Require id=... to be a string

This was documented before, but never enforced.  Passing non-strings could
have strange side-effects and enforcing a string simplifies other
implementation.

src/_pytest/mark/structures.py

@@ -5,6 +5,7 @@ from functools import reduce
 from operator import attrgetter
 
 import attr
+import six
 from six.moves import map
 
 from ..compat import getfslineno
@@ -70,10 +71,12 @@ class ParameterSet(namedtuple("ParameterSet", "values, marks, id")):
         else:
             assert isinstance(marks, (tuple, list, set))
 
-        def param_extract_id(id=None):
-            return id
-
-        id_ = param_extract_id(**kw)
+        id_ = kw.pop("id", None)
+        if id_ is not None:
+            if not isinstance(id_, six.string_types):
+                raise TypeError(
+                    "Expected id to be a string, got {}: {!r}".format(type(id_), id_)
+                )
         return cls(values, marks, id_)
 
     @classmethod

testing/test_mark.py

@@ -5,20 +5,21 @@ from __future__ import print_function
 import os
 import sys
 
+import six
+
+import pytest
+from _pytest.mark import EMPTY_PARAMETERSET_OPTION
+from _pytest.mark import MarkGenerator as Mark
+from _pytest.mark import ParameterSet
+from _pytest.mark import transfer_markers
+from _pytest.nodes import Collector
+from _pytest.nodes import Node
 from _pytest.warnings import SHOW_PYTEST_WARNINGS_ARG
 
 try:
     import mock
 except ImportError:
     import unittest.mock as mock
-import pytest
-from _pytest.mark import (
-    MarkGenerator as Mark,
-    ParameterSet,
-    transfer_markers,
-    EMPTY_PARAMETERSET_OPTION,
-)
-from _pytest.nodes import Node, Collector
 
 ignore_markinfo = pytest.mark.filterwarnings(
     "ignore:MarkInfo objects:pytest.RemovedInPytest4Warning"
@@ -1252,3 +1253,18 @@ def test_markers_from_parametrize(testdir):
 
     result = testdir.runpytest(SHOW_PYTEST_WARNINGS_ARG)
     result.assert_outcomes(passed=4)
+
+
+def test_pytest_param_id_requires_string():
+    with pytest.raises(TypeError) as excinfo:
+        pytest.param(id=True)
+    msg, = excinfo.value.args
+    if six.PY2:
+        assert msg == "Expected id to be a string, got <type 'bool'>: True"
+    else:
+        assert msg == "Expected id to be a string, got <class 'bool'>: True"
+
+
+@pytest.mark.parametrize("s", (None, "hello world"))
+def test_pytest_param_id_allows_none_or_string(s):
+    assert pytest.param(id=s)