Make sure parametrize ids are printable

src/_pytest/compat.py

@@ -182,6 +182,15 @@ def get_default_arg_names(function):
     )
 
 
+_non_printable_ascii_translate_table = {
+    i: u"\\x{:02x}".format(i) for i in range(128) if i not in range(32, 127)
+}
+
+
+def _translate_non_printable(s):
+    return s.translate(_non_printable_ascii_translate_table)
+
+
 if _PY3:
     STRING_TYPES = bytes, str
     UNICODE_TYPES = six.text_type
@@ -221,9 +230,10 @@ if _PY3:
 
         """
         if isinstance(val, bytes):
-            return _bytes_to_ascii(val)
+            ret = _bytes_to_ascii(val)
         else:
-            return val.encode("unicode_escape").decode("ascii")
+            ret = val.encode("unicode_escape").decode("ascii")
+        return _translate_non_printable(ret)
 
 
 else:
@@ -241,11 +251,12 @@ else:
         """
         if isinstance(val, bytes):
             try:
-                return val.encode("ascii")
+                ret = val.decode("ascii")
             except UnicodeDecodeError:
-                return val.encode("string-escape")
+                ret = val.encode("string-escape").decode("ascii")
         else:
-            return val.encode("unicode-escape")
+            ret = val.encode("unicode-escape").decode("ascii")
+        return _translate_non_printable(ret)
 
 
 class _PytestWrapper(object):

testing/python/metafunc.py

@@ -5,6 +5,7 @@ import textwrap
 
 import attr
 import hypothesis
+import six
 from hypothesis import strategies
 
 import pytest
@@ -262,11 +263,8 @@ class TestMetafunc(object):
         from _pytest.python import _idval
 
         escaped = _idval(value, "a", 6, None, item=None, config=None)
-        assert isinstance(escaped, str)
-        if PY3:
-            escaped.encode("ascii")
-        else:
-            escaped.decode("ascii")
+        assert isinstance(escaped, six.text_type)
+        escaped.encode("ascii")
 
     def test_unicode_idval(self):
         """This tests that Unicode strings outside the ASCII character set get
@@ -382,6 +380,20 @@ class TestMetafunc(object):
             "\\xc3\\xb4-other",
         ]
 
+    def test_idmaker_non_printable_characters(self):
+        from _pytest.python import idmaker
+
+        result = idmaker(
+            ("s", "n"),
+            [
+                pytest.param("\x00", 1),
+                pytest.param("\x05", 2),
+                pytest.param(b"\x00", 3),
+                pytest.param(b"\x05", 4),
+            ],
+        )
+        assert result == ["\\x00-1", "\\x05-2", "\\x00-3", "\\x05-4"]
+
     def test_idmaker_enum(self):
         from _pytest.python import idmaker