add assert_type() (#434)

Author: JelleZijlstra

docs/changelog.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Add support for `assert_type()` (#433)
 - `reveal_type()` and `dump_value()` now return their argument,
   the anticipated behavior for `typing.reveal_type()` in Python
   3.11 (#432)

pyanalyze/extensions.py

@@ -392,6 +392,28 @@ def f(x: int) -> None:
     return value
 
 
+def assert_type(val: _T, typ: Any) -> _T:
+    """Assert the inferred static type of an expression.
+
+    When a static type checker encounters a call to this function,
+    it checks that the inferred type of `val` matches the `typ`
+    argument, and if it dooes not, it emits an error.
+
+    Example::
+
+        def f(x: int) -> None:
+            assert_type(x, int)  # ok
+            assert_type(x, str)  # error
+
+    This is useful for checking that the type checker interprets
+    a complicated set of type annotations in the way the user intended.
+
+    At runtime this returns the first argument unchanged.
+
+    """
+    return val
+
+
 _overloads: Dict[str, List[Callable[..., Any]]] = defaultdict(list)
 _type_evaluations: Dict[str, List[Callable[..., Any]]] = defaultdict(list)
 

pyanalyze/implementation.py

@@ -1,8 +1,6 @@
-import typing
-import typing_extensions
 from .annotations import type_from_value
 from .error_code import ErrorCode
-from .extensions import reveal_type
+from .extensions import assert_type, reveal_type
 from .format_strings import parse_format_string
 from .predicates import IsAssignablePredicate
 from .safe import safe_hasattr, safe_isinstance, safe_issubclass
@@ -52,6 +50,7 @@
     concrete_values_from_iterable,
     kv_pairs_from_mapping,
     make_weak,
+    unannotate,
     unite_values,
     flatten_values,
     replace_known_sequence_value,
@@ -66,6 +65,8 @@
 import inspect
 import warnings
 from types import FunctionType
+import typing
+import typing_extensions
 from typing import (
     Sequence,
     TypeVar,
@@ -1042,6 +1043,20 @@ def _cast_impl(ctx: CallContext) -> Value:
     return type_from_value(typ, visitor=ctx.visitor, node=ctx.node)
 
 
+def _assert_type_impl(ctx: CallContext) -> Value:
+    # TODO maybe we should walk over the whole value and remove Annotated.
+    val = unannotate(ctx.vars["val"])
+    typ = ctx.vars["typ"]
+    expected_type = type_from_value(typ, visitor=ctx.visitor, node=ctx.node)
+    if val != expected_type:
+        ctx.show_error(
+            f"Type is {val} (expected {expected_type})",
+            error_code=ErrorCode.inference_failure,
+            arg="obj",
+        )
+    return val
+
+
 def _subclasses_impl(ctx: CallContext) -> Value:
     """Overridden because typeshed types make it (T) => List[T] instead."""
     self_obj = ctx.vars["self"]
@@ -1423,7 +1438,18 @@ def get_default_argspecs() -> Dict[object, Signature]:
             callable=str.format,
         ),
         Signature.make(
-            [SigParameter("typ"), SigParameter("val")], callable=cast, impl=_cast_impl
+            [SigParameter("typ", _POS_ONLY), SigParameter("val", _POS_ONLY)],
+            callable=cast,
+            impl=_cast_impl,
+        ),
+        Signature.make(
+            [
+                SigParameter("val", _POS_ONLY, annotation=TypeVarValue(T)),
+                SigParameter("typ", _POS_ONLY),
+            ],
+            TypeVarValue(T),
+            callable=assert_type,
+            impl=_assert_type_impl,
         ),
         # workaround for https://github.com/python/typeshed/pull/3501
         Signature.make(
@@ -1566,4 +1592,20 @@ def get_default_argspecs() -> Dict[object, Signature]:
                 callable=reveal_type_func,
             )
             signatures.append(sig)
+        # Anticipating that this will be added to the stdlib
+        try:
+            assert_type_func = getattr(mod, "assert_type")
+        except AttributeError:
+            pass
+        else:
+            sig = Signature.make(
+                [
+                    SigParameter("val", _POS_ONLY, annotation=TypeVarValue(T)),
+                    SigParameter("typ", _POS_ONLY),
+                ],
+                TypeVarValue(T),
+                callable=assert_type_func,
+                impl=_assert_type_impl,
+            )
+            signatures.append(sig)
     return {sig.callable: sig for sig in signatures}

pyanalyze/test_implementation.py

@@ -1184,6 +1184,17 @@ def capybara():
             assert_is_value(x, KnownValue(1))
             assert_is_value(y, KnownValue(1))
 
+    @assert_passes()
+    def test_assert_type(self) -> None:
+        from pyanalyze.extensions import assert_type
+        from typing import Any
+
+        def capybara(x: int) -> None:
+            assert_type(x, int)
+            assert_type(x, "int")
+            assert_type(x, Any)  # E: inference_failure
+            assert_type(x, str)  # E: inference_failure
+
 
 class TestCallableGuards(TestNameCheckVisitorBase):
     @assert_passes()