When range checking was added to time.strftime() a check was placed on tm_isdst

to make sure it fell within [-1, 1] just in case someone implementing
strftime() in libc was stupid enough to assume this. Turns out, though, some
OSs (e.g. zOS) are stupid enough to use values outside of this range for time
structs created by the system itself. So instead of throwing a ValueError,
tm_isdst is now normalized before being passed to strftime().

Fixes issue #6823. Thanks Robert Shapiro for diagnosing the problem and
contributing an initial patch.

Lib/test/test_time.py

@@ -87,11 +87,6 @@ class TimeTestCase(unittest.TestCase):
                             (1900, 1, 1, 0, 0, 0, 0, -1, -1))
         self.assertRaises(ValueError, time.strftime, '',
                             (1900, 1, 1, 0, 0, 0, 0, 367, -1))
-        # Check daylight savings flag [-1, 1]
-        self.assertRaises(ValueError, time.strftime, '',
-                            (1900, 1, 1, 0, 0, 0, 0, 1, -2))
-        self.assertRaises(ValueError, time.strftime, '',
-                            (1900, 1, 1, 0, 0, 0, 0, 1, 2))
 
     def test_default_values_for_zero(self):
         # Make sure that using all zeros uses the proper default values.

Misc/ACKS

@@ -669,6 +669,7 @@ Jerry Seutter
 Denis Severson
 Ian Seyer
 Ha Shao
+Richard Shapiro
 Bruce Sherwood
 Pete Shinners
 Michael Shiplett

Misc/NEWS

@@ -1340,6 +1340,10 @@ C-API
 Extension Modules
 -----------------
 
+- Issue #6823: Allow time.strftime() to accept a tuple with a isdst field
+  outside of the range of [-1, 1] by normalizing the value to within that
+  range.
+
 - Issue #6877: Make it possible to link the readline extension to libedit
   on OSX.
 

Modules/timemodule.c

@@ -464,11 +464,13 @@ time_strftime(PyObject *self, PyObject *args)
             PyErr_SetString(PyExc_ValueError, "day of year out of range");
             return NULL;
         }
-        if (buf.tm_isdst < -1 || buf.tm_isdst > 1) {
-            PyErr_SetString(PyExc_ValueError,
-                            "daylight savings flag out of range");
-            return NULL;
-        }
+	/* Normalize tm_isdst just in case someone foolishly implements %Z
+	   based on the assumption that tm_isdst falls within the range of
+	   [-1, 1] */
+        if (buf.tm_isdst < -1)
+	    buf.tm_isdst = -1;
+	else if (buf.tm_isdst > 1)
+	    buf.tm_isdst = 1;
 
 #ifdef MS_WINDOWS
 	/* check that the format string contains only valid directives */