add support for __path__ attribute on modules

Fixes python#1422

Author: Tom Dyas

mypy/nodes.py

@@ -75,7 +75,8 @@ def get_column(self) -> int: pass
 implicit_module_attrs = {'__name__': '__builtins__.str',
                          '__doc__': None,  # depends on Python version, see semanal.py
                          '__file__': '__builtins__.str',
-                         '__package__': '__builtins__.str'}
+                         '__package__': '__builtins__.str',
+                         '__path__': '__builtins__.str', }  # only present in __init__.py files
 
 
 type_aliases = {

mypy/semanal.py

@@ -43,6 +43,7 @@
   traverse the entire AST.
 """
 
+import os.path
 from collections import OrderedDict
 from contextlib import contextmanager
 
@@ -270,6 +271,10 @@ def visit_file(self, file_node: MypyFile, fnam: str, options: Options) -> None:
                 MODULE_REF, self.modules['builtins'], self.cur_mod_id)
 
         for name in implicit_module_attrs:
+            # __path__ is only defined on __init__.py files
+            if name == '__path__' and os.path.basename(fnam) not in ('__init__.py', '__init__.pyi'):
+                continue
+
             v = self.globals[name].node
             if isinstance(v, Var):
                 v.type = self.anal_type(v.type)
@@ -3546,6 +3551,14 @@ def visit_file(self, file: MypyFile, fnam: str, mod_id: str, options: Options) -
                 else:
                     typ = UnionType([UnboundType('__builtins__.str'),
                                      UnboundType('__builtins__.unicode')])
+            elif name == '__path__':
+                # __path__ is only defined on __init__.py files
+                if os.path.basename(fnam).lower() not in ('__init__.py', '__init__.pyi'):
+                    continue
+                # typ = self.sem.named_type('builtins.list', [self.sem.named_type('builtins.str')])
+                # typ = UnboundType('__builtins__.list', args=[UnboundType('__builtins__.str')])
+                # self.visit_import(Import(ids=[('typing', None)]))
+                typ = UnboundType('typing.List', args=[UnboundType('__builtins__.str')])
             else:
                 assert t is not None, 'type should be specified for {}'.format(name)
                 typ = UnboundType(t)

test-data/unit/check-modules.test

@@ -1640,3 +1640,39 @@ m = n  # E: Cannot assign multiple modules to name 'm' without explicit 'types.M
 [file n.py]
 
 [builtins fixtures/module.pyi]
+
+[case testPathAttributePresence]
+import m
+import m.m
+
+x = m.__path__
+x.append('xyzzy')
+
+y = m.m.__path__  # E: Module has no attribute "__path__"
+
+[file m/__init__.py]
+import typing
+
+[file m/m.py]
+
+[builtins fixtures/module_all.pyi]
+
+[typing fixtures/typing-full.pyi]
+
+
+[case testPathAttributePresence_python2]
+import m
+import m.m
+
+x = m.__path__
+x.append('xyzzy')
+
+y = m.m.__path__  # E: Module has no attribute "__path__"
+
+[file m/__init__.py]
+
+[file m/m.py]
+
+[builtins fixtures/module_all.pyi]
+
+[typing fixtures/typing-full.pyi]