Add get_expression_type to CheckerPluginInterface (#15369)

Fixes #14845.

p.s. In the issue above, I was concerned that adding this method would
create an avenue for infinite recursions (if called carelessly), but in
fact I haven't managed to induce it, e.g. FunctionSigContext has `args`
but not the call expression itself.

mypy/checker.py

@@ -6793,6 +6793,9 @@ def has_valid_attribute(self, typ: Type, name: str) -> bool:
             )
         return not watcher.has_new_errors()
 
+    def get_expression_type(self, node: Expression, type_context: Type | None = None) -> Type:
+        return self.expr_checker.accept(node, type_context=type_context)
+
 
 class CollectArgTypeVarTypes(TypeTraverserVisitor):
     """Collects the non-nested argument types in a set."""

mypy/plugin.py

@@ -250,6 +250,11 @@ def named_generic_type(self, name: str, args: list[Type]) -> Instance:
         """Construct an instance of a builtin type with given type arguments."""
         raise NotImplementedError
 
+    @abstractmethod
+    def get_expression_type(self, node: Expression, type_context: Type | None = None) -> Type:
+        """Checks the type of the given expression."""
+        raise NotImplementedError
+
 
 @trait
 class SemanticAnalyzerPluginInterface:

mypy/plugins/attrs.py

@@ -9,7 +9,6 @@
 
 import mypy.plugin  # To avoid circular imports.
 from mypy.applytype import apply_generic_arguments
-from mypy.checker import TypeChecker
 from mypy.errorcodes import LITERAL_REQ
 from mypy.expandtype import expand_type, expand_type_by_instance
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
@@ -1048,13 +1047,7 @@ def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
         return ctx.default_signature  # leave it to the type checker to complain
 
     inst_arg = ctx.args[0][0]
-
-    # <hack>
-    assert isinstance(ctx.api, TypeChecker)
-    inst_type = ctx.api.expr_checker.accept(inst_arg)
-    # </hack>
-
-    inst_type = get_proper_type(inst_type)
+    inst_type = get_proper_type(ctx.api.get_expression_type(inst_arg))
     inst_type_str = format_type_bare(inst_type, ctx.api.options)
 
     attr_types = _get_expanded_attr_types(ctx, inst_type, inst_type, inst_type)
@@ -1074,14 +1067,10 @@ def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
 
 def fields_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> CallableType:
     """Provide the signature for `attrs.fields`."""
-    if not ctx.args or len(ctx.args) != 1 or not ctx.args[0] or not ctx.args[0][0]:
+    if len(ctx.args) != 1 or len(ctx.args[0]) != 1:
         return ctx.default_signature
 
-    # <hack>
-    assert isinstance(ctx.api, TypeChecker)
-    inst_type = ctx.api.expr_checker.accept(ctx.args[0][0])
-    # </hack>
-    proper_type = get_proper_type(inst_type)
+    proper_type = get_proper_type(ctx.api.get_expression_type(ctx.args[0][0]))
 
     # fields(Any) -> Any, fields(type[Any]) -> Any
     if (
@@ -1098,7 +1087,7 @@ def fields_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
         inner = get_proper_type(proper_type.upper_bound)
         if isinstance(inner, Instance):
             # We need to work arg_types to compensate for the attrs stubs.
-            arg_types = [inst_type]
+            arg_types = [proper_type]
             cls = inner.type
     elif isinstance(proper_type, CallableType):
         cls = proper_type.type_object()

test-data/unit/check-custom-plugin.test

@@ -887,7 +887,10 @@ plugins=<ROOT>/test-data/unit/plugins/descriptor.py
 # flags: --config-file tmp/mypy.ini
 
 def dynamic_signature(arg1: str) -> str: ...
-reveal_type(dynamic_signature(1))  # N: Revealed type is "builtins.int"
+a: int = 1
+reveal_type(dynamic_signature(a))  # N: Revealed type is "builtins.int"
+b: bytes = b'foo'
+reveal_type(dynamic_signature(b))  # N: Revealed type is "builtins.bytes"
 [file mypy.ini]
 \[mypy]
 plugins=<ROOT>/test-data/unit/plugins/function_sig_hook.py

test-data/unit/plugins/function_sig_hook.py

@@ -1,26 +1,23 @@
-from mypy.plugin import CallableType, CheckerPluginInterface, FunctionSigContext, Plugin
-from mypy.types import Instance, Type
+from mypy.plugin import CallableType, FunctionSigContext, Plugin
+
 
 class FunctionSigPlugin(Plugin):
     def get_function_signature_hook(self, fullname):
         if fullname == '__main__.dynamic_signature':
             return my_hook
         return None
 
-def _str_to_int(api: CheckerPluginInterface, typ: Type) -> Type:
-    if isinstance(typ, Instance):
-        if typ.type.fullname == 'builtins.str':
-            return api.named_generic_type('builtins.int', [])
-        elif typ.args:
-            return typ.copy_modified(args=[_str_to_int(api, t) for t in typ.args])
-
-    return typ
 
 def my_hook(ctx: FunctionSigContext) -> CallableType:
+    arg1_args = ctx.args[0]
+    if len(arg1_args) != 1:
+        return ctx.default_signature
+    arg1_type = ctx.api.get_expression_type(arg1_args[0])
     return ctx.default_signature.copy_modified(
-        arg_types=[_str_to_int(ctx.api, t) for t in ctx.default_signature.arg_types],
-        ret_type=_str_to_int(ctx.api, ctx.default_signature.ret_type),
+        arg_types=[arg1_type],
+        ret_type=arg1_type,
     )
 
+
 def plugin(version):
     return FunctionSigPlugin