Fix for issue #505 (Better error message for missing keyword argument)

mypy/checkexpr.py

@@ -472,7 +472,7 @@ def check_argument_count(self, callee: Callable, actual_types: List[Type],
             if kind == nodes.ARG_POS and (not formal_to_actual[i] and
                                           not is_error):
                 # No actual for a mandatory positional formal.
-                self.msg.too_few_arguments(callee, context)
+                self.msg.too_few_arguments(callee, context, actual_names)
             elif kind in [nodes.ARG_POS, nodes.ARG_OPT,
                           nodes.ARG_NAMED] and is_duplicate_mapping(
                     formal_to_actual[i], actual_kinds):

mypy/messages.py

@@ -396,17 +396,21 @@ def invalid_index_type(self, index_type: Type, base_str: str,
         self.fail('Invalid index type {} for {}'.format(
             self.format(index_type), base_str), context)
 
-    def invalid_argument_count(self, callee: Callable, num_args: int,
-                               context: Context) -> None:
-        if num_args < len(callee.arg_types):
-            self.too_few_arguments(callee, context)
+    def too_few_arguments(self, callee: Callable, context: Context,
+                          argument_names: List[str]) -> None:
+        if (argument_names is not None and not all(k is None for k in argument_names)
+                and len(argument_names) >= 1):
+            diff = [k for k in callee.arg_names if k not in argument_names]
+            if len(diff) == 1:
+                msg = 'Missing positional argument'
+            else:
+                msg = 'Missing positional arguments'
+            if callee.name and diff:
+                msg += ' "{}" in call to {}'.format('", "'.join(diff), callee.name)
         else:
-            self.too_many_arguments(callee, context)
-
-    def too_few_arguments(self, callee: Callable, context: Context) -> None:
-        msg = 'Too few arguments'
-        if callee.name:
-            msg += ' for {}'.format(callee.name)
+            msg = 'Too few arguments'
+            if callee.name:
+                msg += ' for {}'.format(callee.name)
         self.fail(msg, context)
 
     def too_many_arguments(self, callee: Callable, context: Context) -> None:

mypy/test/data/check-kwargs.test

@@ -153,7 +153,7 @@ import typing
 def f(x, y=A()): pass
 f(x=A(), y=A())
 f(y=A(), x=A())
-f(y=A())      # E: Too few arguments for "f"
+f(y=A())      # E: Missing positional argument "x" in call to "f"
 f(A(), z=A()) # E: Unexpected keyword argument "z" for "f"
 class A: pass
 

mypy/test/data/check-namedtuple.test

@@ -35,7 +35,7 @@ X = namedtuple('X', ['x', 'y'])
 x = X(x=1, y='x')
 x = X(1, y='x')
 x = X(x=1, z=1) # E: Unexpected keyword argument "z" for "X"
-x = X(y=1) # E: Too few arguments for "X"
+x = X(y=1) # E: Missing positional argument "x" in call to "X"
 
 [case testCreateNamedTupleAndUseAsTuple]
 from collections import namedtuple