Adds doc

Author: hippo91

astroid/brain/brain_type.py

@@ -1,4 +1,21 @@
 # -*- coding: utf-8 -*-
+"""
+Astroid hooks for type support.
+
+Starting from python3.9, type object behaves as it had __class_getitem__ method.
+However it was not possible to simply add this method inside type's body, otherwise
+all types would also have this method. In this case it would have been possible
+to write str[int].
+Guido Van Rossum proposed a hack to handle this in the interpreter:
+https://github.com/python/cpython/blob/master/Objects/abstract.c#L186-L189
+
+This brain follows the same logic. It is no wise to add permanently the __class_getitem__ method 
+to the type object. Instead we choose to add it only in the case of a subscript node
+which inside name node is type. 
+Doing this type[int] is allowed whereas str[int] is not.
+
+Thanks to Lukasz Langa for fruitful discussion.
+"""
 import sys
 
 from astroid import (
@@ -10,20 +27,30 @@
 
 
 def _looks_like_type_subscript(node):
-    """Try to figure out if a Subscript node *might* be a typing-related subscript"""
-    if isinstance(node, nodes.Name):
+    """
+    Try to figure out if a Name node is used inside a type related subscript
+
+    :param node: node to check
+    :type node: nodes.Name
+    :return: true if the node is a Name node inside a type related subscript
+    :rtype: bool
+    """
+    if isinstance(node, nodes.Name) and isinstance(node.parent, nodes.Subscript):
         return node.name == "type"
-    if isinstance(node, nodes.Subscript):
-        if isinstance(node.value, Name) and node.value.name == "type":
-            return True
     return False
 
 
 def infer_type_sub(node, context=None):
-    """Infer a typing.X[...] subscript"""
-    sub_node = node.parent
-    if not isinstance(sub_node, nodes.Subscript):
-        raise UseInferenceDefault
+    """
+    Infer a type[...] subscript
+
+    :param node: node to infer
+    :type node: astroid.node_classes.NodeNG
+    :param context: inference context
+    :type context: astroid.context.InferenceContext
+    :return: the inferred node
+    :rtype: nodes.NodeNG
+    """
     class_src = """
     class type:
         def __class_getitem__(cls, key):

tests/unittest_brain.py

@@ -940,6 +940,10 @@ def test_sys_streams(self):
 @test_utils.require_version("3.9")
 class TypeBrain(unittest.TestCase):
     def test_type_subscript(self):
+        """
+        Check that type object has the __class_getitem__ method
+        when it is used as a subscript
+        """
         src = builder.extract_node(
             """
             a: type[int] = int
@@ -952,6 +956,11 @@ def test_type_subscript(self):
         self.assertIsInstance(meth_inf, astroid.FunctionDef)
 
     def test_invalid_type_subscript(self):
+        """
+        Check that a type (str for example) that inherits
+        from type does not have __class_getitem__ method even
+        when it is used as a subscript
+        """
         src = builder.extract_node(
             """
             a: str[int] = "abc"