23 KiB
Terminal statements
Introduction
Terminal statements complicate a naive control-flow analysis.
As a simple example:
def f(cond: bool) -> str:
if cond:
x = "test"
else:
raise ValueError
return x
def g(cond: bool):
if cond:
x = "test"
reveal_type(x) # revealed: Literal["test"]
else:
x = "terminal"
reveal_type(x) # revealed: Literal["terminal"]
raise ValueError
reveal_type(x) # revealed: Literal["test"]
In f, we should be able to determine that the else branch ends in a terminal statement, and that
the return statement can only be executed when the condition is true. We should therefore consider
the reference always bound, even though x is only bound in the true branch.
Similarly, in g, we should see that the assignment of the value "terminal" can never be seen by
the final reveal_type.
return
A return statement is terminal; bindings that occur before it are not visible after it.
def resolved_reference(cond: bool) -> str:
if cond:
x = "test"
else:
return "early"
return x # no possibly-unresolved-reference diagnostic!
def return_in_then_branch(cond: bool):
if cond:
x = "terminal"
reveal_type(x) # revealed: Literal["terminal"]
return
else:
x = "test"
reveal_type(x) # revealed: Literal["test"]
reveal_type(x) # revealed: Literal["test"]
def return_in_else_branch(cond: bool):
if cond:
x = "test"
reveal_type(x) # revealed: Literal["test"]
else:
x = "terminal"
reveal_type(x) # revealed: Literal["terminal"]
return
reveal_type(x) # revealed: Literal["test"]
def return_in_both_branches(cond: bool):
if cond:
x = "terminal1"
reveal_type(x) # revealed: Literal["terminal1"]
return
else:
x = "terminal2"
reveal_type(x) # revealed: Literal["terminal2"]
return
def return_in_try(cond: bool):
x = "before"
try:
if cond:
x = "test"
return
except:
# TODO: Literal["before"]
reveal_type(x) # revealed: Literal["before", "test"]
else:
reveal_type(x) # revealed: Literal["before"]
finally:
reveal_type(x) # revealed: Literal["before", "test"]
reveal_type(x) # revealed: Literal["before", "test"]
def return_in_nested_then_branch(cond1: bool, cond2: bool):
if cond1:
x = "test1"
reveal_type(x) # revealed: Literal["test1"]
else:
if cond2:
x = "terminal"
reveal_type(x) # revealed: Literal["terminal"]
return
else:
x = "test2"
reveal_type(x) # revealed: Literal["test2"]
reveal_type(x) # revealed: Literal["test2"]
reveal_type(x) # revealed: Literal["test1", "test2"]
def return_in_nested_else_branch(cond1: bool, cond2: bool):
if cond1:
x = "test1"
reveal_type(x) # revealed: Literal["test1"]
else:
if cond2:
x = "test2"
reveal_type(x) # revealed: Literal["test2"]
else:
x = "terminal"
reveal_type(x) # revealed: Literal["terminal"]
return
reveal_type(x) # revealed: Literal["test2"]
reveal_type(x) # revealed: Literal["test1", "test2"]
def return_in_both_nested_branches(cond1: bool, cond2: bool):
if cond1:
x = "test"
reveal_type(x) # revealed: Literal["test"]
else:
x = "terminal0"
if cond2:
x = "terminal1"
reveal_type(x) # revealed: Literal["terminal1"]
return
else:
x = "terminal2"
reveal_type(x) # revealed: Literal["terminal2"]
return
reveal_type(x) # revealed: Literal["test"]
continue
A continue statement jumps back to the top of the innermost loop. This makes it terminal within
the loop body: definitions before it are not visible after it within the rest of the loop body. They
are likely visible after the loop body, since loops do not introduce new scopes. (Statically known
infinite loops are one exception — if control never leaves the loop body, bindings inside of the
loop are not visible outside of it.)
TODO: We are not currently modeling the cyclic control flow for loops, pending fixpoint support in Salsa. The false positives in this section are because of that, and not our terminal statement support. See ruff#14160 for more details.
def resolved_reference(cond: bool) -> str:
while True:
if cond:
x = "test"
else:
continue
return x
def continue_in_then_branch(cond: bool, i: int):
x = "before"
for _ in range(i):
if cond:
x = "continue"
reveal_type(x) # revealed: Literal["continue"]
continue
else:
x = "loop"
reveal_type(x) # revealed: Literal["loop"]
reveal_type(x) # revealed: Literal["loop"]
# TODO: Should be Literal["before", "loop", "continue"]
reveal_type(x) # revealed: Literal["before", "loop"]
def continue_in_else_branch(cond: bool, i: int):
x = "before"
for _ in range(i):
if cond:
x = "loop"
reveal_type(x) # revealed: Literal["loop"]
else:
x = "continue"
reveal_type(x) # revealed: Literal["continue"]
continue
reveal_type(x) # revealed: Literal["loop"]
# TODO: Should be Literal["before", "loop", "continue"]
reveal_type(x) # revealed: Literal["before", "loop"]
def continue_in_both_branches(cond: bool, i: int):
x = "before"
for _ in range(i):
if cond:
x = "continue1"
reveal_type(x) # revealed: Literal["continue1"]
continue
else:
x = "continue2"
reveal_type(x) # revealed: Literal["continue2"]
continue
# TODO: Should be Literal["before", "continue1", "continue2"]
reveal_type(x) # revealed: Literal["before"]
def continue_in_nested_then_branch(cond1: bool, cond2: bool, i: int):
x = "before"
for _ in range(i):
if cond1:
x = "loop1"
reveal_type(x) # revealed: Literal["loop1"]
else:
if cond2:
x = "continue"
reveal_type(x) # revealed: Literal["continue"]
continue
else:
x = "loop2"
reveal_type(x) # revealed: Literal["loop2"]
reveal_type(x) # revealed: Literal["loop2"]
reveal_type(x) # revealed: Literal["loop1", "loop2"]
# TODO: Should be Literal["before", "loop1", "loop2", "continue"]
reveal_type(x) # revealed: Literal["before", "loop1", "loop2"]
def continue_in_nested_else_branch(cond1: bool, cond2: bool, i: int):
x = "before"
for _ in range(i):
if cond1:
x = "loop1"
reveal_type(x) # revealed: Literal["loop1"]
else:
if cond2:
x = "loop2"
reveal_type(x) # revealed: Literal["loop2"]
else:
x = "continue"
reveal_type(x) # revealed: Literal["continue"]
continue
reveal_type(x) # revealed: Literal["loop2"]
reveal_type(x) # revealed: Literal["loop1", "loop2"]
# TODO: Should be Literal["before", "loop1", "loop2", "continue"]
reveal_type(x) # revealed: Literal["before", "loop1", "loop2"]
def continue_in_both_nested_branches(cond1: bool, cond2: bool, i: int):
x = "before"
for _ in range(i):
if cond1:
x = "loop"
reveal_type(x) # revealed: Literal["loop"]
else:
if cond2:
x = "continue1"
reveal_type(x) # revealed: Literal["continue1"]
continue
else:
x = "continue2"
reveal_type(x) # revealed: Literal["continue2"]
continue
reveal_type(x) # revealed: Literal["loop"]
# TODO: Should be Literal["before", "loop", "continue1", "continue2"]
reveal_type(x) # revealed: Literal["before", "loop"]
break
A break statement jumps to the end of the innermost loop. This makes it terminal within the loop
body: definitions before it are not visible after it within the rest of the loop body. They are
likely visible after the loop body, since loops do not introduce new scopes. (Statically known
infinite loops are one exception — if control never leaves the loop body, bindings inside of the
loop are not visible outside of it.)
def resolved_reference(cond: bool) -> str:
while True:
if cond:
x = "test"
else:
break
return x
return x # error: [unresolved-reference]
def break_in_then_branch(cond: bool, i: int):
x = "before"
for _ in range(i):
if cond:
x = "break"
reveal_type(x) # revealed: Literal["break"]
break
else:
x = "loop"
reveal_type(x) # revealed: Literal["loop"]
reveal_type(x) # revealed: Literal["loop"]
reveal_type(x) # revealed: Literal["before", "break", "loop"]
def break_in_else_branch(cond: bool, i: int):
x = "before"
for _ in range(i):
if cond:
x = "loop"
reveal_type(x) # revealed: Literal["loop"]
else:
x = "break"
reveal_type(x) # revealed: Literal["break"]
break
reveal_type(x) # revealed: Literal["loop"]
reveal_type(x) # revealed: Literal["before", "loop", "break"]
def break_in_both_branches(cond: bool, i: int):
x = "before"
for _ in range(i):
if cond:
x = "break1"
reveal_type(x) # revealed: Literal["break1"]
break
else:
x = "break2"
reveal_type(x) # revealed: Literal["break2"]
break
reveal_type(x) # revealed: Literal["before", "break1", "break2"]
def break_in_nested_then_branch(cond1: bool, cond2: bool, i: int):
x = "before"
for _ in range(i):
if cond1:
x = "loop1"
reveal_type(x) # revealed: Literal["loop1"]
else:
if cond2:
x = "break"
reveal_type(x) # revealed: Literal["break"]
break
else:
x = "loop2"
reveal_type(x) # revealed: Literal["loop2"]
reveal_type(x) # revealed: Literal["loop2"]
reveal_type(x) # revealed: Literal["loop1", "loop2"]
reveal_type(x) # revealed: Literal["before", "loop1", "break", "loop2"]
def break_in_nested_else_branch(cond1: bool, cond2: bool, i: int):
x = "before"
for _ in range(i):
if cond1:
x = "loop1"
reveal_type(x) # revealed: Literal["loop1"]
else:
if cond2:
x = "loop2"
reveal_type(x) # revealed: Literal["loop2"]
else:
x = "break"
reveal_type(x) # revealed: Literal["break"]
break
reveal_type(x) # revealed: Literal["loop2"]
reveal_type(x) # revealed: Literal["loop1", "loop2"]
reveal_type(x) # revealed: Literal["before", "loop1", "loop2", "break"]
def break_in_both_nested_branches(cond1: bool, cond2: bool, i: int):
x = "before"
for _ in range(i):
if cond1:
x = "loop"
reveal_type(x) # revealed: Literal["loop"]
else:
if cond2:
x = "break1"
reveal_type(x) # revealed: Literal["break1"]
break
else:
x = "break2"
reveal_type(x) # revealed: Literal["break2"]
break
reveal_type(x) # revealed: Literal["loop"]
reveal_type(x) # revealed: Literal["before", "loop", "break1", "break2"]
raise
A raise statement is terminal. If it occurs in a lexically containing try statement, it will
jump to one of the except clauses (if it matches the value being raised), or to the else clause
(if none match). Currently, we assume definitions from before the raise are visible in all
except and else clauses. (In the future, we might analyze the except clauses to see which ones
match the value being raised, and limit visibility to those clauses.) Definitions from before the
raise are not visible in any else clause, but are visible in except clauses or after the
containing try statement (since control flow may have passed through an except).
Currently we assume that an exception could be raised anywhere within a try block. We may want to
implement a more precise understanding of where exceptions (barring KeyboardInterrupt and
MemoryError) can and cannot actually be raised.
def raise_in_then_branch(cond: bool):
x = "before"
try:
if cond:
x = "raise"
reveal_type(x) # revealed: Literal["raise"]
raise ValueError
else:
x = "else"
reveal_type(x) # revealed: Literal["else"]
reveal_type(x) # revealed: Literal["else"]
except ValueError:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "raise", "else"]
except:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "raise", "else"]
else:
reveal_type(x) # revealed: Literal["else"]
finally:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "raise", "else"]
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "raise", "else"]
def raise_in_else_branch(cond: bool):
x = "before"
try:
if cond:
x = "else"
reveal_type(x) # revealed: Literal["else"]
else:
x = "raise"
reveal_type(x) # revealed: Literal["raise"]
raise ValueError
reveal_type(x) # revealed: Literal["else"]
except ValueError:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else", "raise"]
except:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else", "raise"]
else:
reveal_type(x) # revealed: Literal["else"]
finally:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else", "raise"]
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else", "raise"]
def raise_in_both_branches(cond: bool):
x = "before"
try:
if cond:
x = "raise1"
reveal_type(x) # revealed: Literal["raise1"]
raise ValueError
else:
x = "raise2"
reveal_type(x) # revealed: Literal["raise2"]
raise ValueError
except ValueError:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "raise1", "raise2"]
except:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "raise1", "raise2"]
else:
# This branch is unreachable, since all control flows in the `try` clause raise exceptions.
# As a result, this binding should never be reachable, since new bindings are visible only
# when they are reachable.
x = "unreachable"
finally:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "raise1", "raise2"]
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "raise1", "raise2"]
def raise_in_nested_then_branch(cond1: bool, cond2: bool):
x = "before"
try:
if cond1:
x = "else1"
reveal_type(x) # revealed: Literal["else1"]
else:
if cond2:
x = "raise"
reveal_type(x) # revealed: Literal["raise"]
raise ValueError
else:
x = "else2"
reveal_type(x) # revealed: Literal["else2"]
reveal_type(x) # revealed: Literal["else2"]
reveal_type(x) # revealed: Literal["else1", "else2"]
except ValueError:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else1", "raise", "else2"]
except:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else1", "raise", "else2"]
else:
reveal_type(x) # revealed: Literal["else1", "else2"]
finally:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else1", "raise", "else2"]
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else1", "raise", "else2"]
def raise_in_nested_else_branch(cond1: bool, cond2: bool):
x = "before"
try:
if cond1:
x = "else1"
reveal_type(x) # revealed: Literal["else1"]
else:
if cond2:
x = "else2"
reveal_type(x) # revealed: Literal["else2"]
else:
x = "raise"
reveal_type(x) # revealed: Literal["raise"]
raise ValueError
reveal_type(x) # revealed: Literal["else2"]
reveal_type(x) # revealed: Literal["else1", "else2"]
except ValueError:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else1", "else2", "raise"]
except:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else1", "else2", "raise"]
else:
reveal_type(x) # revealed: Literal["else1", "else2"]
finally:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else1", "else2", "raise"]
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else1", "else2", "raise"]
def raise_in_both_nested_branches(cond1: bool, cond2: bool):
x = "before"
try:
if cond1:
x = "else"
reveal_type(x) # revealed: Literal["else"]
else:
if cond2:
x = "raise1"
reveal_type(x) # revealed: Literal["raise1"]
raise ValueError
else:
x = "raise2"
reveal_type(x) # revealed: Literal["raise2"]
raise ValueError
reveal_type(x) # revealed: Literal["else"]
except ValueError:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else", "raise1", "raise2"]
except:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else", "raise1", "raise2"]
else:
reveal_type(x) # revealed: Literal["else"]
finally:
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else", "raise1", "raise2"]
# Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
reveal_type(x) # revealed: Literal["before", "else", "raise1", "raise2"]
Terminal in try with finally clause
TODO: we don't yet model that a break or continue in a try block will jump to a finally
clause before it jumps to end/start of the loop.
def f():
x = 1
while True:
try:
break
finally:
x = 2
# TODO: should be Literal[2]
reveal_type(x) # revealed: Literal[1]
Nested functions
Free references inside of a function body refer to variables defined in the containing scope.
Function bodies are lazy scopes: at runtime, these references are not resolved immediately at the
point of the function definition. Instead, they are resolved at the time of the call, which means
that their values (and types) can be different for different invocations. For simplicity, we instead
resolve free references at the end of the containing scope. That means that in the examples below,
all of the x bindings should be visible to the reveal_type, regardless of where we place the
return statements.
TODO: These currently produce the wrong results, but not because of our terminal statement support. See ruff#15777 for more details.
def top_level_return(cond1: bool, cond2: bool):
x = 1
def g():
# TODO eliminate Unknown
reveal_type(x) # revealed: Unknown | Literal[1, 2, 3]
if cond1:
if cond2:
x = 2
else:
x = 3
return
def return_from_if(cond1: bool, cond2: bool):
x = 1
def g():
# TODO: Literal[1, 2, 3]
reveal_type(x) # revealed: Unknown | Literal[1]
if cond1:
if cond2:
x = 2
else:
x = 3
return
def return_from_nested_if(cond1: bool, cond2: bool):
x = 1
def g():
# TODO: Literal[1, 2, 3]
reveal_type(x) # revealed: Unknown | Literal[1, 3]
if cond1:
if cond2:
x = 2
return
else:
x = 3
Statically known terminal statements
We model reachability using the same visibility constraints that we use to model statically known
bounds. In this example, we see that the return statement is always executed, and therefore that
the "b" assignment is not visible to the reveal_type.
def _(cond: bool):
x = "a"
if cond:
x = "b"
if True:
return
reveal_type(x) # revealed: Literal["a"]
Bindings after a terminal statement are unreachable
Any bindings introduced after a terminal statement are unreachable, and are currently considered not visible. We anticipate that we want to provide a more useful analysis for code after terminal statements.
def f(cond: bool) -> str:
x = "before"
if cond:
reveal_type(x) # revealed: Literal["before"]
return "a"
x = "after-return"
reveal_type(x) # revealed: Never
else:
x = "else"
return reveal_type(x) # revealed: Literal["else"]