Fix a 'duplicate top-level identifier' pyi issue.

We were generating a bad pyi for 'from datetime import datetime' that defined
'datetime' twice (once via an 'import datetime' line and again as a 'datetime:
Type[datetime.datetime' constant).

Fixed by changing the import statement to 'import datetime as _datetime'.

Resolves #1057.

PiperOrigin-RevId: 411930502

Author: rchen152

pytype/pytd/printer.py

@@ -441,13 +441,14 @@ def _UseExistingModuleAlias(self, name):
   def _GuessModule(self, maybe_module):
     """Guess which part of the given name is the module prefix."""
     if "." not in maybe_module:
-      return maybe_module
+      return maybe_module, ""
     prefix, suffix = maybe_module.rsplit(".", 1)
     # Heuristic: modules are typically lowercase, classes uppercase.
     if suffix[0].islower():
-      return maybe_module
+      return maybe_module, ""
     else:
-      return self._GuessModule(prefix)
+      module, rest = self._GuessModule(prefix)
+      return module, f"{rest}.{suffix}" if rest else suffix
 
   def VisitNamedType(self, node):
     """Convert a type to a string."""
@@ -467,8 +468,16 @@ def VisitNamedType(self, node):
           if aliased_name:
             node_name = aliased_name
           else:
-            self._RequireImport(self._GuessModule(prefix))
-            node_name = node.name
+            module, rest = self._GuessModule(prefix)
+            module_alias = module
+            while self._NameCollision(module_alias):
+              module_alias = f"_{module_alias}"
+            if module_alias == module:
+              self._RequireImport(module)
+              node_name = node.name
+            else:
+              self._RequireImport(f"{module} as {module_alias}")
+              node_name = ".".join(filter(bool, (module_alias, rest, suffix)))
         else:
           node_name = node.name
       else:

pytype/tests/test_import2.py

@@ -65,6 +65,13 @@ def test_import_attribute_error(self):
         print(err.name)
     """)
 
+  def test_datetime_datetime(self):
+    with self.DepTree([("foo.py", "from datetime import datetime")]):
+      self.Check("""
+        import foo
+        assert_type(foo.datetime(1, 1, 1), "datetime.datetime")
+      """)
+
 
 if __name__ == "__main__":
   test_base.main()