Fix string formatting not supporting strftime format

Author: sdispater

CHANGELOG.md

@@ -9,6 +9,7 @@
 - Fixed errors on some systems when retrieving timezone from clock files.
 - Fixed parsing of partial time.
 - Fixed parsing not raising an error for week 53 for ordinary years.
+- Fixed string formatting not supporting `strftime` format.
 
 
 ## [2.0.1] - 2018-05-10

pendulum/mixins/default.py

@@ -31,6 +31,9 @@ def for_json(self):
 
     def __format__(self, format_spec):
         if len(format_spec) > 0:
+            if '%' in format_spec:
+                return self.strftime(format_spec)
+
             return self.format(format_spec)
 
         return str(self)

tests/date/test_strings.py

@@ -44,4 +44,4 @@ def test_format():
     d = pendulum.Date(1975, 12, 25)
     assert '{}'.format(d) == '1975-12-25'
     assert '{:YYYY}'.format(d) == '1975'
-    assert '{:%Y}'.format(d) == '%1975'
+    assert '{:%Y}'.format(d) == '1975'

tests/datetime/test_strings.py

@@ -135,4 +135,6 @@ def test_format():
     d = pendulum.datetime(1975, 12, 25, 14, 15, 16, tz='Europe/Paris')
     assert '{}'.format(d) == '1975-12-25T14:15:16+01:00'
     assert '{:YYYY}'.format(d) == '1975'
-    assert '{:%Y}'.format(d) == '%1975'
+    assert '{:%Y}'.format(d) == '1975'
+    assert '{:%H:%M %d.%m.%Y}'.format(d) == '14:15 25.12.1975'
+