Show a warning when non-str is given to Monkeypatch.setenv

Author: nicoddemus

src/_pytest/monkeypatch.py

@@ -225,7 +225,15 @@ def setenv(self, name, value, prepend=None):
         """ Set environment variable ``name`` to ``value``.  If ``prepend``
         is a character, read the current environment variable value
         and prepend the ``value`` adjoined with the ``prepend`` character."""
-        value = str(value)
+        if not isinstance(value, str):
+            warnings.warn(
+                pytest.PytestWarning(
+                    "Environment variable value {!r} should be str, converted to str implicitly".format(
+                        value
+                    )
+                )
+            )
+            value = str(value)
         if prepend and name in os.environ:
             value = value + prepend + os.environ[name]
         self._warn_if_env_name_is_not_str(name)

testing/test_monkeypatch.py

@@ -194,29 +194,39 @@ def test_delenv():
             del os.environ[name]
 
 
-@pytest.mark.skipif(six.PY3, reason="Python 2 only test")
-class TestEnvironKeysWarning(object):
+class TestEnvironWarnings(object):
     """
-    os.environ needs keys to be native strings, otherwise it will cause problems with other modules (notably
-    subprocess). We only test this behavior on Python 2, because Python 3 raises an error right away.
+    os.environ keys and values should be native strings, otherwise it will cause problems with other modules (notably
+    subprocess). On Python 2 os.environ accepts anything without complaining, while Python 3 does the right thing
+    and raises an error.
     """
 
     VAR_NAME = u"PYTEST_INTERNAL_MY_VAR"
 
+    @pytest.mark.skipif(six.PY3, reason="Python 2 only test")
     def test_setenv_unicode_key(self, monkeypatch):
         with pytest.warns(
             pytest.PytestWarning,
             match="Environment variable name {!r} should be str".format(self.VAR_NAME),
         ):
             monkeypatch.setenv(self.VAR_NAME, "2")
 
+    @pytest.mark.skipif(six.PY3, reason="Python 2 only test")
     def test_delenv_unicode_key(self, monkeypatch):
         with pytest.warns(
             pytest.PytestWarning,
             match="Environment variable name {!r} should be str".format(self.VAR_NAME),
         ):
             monkeypatch.delenv(self.VAR_NAME, raising=False)
 
+    def test_setenv_non_str_warning(self, monkeypatch):
+        value = u"2" if six.PY2 else b"2"
+        msg = (
+            "Environment variable value {!r} should be str, converted to str implicitly"
+        )
+        with pytest.warns(pytest.PytestWarning, match=msg.format(value)):
+            monkeypatch.setenv(str(self.VAR_NAME), value)
+
 
 def test_setenv_prepend():
     import os