Merge pull request #237 from Unidata/issue236

Change ValueError to TypeError in datetime.__sub__ (closes issue #236)

Changelog

@@ -15,6 +15,7 @@ version 1.5.0 (not yet released)
  * remove legacy `utime` class, and legacy `JulianDayFromDate` and 
    `DateFromJulianDay` functions (replaced by `cftime.datetime.toordinal`
    and `cftime.datetime.fromordinal`).  PR #235.
+ * Change ValueError to TypeError in __sub__ (issue #236, PR #236).
 
 version 1.4.1 (release tag v1.4.1.rel)
 ======================================

src/cftime/_cftime.pyx

@@ -1366,11 +1366,11 @@ The default format of the string produced by strftime is controlled by self.form
             if isinstance(other, datetime):
                 # datetime - datetime
                 if dt.calendar != other.calendar:
-                    raise ValueError("cannot compute the time difference between dates with different calendars")
+                    raise TypeError("cannot compute the time difference between dates with different calendars")
                 if dt.calendar == "":
-                    raise ValueError("cannot compute the time difference between dates that are not calendar-aware")
+                    raise TypeError("cannot compute the time difference between dates that are not calendar-aware")
                 if dt.has_year_zero != other.has_year_zero:
-                    raise ValueError("cannot compute the time difference between dates with year zero conventions")
+                    raise TypeError("cannot compute the time difference between dates with year zero conventions")
                 ordinal_self = self.toordinal() # julian day
                 ordinal_other = other.toordinal()
                 days = ordinal_self - ordinal_other
@@ -1387,7 +1387,7 @@ Cannot compute the time difference between dates with different calendars.
 One of the datetime objects may have been converted to a native python
 datetime instance.  Try using only_use_cftime_datetimes=True when creating the
 datetime object."""
-                    raise ValueError(msg)
+                    raise TypeError(msg)
                 return dt._to_real_datetime() - other
             elif isinstance(other, timedelta):
                 # datetime - timedelta
@@ -1428,7 +1428,7 @@ Cannot compute the time difference between dates with different calendars.
 One of the datetime objects may have been converted to a native python
 datetime instance.  Try using only_use_cftime_datetimes=True when creating the
 datetime object."""
-                    raise ValueError(msg)
+                    raise TypeError(msg)
                 return self - other._to_real_datetime()
             else:
                 return NotImplemented

test/test_cftime.py

@@ -1284,12 +1284,9 @@ def invalid_sub_5():
         def invalid_sub_6():
             self.date3_gregorian - self.date3_gregorian_yearzero
 
-        for func in [invalid_sub_1, invalid_sub_2]:
+        for func in [invalid_sub_1, invalid_sub_2, invalid_sub_3, invalid_sub_4, invalid_sub_5, invalid_sub_6]:
             self.assertRaises(TypeError, func)
 
-        for func in [invalid_sub_3, invalid_sub_4, invalid_sub_5, invalid_sub_6] :
-            self.assertRaises(ValueError, func)
-
     def test_replace(self):
         self.assertEqual(self.date1_365_day.replace(year=4000).year, 4000)
         self.assertEqual(self.date1_365_day.replace(month=3).month, 3)