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 is True:
            x = "test"
            return
    except:
        reveal_type(x)  # revealed: Never
    else:
        reveal_type(x)  # revealed: Literal["before"]
    finally:
        # TODO: should include `Literal["test"]` when the return passes through `finally`
        # https://github.com/astral-sh/ty/issues/233
        reveal_type(x)  # revealed: Literal["before"]
    reveal_type(x)  # revealed: Literal["before"]

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.)

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"]
    reveal_type(x)  # revealed: Literal["before", "continue", "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"]
    reveal_type(x)  # revealed: Literal["before", "loop", "continue"]

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
    reveal_type(x)  # revealed: Literal["before", "continue1", "continue2"]

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"]
    reveal_type(x)  # revealed: Literal["before", "loop1", "continue", "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"]
    reveal_type(x)  # revealed: Literal["before", "loop1", "loop2", "continue"]

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"]
    reveal_type(x)  # revealed: Literal["before", "loop", "continue1", "continue2"]

`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. Inside a try statement, it jumps to a matching except clause or propagates out of the statement. We do not yet determine which typed handler matches the exception, so every handler sees the same possible values.

When only one branch raises, the exception handler sees only the value assigned in that branch:

def raise_in_then_branch(cond: bool):
    x = "before"
    try:
        if cond is True:
            x = "raise"
            raise ValueError
        x = "else"
    except ValueError:
        reveal_type(x)  # revealed: Literal["raise"]
    else:
        reveal_type(x)  # revealed: Literal["else"]
    reveal_type(x)  # revealed: Literal["raise", "else"]

If both branches raise, the handler sees either value and the else clause cannot run:

def raise_in_both_branches(cond: bool):
    x = "before"
    try:
        if cond is True:
            x = "raise1"
            raise ValueError
        else:
            x = "raise2"
            raise ValueError
    except ValueError:
        reveal_type(x)  # revealed: Literal["raise1", "raise2"]
    else:
        x = "unreachable"
    reveal_type(x)  # revealed: Literal["raise1", "raise2"]

Nested conditions do not make values from non-raising branches visible to the exception handler:

def raise_in_nested_branch(cond1: bool, cond2: bool):
    x = "before"
    try:
        if cond1 is True:
            x = "else1"
        elif cond2 is True:
            x = "raise"
            raise ValueError
        else:
            x = "else2"
    except ValueError:
        reveal_type(x)  # revealed: Literal["raise"]
    else:
        reveal_type(x)  # revealed: Literal["else1", "else2"]
    reveal_type(x)  # revealed: Literal["else1", "raise", "else2"]

Multiple raising branches inside a nested condition remain visible to the handler:

def raise_in_both_nested_branches(cond1: bool, cond2: bool):
    x = "before"
    try:
        if cond1 is True:
            x = "else"
        elif cond2 is True:
            x = "raise1"
            raise ValueError
        else:
            x = "raise2"
            raise ValueError
    except ValueError:
        reveal_type(x)  # revealed: Literal["raise1", "raise2"]
    else:
        reveal_type(x)  # revealed: Literal["else"]
    reveal_type(x)  # revealed: Literal["else", "raise1", "raise2"]

Terminal in `try` with `finally` clause

We model terminal control flow in a try, except, or else block as jumping to a finally clause before it terminates the current scope or jumps to its final destination when there are no normal paths into the finally block.

TODO: we don't yet consider both normal and terminal entry states when checking a finally block that has a mix of normal and terminal entry paths. See ty#233.

def finally_runs_after_return():
    x = "before"
    try:
        x = "return"
        return
    finally:
        reveal_type(x)  # revealed: Literal["return"]

def finally_runs_after_try_and_except_are_terminal(cond: bool):
    x = "before"
    try:
        if cond:
            x = "try-return"
            return
        else:
            x = "try-raise"
            raise ValueError
    except ValueError:
        x = "except-return"
        return
    finally:
        reveal_type(x)  # revealed: Literal["try-return", "try-raise", "except-return"]

def finally_runs_after_except_and_else_are_terminal():
    x = "before"
    try:
        x = "try-normal"
    except ValueError:
        x = "except-return"
        return
    else:
        x = "else-return"
        return
    finally:
        reveal_type(x)  # revealed: Literal["else-return"]

def finally_runs_after_mixed_except_paths(cond: bool):
    x = "before"
    try:
        raise ValueError
    except ValueError:
        if cond:
            x = "except-return"
            return
        x = "except-normal"
    finally:
        # TODO: should also include `Literal["except-return"]`
        reveal_type(x)  # revealed: Literal["except-normal"]

def finally_runs_after_mixed_try_paths(cond: bool):
    x = "before"
    try:
        if cond:
            x = "try-return"
            return
        x = "try-normal"
    finally:
        # TODO: should also include `Literal["try-return"]`
        reveal_type(x)  # revealed: Literal["try-normal"]

def finally_runs_after_mixed_break_paths(cond: bool):
    x = "before"
    while True:
        try:
            if cond:
                x = "break"
                break
            x = "normal"
        finally:
            # TODO: should also include `Literal["break"]`
            reveal_type(x)  # revealed: Literal["normal"]
        break

def finally_runs_before_break():
    x = "before"
    while True:
        try:
            x = "break"
            break
        finally:
            reveal_type(x)  # revealed: Literal["break"]

def finally_runs_before_continue(cond: bool):
    while cond:
        x = "before"
        try:
            x = "continue"
            continue
        finally:
            reveal_type(x)  # revealed: Literal["continue"]

def nested_finally_runs_after_return():
    x = "before"
    try:
        try:
            x = "return"
            return
        finally:
            reveal_type(x)  # revealed: Literal["return"]
    finally:
        reveal_type(x)  # revealed: Literal["return"]

def nested_outer_finally_sees_inner_finally_assignments():
    x = "before"
    try:
        try:
            x = "return"
            return
        finally:
            x = "inner-finally"
    finally:
        reveal_type(x)  # revealed: Literal["inner-finally"]

def finally_assignment_runs_before_break():
    x = 1
    while True:
        try:
            break
        finally:
            x = 2
    # TODO: should be Literal[2]
    reveal_type(x)  # revealed: Literal[1]

Calls to functions returning `Never` / `NoReturn`

These calls should be treated as terminal statements.

No implicit return

If we see a call to a function returning Never, we should be able to understand that the function cannot implicitly return None. In the below examples, verify that there are no errors emitted for invalid return type.

from typing import NoReturn
import sys

def f() -> NoReturn:
    sys.exit(1)

Let's try cases where the function annotated with NoReturn is some sub-expression.

from typing import NoReturn
import sys

# TODO: this is currently not yet supported
# error: [invalid-return-type]
def _() -> NoReturn:
    3 + sys.exit(1)

# TODO: this is currently not yet supported
# error: [invalid-return-type]
def _() -> NoReturn:
    3 if sys.exit(1) else 4

Type narrowing

If a variable's type is a union, and some types in the union result in a function marked with NoReturn being called, then we should correctly narrow the variable's type.

from typing import NoReturn
import sys

def g(x: int | None):
    if x is None:
        sys.exit(1)

    reveal_type(x)  # revealed: int

Possibly unresolved diagnostics

If the codepath on which a variable is not defined eventually returns Never, use of the variable should not give any diagnostics.

import sys

def _(flag: bool):
    if flag:
        x = 3
    else:
        sys.exit()

    x  # No possibly-unresolved-references diagnostic here.

Similarly, there shouldn't be any diagnostics if the except block of a try/except construct has a call with NoReturn.

import sys

def _():
    try:
        x = 3
    except:
        sys.exit()

    x  # No possibly-unresolved-references diagnostic here.

Bindings in branches

In case of a NoReturn call being present in conditionals, the revealed type of the end of the branch should reflect the path which did not hit any of the NoReturn calls. These tests are similar to the ones for return above.

import sys

def call_in_then_branch(cond: bool):
    if cond:
        x = "terminal"
        reveal_type(x)  # revealed: Literal["terminal"]
        sys.exit()
    else:
        x = "test"
        reveal_type(x)  # revealed: Literal["test"]
    reveal_type(x)  # revealed: Literal["test"]

def call_in_else_branch(cond: bool):
    if cond:
        x = "test"
        reveal_type(x)  # revealed: Literal["test"]
    else:
        x = "terminal"
        reveal_type(x)  # revealed: Literal["terminal"]
        sys.exit()
    reveal_type(x)  # revealed: Literal["test"]

def call_in_both_branches(cond: bool):
    if cond:
        x = "terminal1"
        reveal_type(x)  # revealed: Literal["terminal1"]
        sys.exit()
    else:
        x = "terminal2"
        reveal_type(x)  # revealed: Literal["terminal2"]
        sys.exit()

    reveal_type(x)  # revealed: Never

def call_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"]
            sys.exit()
        else:
            x = "test2"
            reveal_type(x)  # revealed: Literal["test2"]
        reveal_type(x)  # revealed: Literal["test2"]
    reveal_type(x)  # revealed: Literal["test1", "test2"]

def call_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"]
            sys.exit()
        reveal_type(x)  # revealed: Literal["test2"]
    reveal_type(x)  # revealed: Literal["test1", "test2"]

def call_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"]
            sys.exit()
        else:
            x = "terminal2"
            reveal_type(x)  # revealed: Literal["terminal2"]
            sys.exit()
    reveal_type(x)  # revealed: Literal["test"]

Overloads

If only some overloads of a function are marked with NoReturn, we should run the overload evaluation algorithm when evaluating the constraints.

from typing import NoReturn, overload

@overload
def f(x: int) -> NoReturn: ...
@overload
def f(x: str) -> int: ...
def f(x): ...

# No errors
def _() -> NoReturn:
    f(3)

# This should be an error because of implicitly returning `None`
# error: [invalid-return-type]
def _() -> NoReturn:
    f("")

Generic functions

If a generic function's return type depends on a type variable, and the argument passed resolves that type variable to Never, the call should still be treated as terminal.

from typing import TypeVar, NoReturn

T = TypeVar("T")

def identity(x: T) -> T:
    return x

# No "implicitly returns `None`" diagnostic
def _() -> NoReturn:
    identity(exit())

def _(flag: bool):
    if flag:
        x = "test"
    else:
        x = "terminal"
        identity(exit())

    reveal_type(x)  # revealed: Literal["test"]

Other callables

If other types of callables are annotated with NoReturn, we should still be able to infer correct reachability.

import sys

from typing import NoReturn

class C:
    def __call__(self) -> NoReturn:
        sys.exit()

    def die(self) -> NoReturn:
        sys.exit()

# No "implicitly returns `None`" diagnostic
def _() -> NoReturn:
    C()()

# No "implicitly returns `None`" diagnostic
def _() -> NoReturn:
    C().die()

Awaiting async `NoReturn` functions

Awaiting an async function annotated as returning NoReturn should be treated as terminal, just like calling a synchronous NoReturn function.

from typing import NoReturn

async def stop() -> NoReturn:
    raise NotImplementedError

async def main(flag: bool):
    if flag:
        x = "terminal"
        await stop()
    else:
        x = "test"
        pass

    reveal_type(x)  # revealed: Literal["test"]

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 currently consider all reachable bindings in the containing scope:

def top_level_return(cond1: bool, cond2: bool):
    x = 1

    def g():
        reveal_type(x)  # revealed: 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():
        reveal_type(x)  # revealed: Literal[1, 2, 3]
    if cond1:
        if cond2:
            x = 2
        else:
            x = 3
        return

def return_from_nested_if(cond1: bool, cond2: bool):
    x = 1

    def g():
        reveal_type(x)  # revealed: Literal[1, 2, 3]
    if cond1:
        if cond2:
            x = 2
            return
        else:
            x = 3

Statically known terminal statements

We model reachability using the same 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"]

Terminal statements

Terminal statements

Introduction

return

continue

break

raise

Terminal in try with finally clause

Calls to functions returning Never / NoReturn

No implicit return

Type narrowing

Possibly unresolved diagnostics

Bindings in branches

Overloads

Generic functions

Other callables

Awaiting async NoReturn functions

Nested functions

Statically known terminal statements

Bindings after a terminal statement are unreachable

`return`

`continue`

`break`

`raise`

Terminal in `try` with `finally` clause

Calls to functions returning `Never` / `NoReturn`

Awaiting async `NoReturn` functions