Don't reexport star-imported symbols when implicit reexports are disabled (#7361)

Based on #7099.

Currently, in regular files, symbols imported into module A with `from B
import *` are considered explicitly exported from module A as well, i.e.
they are considered part of module's A public API.

This behavior makes sense in stub files as a shorthand (and is specified
in PEP 484), but for regular files I think it is better to be explicit:
add the symbols to `__all__`.

Fixes #7042
Further discussion: #7042 (comment)

Author: bluetech

docs/source/command_line.rst

@@ -429,15 +429,18 @@ of the above sections.
 ``--no-implicit-reexport``
     By default, imported values to a module are treated as exported and mypy allows
     other modules to import them. This flag changes the behavior to not re-export unless
-    the item is imported using from-as. Note this is always treated as enabled for
-    stub files. For example:
+    the item is imported using from-as or is included in ``__all__``. Note this is
+    always treated as enabled for stub files. For example:
 
     .. code-block:: python
 
        # This won't re-export the value
        from foo import bar
        # This will re-export it as bar and allow other modules to import it
        from foo import bar as bar
+       # This will also re-export bar
+       from foo import bar
+       __all__ = ['bar']
 
 
 ``--strict-equality``

docs/source/config_file.rst

@@ -310,15 +310,18 @@ Miscellaneous strictness flags
 ``implicit_reexport`` (bool, default True)
     By default, imported values to a module are treated as exported and mypy allows
     other modules to import them. When false, mypy will not re-export unless
-    the item is imported using from-as. Note that mypy treats stub files as if this
-    is always disabled. For example:
+    the item is imported using from-as or is included in ``__all__``. Note that mypy
+    treats stub files as if this is always disabled. For example:
 
     .. code-block:: python
 
        # This won't re-export the value
        from foo import bar
        # This will re-export it as bar and allow other modules to import it
        from foo import bar as bar
+       # This will also re-export bar
+       from foo import bar
+       __all__ = ['bar']
 
 ``strict_equality``  (bool, default False)
    Prohibit equality checks, identity checks, and container checks between

mypy/semanal.py

@@ -463,10 +463,16 @@ def add_builtin_aliases(self, tree: MypyFile) -> None:
                     del tree.names[name]
 
     def adjust_public_exports(self) -> None:
-        """Make variables not in __all__ not be public"""
+        """Adjust the module visibility of globals due to __all__."""
         if '__all__' in self.globals:
             for name, g in self.globals.items():
-                if name not in self.all_exports:
+                # Being included in __all__ explicitly exports and makes public.
+                if name in self.all_exports:
+                    g.module_public = True
+                    g.module_hidden = False
+                # But when __all__ is defined, and a symbol is not included in it,
+                # it cannot be public.
+                else:
                     g.module_public = False
 
     @contextmanager
@@ -1863,7 +1869,12 @@ def visit_import_all(self, i: ImportAll) -> None:
                         if self.process_import_over_existing_name(
                                 name, existing_symbol, node, i):
                             continue
-                    self.add_imported_symbol(name, node, i)
+                    # In stub files, `from x import *` always reexports the symbols.
+                    # In regular files, only if implicit reexports are enabled.
+                    module_public = self.is_stub_file or self.options.implicit_reexport
+                    self.add_imported_symbol(name, node, i,
+                                             module_public=module_public,
+                                             module_hidden=not module_public)
         else:
             # Don't add any dummy symbols for 'from x import *' if 'x' is unknown.
             pass

test-data/unit/check-flags.test

@@ -1145,6 +1145,42 @@ from other_module_1 import a
 [out]
 main:2: error: Module 'other_module_2' has no attribute 'a'
 
+[case testNoImplicitReexportRespectsAll]
+# flags: --no-implicit-reexport
+from other_module_2 import a
+from other_module_2 import b
+[file other_module_1.py]
+a = 5
+b = 6
+[file other_module_2.py]
+from other_module_1 import a, b
+__all__ = ('b',)
+[out]
+main:2: error: Module 'other_module_2' has no attribute 'a'
+
+[case testNoImplicitReexportStarConsideredImplicit]
+# flags: --no-implicit-reexport
+from other_module_2 import a
+[file other_module_1.py]
+a = 5
+[file other_module_2.py]
+from other_module_1 import *
+[out]
+main:2: error: Module 'other_module_2' has no attribute 'a'
+
+[case testNoImplicitReexportStarCanBeReexportedWithAll]
+# flags: --no-implicit-reexport
+from other_module_2 import a
+from other_module_2 import b
+[file other_module_1.py]
+a = 5
+b = 6
+[file other_module_2.py]
+from other_module_1 import *
+__all__ = ('b',)
+[out]
+main:2: error: Module 'other_module_2' has no attribute 'a'
+
 [case testNoImplicitReexportMypyIni]
 # flags: --config-file tmp/mypy.ini
 from other_module_2 import a