added indicative error when parametrizing an argument with a default value

_pytest/compat.py

@@ -135,6 +135,14 @@ def getfuncargnames(function, is_method=False, cls=None):
     return arg_names
 
 
+def get_default_arg_names(function):
+    # Note: this code intentionally mirrors the code at the beginning of getfuncargnames,
+    # to get the arguments which were excluded from its result because they had default values
+    return tuple(p.name for p in signature(function).parameters.values()
+                 if p.kind in (Parameter.POSITIONAL_OR_KEYWORD, Parameter.KEYWORD_ONLY) and
+                 p.default is not Parameter.empty)
+
+
 if _PY3:
     STRING_TYPES = bytes, str
     UNICODE_TYPES = str,

_pytest/python.py

@@ -25,7 +25,7 @@ from _pytest.compat import (
     isclass, isfunction, is_generator, ascii_escaped,
     REGEX_TYPE, STRING_TYPES, NoneType, NOTSET,
     get_real_func, getfslineno, safe_getattr,
-    safe_str, getlocation, enum,
+    safe_str, getlocation, enum, get_default_arg_names
 )
 from _pytest.outcomes import fail
 from _pytest.mark.structures import transfer_markers
@@ -789,6 +789,7 @@ class Metafunc(fixtures.FuncargnamesCompatAttr):
         argnames, parameters = ParameterSet._for_parametrize(
             argnames, argvalues, self.function, self.config)
         del argvalues
+        default_arg_names = set(get_default_arg_names(self.function))
 
         if scope is None:
             scope = _find_parametrized_scope(argnames, self._arg2fixturedefs, indirect)
@@ -797,13 +798,16 @@ class Metafunc(fixtures.FuncargnamesCompatAttr):
         valtypes = {}
         for arg in argnames:
             if arg not in self.fixturenames:
-                if isinstance(indirect, (tuple, list)):
-                    name = 'fixture' if arg in indirect else 'argument'
+                if arg in default_arg_names:
+                    raise ValueError("%r already takes an argument %r with a default value" % (self.function, arg))
                 else:
-                    name = 'fixture' if indirect else 'argument'
-                raise ValueError(
-                    "%r uses no %s %r" % (
-                        self.function, name, arg))
+                    if isinstance(indirect, (tuple, list)):
+                        name = 'fixture' if arg in indirect else 'argument'
+                    else:
+                        name = 'fixture' if indirect else 'argument'
+                    raise ValueError(
+                        "%r uses no %s %r" % (
+                            self.function, name, arg))
 
         if indirect is True:
             valtypes = dict.fromkeys(argnames, "params")

changelog/3221.feature

@@ -0,0 +1 @@
+Added a more indicative error message when parametrizing a function whose argument takes a default value.

testing/python/metafunc.py

@@ -616,6 +616,19 @@ class TestMetafunc(object):
             "*uses no argument 'y'*",
         ])
 
+    def test_parametrize_gives_indicative_error_on_function_with_default_argument(self, testdir):
+        testdir.makepyfile("""
+            import pytest
+
+            @pytest.mark.parametrize('x, y', [('a', 'b')])
+            def test_simple(x, y=1):
+                assert len(x) == 1
+        """)
+        result = testdir.runpytest("--collect-only")
+        result.stdout.fnmatch_lines([
+            "*already takes an argument 'y' with a default value",
+        ])
+
     def test_addcalls_and_parametrize_indirect(self):
         def func(x, y):
             pass