Add __path__ to package __init__

Resolves python#1422

Author: hauntsaninja

mypy/nodes.py

@@ -97,10 +97,13 @@ def get_column(self) -> int:
 inverse_node_kinds = {_kind: _name for _name, _kind in node_kinds.items()}  # type: Final
 
 
-implicit_module_attrs = {'__name__': '__builtins__.str',
-                         '__doc__': None,  # depends on Python version, see semanal.py
-                         '__file__': '__builtins__.str',
-                         '__package__': '__builtins__.str'}  # type: Final
+implicit_module_attrs = {
+    '__name__': '__builtins__.str',
+    '__doc__': None,  # depends on Python version, see semanal.py
+    '__path__': None,  # depends on if the module is a package
+    '__file__': '__builtins__.str',
+    '__package__': '__builtins__.str'
+}  # type: Final
 
 
 # These aliases exist because built-in class objects are not subscriptable.

mypy/semanal.py

@@ -400,6 +400,12 @@ def add_implicit_module_attrs(self, file_node: MypyFile) -> None:
                 else:
                     typ = UnionType([UnboundType('__builtins__.str'),
                                      UnboundType('__builtins__.unicode')])
+            elif name == '__path__':
+                if not file_node.is_package_init_file():
+                    continue
+                # Need to construct the type ourselves, to avoid issues with __builtins__.list
+                # not being subscriptable or typing.List not getting bound
+                typ = self.named_type("__builtins__.list", [self.str_type()])
             else:
                 assert t is not None, 'type should be specified for {}'.format(name)
                 typ = UnboundType(t)

test-data/unit/check-modules.test

@@ -2825,3 +2825,13 @@ from mystery import a, b as b, c as d
 [out]
 tmp/stub.pyi:1: error: Cannot find implementation or library stub for module named 'mystery'
 tmp/stub.pyi:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
+
+[case testPackagePath]
+import p
+reveal_type(p.__path__)  # N: Revealed type is 'builtins.list[builtins.str]'
+p.m.__path__  # E: "object" has no attribute "__path__"
+
+[file p/__init__.py]
+from . import m as m
+[file p/m.py]
+[builtins fixtures/list.pyi]