Add TimestampTemplate config type

No more crash when 'completion->timestamp-format' is set to '%'.

Fixes #1039.

qutebrowser/config/configdata.py

@@ -381,7 +381,7 @@ def data(readonly=False):
              "What to display in the download filename input."),
 
             ('timestamp-format',
-             SettingValue(typ.String(none_ok=True), '%Y-%m-%d'),
+             SettingValue(typ.TimestampTemplate(none_ok=True), '%Y-%m-%d'),
              "How to format timestamps (e.g. for history)"),
 
             ('show',

qutebrowser/config/configtypes.py

@@ -27,6 +27,7 @@ import os.path
 import itertools
 import collections
 import warnings
+import datetime
 
 from PyQt5.QtCore import QUrl
 from PyQt5.QtGui import QColor, QFont
@@ -1630,3 +1631,25 @@ class URLSegmentList(FlagList):
     """A list of URL segments."""
 
     valid_values = ValidValues('host', 'path', 'query', 'anchor')
+
+
+class TimestampTemplate(BaseType):
+
+    """A strftime-like template for timestamps.
+
+    See
+    https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior
+    for reference.
+    """
+
+    def validate(self, value):
+        self._basic_validation(value)
+        if not value:
+            return
+        try:
+            # Dummy check to see if the template is valid
+            datetime.datetime.now().strftime(value)
+        except ValueError as error:
+            # thrown on invalid template string
+            raise configexc.ValidationError(
+                value, "Invalid format string: {}".format(error))

tests/unit/config/test_configtypes.py

@@ -2042,6 +2042,24 @@ class TestUserAgent:
         klass().complete()
 
 
+class TestTimestampTemplate:
+
+    """Test TimestampTemplate."""
+
+    @pytest.fixture
+    def klass(self):
+        return configtypes.TimestampTemplate
+
+    @pytest.mark.parametrize('val', ['', 'foobar', '%H:%M', 'foo %H bar %M'])
+    def test_validate_valid(self, klass, val):
+        klass(none_ok=True).validate(val)
+
+    @pytest.mark.parametrize('val', ['', '%'])
+    def test_validate_invalid(self, klass, val):
+        with pytest.raises(configexc.ValidationError):
+            klass().validate(val)
+
+
 @pytest.mark.parametrize('first, second, equal', [
     (re.compile('foo'), RegexEq('foo'), True),
     (RegexEq('bar'), re.compile('bar'), True),