Improve error message when passing non-string ids to pytest.mark.parametrize

Fix #1857

CHANGELOG.rst

@@ -3,13 +3,17 @@
 
 *
 
-*
+* Improve error message when passing non-string ids to ``pytest.mark.parametrize`` (`#1857`_).
+  Thanks `@okken`_ for the report and `@nicoddemus`_ for the PR.
 
 *
 
 *
 
 
+.. _#1857: https://github.com/pytest-dev/pytest/issues/1857
+
+
 3.0.1
 =====
 

_pytest/python.py

@@ -778,6 +778,7 @@ class Metafunc(fixtures.FuncargnamesCompatAttr):
         """
         from _pytest.fixtures import scopes
         from _pytest.mark import extract_argvalue
+        from py.io import saferepr
 
         unwrapped_argvalues = []
         newkeywords = []
@@ -831,9 +832,14 @@ class Metafunc(fixtures.FuncargnamesCompatAttr):
         if callable(ids):
             idfn = ids
             ids = None
-        if ids and len(ids) != len(argvalues):
-            raise ValueError('%d tests specified with %d ids' %(
-                             len(argvalues), len(ids)))
+        if ids:
+            if len(ids) != len(argvalues):
+                raise ValueError('%d tests specified with %d ids' %(
+                                 len(argvalues), len(ids)))
+            for id_value in ids:
+                if id_value is not None and not isinstance(id_value, str):
+                    msg = 'ids must be list of strings, found: %s (type: %s)'
+                    raise ValueError(msg % (saferepr(id_value), type(id_value).__name__))
         ids = idmaker(argnames, argvalues, idfn, ids, self.config)
         newcalls = []
         for callspec in self._calls or [CallSpec2(self)]:

testing/python/metafunc.py

@@ -916,6 +916,18 @@ class TestMetafuncFunctional:
         result = testdir.runpytest()
         result.stdout.fnmatch_lines(['* 1 skipped *'])
 
+    def test_parametrized_ids_invalid_type(self, testdir):
+        """Tests parametrized with ids as non-strings (#1857)."""
+        testdir.makepyfile('''
+            import pytest
+
+            @pytest.mark.parametrize("x, expected", [(10, 20), (40, 80)], ids=(None, 2))
+            def test_ids_numbers(x,expected):
+                assert x * 2 == expected
+        ''')
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(['*ids must be list of strings, found: 2 (type: int)*'])
+
     def test_parametrize_with_identical_ids_get_unique_names(self, testdir):
         testdir.makepyfile("""
             import pytest