Function compatibility rewrite (#2521)

* New algorithm for checking function compatibility.

The new algorithm correctly handles argument names.  It pays attention to them
when matching up arguments for function assignments, but not for overrides
(because too many libraries vary the agument names between superclass and
subclass).

I'll include a full writeup in prose of the new algorithm in the pull request.

Print callable types with named arguments differently from each other when important

Fix importFunctionAndAssignFunction test to match var names

* Rename argument shorter

* Explain what i am doing better

* Stop checking argument names for multiple base classes

* Elide __foo argument names from funcs where args are not typed

* Cosmetic

* Make the text descriptions of args more concise

* Account for one left argument with two corresponding right args

* cosmetic

* cosmetic

* Remove `In class foo`

* Factor out corresponding_argument method

* Optional for the name and pos

* typo

* Ignore implicitly typed function arg names

* Test for ignoring arg names for implicit defs

* Fix tests

* Delete tests for implicit callable lacking names as they make no sense now

mypy/checker.py

@@ -40,8 +40,8 @@
 from mypy.checkmember import map_type_from_supertype, bind_self, erase_to_bound
 from mypy import messages
 from mypy.subtypes import (
-    is_subtype, is_equivalent, is_proper_subtype, is_more_precise, restrict_subtype_away,
-    is_subtype_ignoring_tvars
+    is_subtype, is_equivalent, is_proper_subtype, is_more_precise,
+    restrict_subtype_away, is_subtype_ignoring_tvars
 )
 from mypy.maptype import map_instance_to_supertype
 from mypy.semanal import fill_typevars, set_callable_name, refers_to_fullname
@@ -835,7 +835,7 @@ def check_inplace_operator_method(self, defn: FuncBase) -> None:
     def check_getattr_method(self, typ: CallableType, context: Context) -> None:
         method_type = CallableType([AnyType(), self.named_type('builtins.str')],
                                    [nodes.ARG_POS, nodes.ARG_POS],
-                                   [None],
+                                   [None, None],
                                    AnyType(),
                                    self.named_type('builtins.function'))
         if not is_subtype(typ, method_type):
@@ -936,7 +936,7 @@ def check_override(self, override: FunctionLike, original: FunctionLike,
         """
         # Use boolean variable to clarify code.
         fail = False
-        if not is_subtype(override, original):
+        if not is_subtype(override, original, ignore_pos_arg_names=True):
             fail = True
         elif (not isinstance(original, Overloaded) and
               isinstance(override, Overloaded) and
@@ -1043,7 +1043,7 @@ def check_compatibility(self, name: str, base1: TypeInfo,
             # Method override
             first_sig = bind_self(first_type)
             second_sig = bind_self(second_type)
-            ok = is_subtype(first_sig, second_sig)
+            ok = is_subtype(first_sig, second_sig, ignore_pos_arg_names=True)
         elif first_type and second_type:
             ok = is_equivalent(first_type, second_type)
         else:

mypy/messages.py

@@ -15,7 +15,7 @@
 )
 from mypy.nodes import (
     TypeInfo, Context, MypyFile, op_methods, FuncDef, reverse_type_aliases,
-    ARG_STAR, ARG_STAR2
+    ARG_POS, ARG_OPT, ARG_NAMED, ARG_NAMED_OPT, ARG_STAR, ARG_STAR2
 )
 
 
@@ -77,6 +77,15 @@
 TYPEDDICT_ITEM_NAME_MUST_BE_STRING_LITERAL = \
     'Expected TypedDict item name to be string literal'
 
+ARG_CONSTRUCTOR_NAMES = {
+    ARG_POS: "Arg",
+    ARG_OPT: "DefaultArg",
+    ARG_NAMED: "NamedArg",
+    ARG_NAMED_OPT: "DefaultNamedArg",
+    ARG_STAR: "StarArg",
+    ARG_STAR2: "KwArg",
+}
+
 
 class MessageBuilder:
     """Helper class for reporting type checker error messages with parameters.
@@ -176,8 +185,30 @@ def format(self, typ: Type, verbosity: int = 0) -> str:
                 return_type = strip_quotes(self.format(func.ret_type))
                 if func.is_ellipsis_args:
                     return 'Callable[..., {}]'.format(return_type)
-                arg_types = [strip_quotes(self.format(t)) for t in func.arg_types]
-                return 'Callable[[{}], {}]'.format(", ".join(arg_types), return_type)
+                arg_strings = []
+                for arg_name, arg_type, arg_kind in zip(
+                        func.arg_names, func.arg_types, func.arg_kinds):
+                    if (arg_kind == ARG_POS and arg_name is None
+                            or verbosity == 0 and arg_kind in (ARG_POS, ARG_OPT)):
+
+                        arg_strings.append(
+                            strip_quotes(
+                                self.format(
+                                    arg_type,
+                                    verbosity = max(verbosity - 1, 0))))
+                    else:
+                        constructor = ARG_CONSTRUCTOR_NAMES[arg_kind]
+                        if arg_kind in (ARG_STAR, ARG_STAR2):
+                            arg_strings.append("{}({})".format(
+                                constructor,
+                                strip_quotes(self.format(arg_type))))
+                        else:
+                            arg_strings.append("{}('{}', {})".format(
+                                constructor,
+                                arg_name,
+                                strip_quotes(self.format(arg_type))))
+
+                return 'Callable[[{}], {}]'.format(", ".join(arg_strings), return_type)
             else:
                 # Use a simple representation for function types; proper
                 # function types may result in long and difficult-to-read