FALSE, MYPY_TRUE) and guard_value in (
ALWAYS_TRUE,
MYPY_TRUE,
):
for body in s.bodies[i + 1 :]:
mark_block_unreachable(body)
if guard_value == MYPY_TRUE:
# This condition is false at runtime; this will affect
# import priorities.
mark_block_mypy_only(s.bodies[i])
def assert_will_always_fail(s: AssertStmt, options: Options) -> bool:
return infer_condition_value(s.expr, options) in (ALWAYS_FALSE, MYPY_FALSE)
def infer_condition_value(expr: Expression, options: Options) -> int:
"""Infer whether the given condition is always true/false.
Return ALWAYS_TRUE if always true, ALWAYS_FALSE if always false,
MYPY_TRUE if true under mypy and false at runtime, MYPY_FALSE if
false under mypy and true at runtime, else TRUTH_VALUE_UNKNOWN.
"""
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"):
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
else:
result = consider_sys_version_info(expr, pyversion)
if result == TRUTH_VALUE_UNKNOWN:
result = consider_sys_platform(expr, options.platform)
if result == TRUTH_VALUE_UNKNOWN:
if name == "PY2":
result = ALWAYS_FALSE
elif name == "PY3":
result = ALWAYS_TRUE
elif name == "MYPY" or name == "TYPE_CHECKING":
result = MYPY_TRUE
elif name in options.always_true:
result = ALWAYS_TRUE
elif name in options.always_false:
result = ALWAYS_FALSE
if negated:
result = inverted_truth_mapping[result]
return result
def infer_pattern_value(pattern: Pattern) -> int:
if isinstance(pattern, AsPattern) and pattern.pattern is None:
return ALWAYS_TRUE
elif isinstance(pattern, OrPattern) and any(
infer_pattern_value(p) == ALWAYS_TRUE for p in pattern.patterns
):
return ALWAYS_TRUE
else:
return TRUTH_VALUE_UNKNOWN
def consider_sys_version_info(expr: Expression, pyversion: tuple[int, ...]) -> int:
"""Consider whether expr is a comparison involving sys.version_info.
Return ALWAYS_TRUE, ALWAYS_FALSE, or TRUTH_VALUE_UNKNOWN.
"""
# Cases supported:
# - sys.version_info[<int>] <compare_op> <int>
# - sys.version_info[:<int>] <compare_op> <tuple_of_n_ints>
# - sys.version_info <compare_op> <tuple_of_1_or_2_ints>
# (in this case <compare_op> must be >, >=, <, <=, but cannot be ==, !=)
if not isinstance(expr, ComparisonExpr):
return TRUTH_VALUE_UNKNOWN
# Let's not yet support chained comparisons.
if len(expr.operators) > 1:
return TRUTH_VALUE_UNKNOWN
op = expr.operators[0]
if op not in ("==", "!=", "<=", ">=", "<", ">"):
return TRUTH_VALUE_UNKNOWN
index = contains_sys_version_info(expr.operands[0])
thing = contains_int_or_tuple_of_ints(expr.operands[1])
if index is None or thing is None:
index = contains_sys_version_info(expr.operands[1])
thing = contains_int_or_tuple_of_ints(expr.operands[0])
op = reverse_op[op]
if isinstance(index, int) and isinstance(thing, int):
# sys.version_info[i] <compare_op> k
if 0 <= index <= 1:
return fixed_comparison(pyversion[index], op, thing)
else:
return TRUTH_VALUE_UNKNOWN
elif isinstance(index, tuple) and isinstance(thing, tuple):
lo, hi = index
if lo is None:
lo = 0
if hi is None:
hi = 2
if 0 <= lo < hi <= 2:
val = pyversion[lo:hi]
if len(val) == len(thing) or len(val) > len(thing) and op not in ("==", "!="):
return fixed_comparison(val, op, thing)
return TRUTH_VALUE_UNKNOWN
def consider_sys_platform(expr: Expression, platform: str) -> int:
"""Consider whether expr is a comparison involving sys.platform.
Return ALWAYS_TRUE, ALWAYS_FALSE, or TRUTH_VALUE_UNKNOWN.
"""
# Cases supported:
# - sys.platform == 'posix'
# - sys.platform != 'win32'
# - sys.platform.startswith('win')
if isinstance(expr, ComparisonExpr):
# Let's not yet support chained comparisons.
if len(expr.operators) > 1:
return TRUTH_VALUE_UNKNOWN
op = expr.operators[0]
if op not in ("==", "!="):
return TRUTH_VALUE_UNKNOWN
if not is_sys_attr(expr.operands[0], "platform"):
return TRUTH_VALUE_UNKNOWN
right = expr.operands[1]
if not isinstance(right, StrExpr):
return TRUTH_VALUE_UNKNOWN
return fixed_comparison(platform, op, right.value)
elif isinstance(expr, CallExpr):
if not isinstance(expr.callee, MemberExpr):
return TRUTH_VALUE_UNKNOWN
if len(expr.args) != 1 or not isinstance(expr.args[0], StrExpr):
return TRUTH_VALUE_UNKNOWN
if not is_sys_attr(expr.callee.expr, "platform"):
return TRUTH_VALUE_UNKNOWN
if expr.callee.name != "startswith":
return TRUTH_VALUE_UNKNOWN
if platform.startswith(expr.args[0].value):
return ALWAYS_TRUE
else:
return ALWAYS_FALSE
else:
return TRUTH_VALUE_UNKNOWN
Targ = TypeVar("Targ", int, str, Tuple[int, ...])
def fixed_comparison(left: Targ, op: str, right: Targ) -> int:
rmap = {False: ALWAYS_FALSE, True: ALWAYS_TRUE}
if op == "==":
return rmap[left == right]
if op == "!=":
return rmap[left != right]
if op == "<=":
return rmap[left <= right]
if op == ">=":
return rmap[left >= right]
if op == "<":
return rmap[left < right]
if op == ">":
return rmap[left > right]
return TRUTH_VALUE_UNKNOWN
def contains_int_or_tuple_of_ints(expr: Expression) -> None | int | tuple[int, ...]:
if isinstance(expr, IntExpr):
return expr.value
if isinstance(expr, TupleExpr):
if literal(expr) == LITERAL_YES:
thing = []
for x in expr.items:
if not isinstance(x, IntExpr):
return None
thing.append(x.value)
return tuple(thing)
return None
def contains_sys_version_info(expr: Expression) -> None | int | tuple[int | None, int | None]:
if is_sys_attr(expr, "version_info"):
return (None, None) # Same as sys.version_info[:]
if isinstance(expr, IndexExpr) and is_sys_attr(expr.base, "version_info"):
index = expr.index
if isinstance(index, IntExpr):
return index.value
if isinstance(index, SliceExpr):
if index.stride is not None:
if not isinstance(index.stride, IntExpr) or index.stride.value != 1:
return None
begin = end = None
if index.begin_index is not None:
if not isinstance(index.begin_index, IntExpr):
return None
begin = index.begin_index.value
if index.end_index is not None:
if not isinstance(index.end_index, IntExpr):
return None
end = index.end_index.value
return (begin, end)
return None
def is_sys_attr(expr: Expression, name: str) -> bool:
# TODO: This currently doesn't work with code like this:
# - import sys as _sys
# - from sys import version_info
if isinstance(expr, MemberExpr) and expr.name == name:
if isinstance(expr.expr, NameExpr) and expr.expr.name == "sys":
# TODO: Guard against a local named sys, etc.
# (Though later passes will still do most checking.)
return True
return False
def mark_block_unreachable(block: Block) -> None:
block.is_unreachable = True
block.accept(MarkImportsUnreachableVisitor())
class MarkImportsUnreachableVisitor(TraverserVisitor):
"""Visitor that flags all imports nested within a node as unreachable."""
def visit_import(self, node: Import) -> None:
node.is_unreachable = True
def visit_import_from(self, node: ImportFrom) -> None:
node.is_unreachable = True
def visit_import_all(self, node: ImportAll) -> None:
node.is_unreachable = True
def mark_block_mypy_only(block: Block) -> None:
block.accept(MarkImportsMypyOnlyVisitor())
class MarkImportsMypyOnlyVisitor(TraverserVisitor):
"""Visitor that sets is_mypy_only (which affects priority)."""
def visit_import(self, node: Import) -> None:
node.is_mypy_only = True
def visit_import_from(self, node: ImportFrom) -> None:
node.is_mypy_only = True
def visit_import_all(self, node: ImportAll) -> None:
node.is_mypy_only = True
def visit_func_def(self, node: FuncDef) -> None:
node.is_mypy_only = True