Add ability to specify output format for DurationField

docs/api-guide/fields.md

@@ -377,13 +377,16 @@ A Duration representation.
 Corresponds to `django.db.models.fields.DurationField`
 
 The `validated_data` for these fields will contain a `datetime.timedelta` instance.
-The representation is a string following this format `'[DD] [HH:[MM:]]ss[.uuuuuu]'`.
 
-**Signature:** `DurationField(max_value=None, min_value=None)`
+**Signature:** `DurationField(format=api_settings.DURATION_FORMAT, max_value=None, min_value=None)`
 
+* `format` - A string representing the output format.  If not specified, this defaults to the same value as the `DURATION_FORMAT` settings key, which will be `'django'` unless set. Formats are described below. Setting this value to `None` indicates that Python `timedelta` objects should be returned by `to_representation`. In this case the date encoding will be determined by the renderer.
 * `max_value` Validate that the duration provided is no greater than this value.
 * `min_value` Validate that the duration provided is no less than this value.
 
+#### `DurationField` formats
+Format may either be the special string `'iso-8601'`, which indicates that [ISO 8601][iso8601] style intervals should be used (eg `'P4DT1H15M20S'`), or `'django'` which indicates that Django interval format `'[DD] [HH:[MM:]]ss[.uuuuuu]'` should be used (eg: `'4 1:15:20'`).
+
 ---
 
 # Choice selection fields

docs/api-guide/settings.md

@@ -314,6 +314,15 @@ May be a list including the string `'iso-8601'` or Python [strftime format][strf
 
 Default: `['iso-8601']`
 
+
+#### DURATION_FORMAT
+
+A format string that should be used by default for rendering the output of `DurationField` serializer fields.  If `None`, then `DurationField` serializer fields will return Python `timedelta` objects, and the duration encoding will be determined by the renderer.
+
+May be any of `None`, `'iso-8601'` or `'standard'` (the format accepted by `django.utils.dateparse.parse_duration`).
+
+Default: `'standard'`
+
 ---
 
 ## Encodings

rest_framework/__init__.py

@@ -21,6 +21,7 @@
 
 # Default datetime input and output formats
 ISO_8601 = 'iso-8601'
+DJANGO_DURATION_FORMAT = 'django'
 
 
 class RemovedInDRF317Warning(PendingDeprecationWarning):

rest_framework/fields.py

@@ -24,7 +24,7 @@
 from django.utils.dateparse import (
     parse_date, parse_datetime, parse_duration, parse_time
 )
-from django.utils.duration import duration_string
+from django.utils.duration import duration_iso_string, duration_string
 from django.utils.encoding import is_protected_type, smart_str
 from django.utils.formats import localize_input, sanitize_separators
 from django.utils.ipv6 import clean_ipv6_address
@@ -35,7 +35,7 @@
 except ImportError:
     pytz = None
 
-from rest_framework import ISO_8601
+from rest_framework import DJANGO_DURATION_FORMAT, ISO_8601
 from rest_framework.compat import ip_address_validators
 from rest_framework.exceptions import ErrorDetail, ValidationError
 from rest_framework.settings import api_settings
@@ -1351,9 +1351,17 @@ class DurationField(Field):
         'overflow': _('The number of days must be between {min_days} and {max_days}.'),
     }
 
-    def __init__(self, **kwargs):
+    def __init__(self, *, format=empty, **kwargs):
         self.max_value = kwargs.pop('max_value', None)
         self.min_value = kwargs.pop('min_value', None)
+        if format is not empty:
+            if format is None or (isinstance(format, str) and format.lower() in (ISO_8601, DJANGO_DURATION_FORMAT)):
+                self.format = format
+            else:
+                raise ValueError(
+                    f"Unknown duration format provided, got '{format}'"
+                    " while expecting 'django', 'iso-8601' or `None`."
+                )
         super().__init__(**kwargs)
         if self.max_value is not None:
             message = lazy_format(self.error_messages['max_value'], max_value=self.max_value)
@@ -1376,7 +1384,22 @@ def to_internal_value(self, value):
         self.fail('invalid', format='[DD] [HH:[MM:]]ss[.uuuuuu]')
 
     def to_representation(self, value):
-        return duration_string(value)
+        output_format = getattr(self, 'format', api_settings.DURATION_FORMAT)
+
+        if output_format is None:
+            return value
+
+        if isinstance(output_format, str):
+            if output_format.lower() == ISO_8601:
+                return duration_iso_string(value)
+
+            if output_format.lower() == DJANGO_DURATION_FORMAT:
+                return duration_string(value)
+
+        raise ValueError(
+            f"Unknown duration format provided, got '{output_format}'"
+            " while expecting 'django', 'iso-8601' or `None`."
+        )
 
 
 # Choice types...

rest_framework/settings.py

@@ -24,7 +24,7 @@
 from django.core.signals import setting_changed
 from django.utils.module_loading import import_string
 
-from rest_framework import ISO_8601
+from rest_framework import DJANGO_DURATION_FORMAT, ISO_8601
 
 DEFAULTS = {
     # Base API policies
@@ -109,6 +109,8 @@
     'TIME_FORMAT': ISO_8601,
     'TIME_INPUT_FORMATS': [ISO_8601],
 
+    'DURATION_FORMAT': DJANGO_DURATION_FORMAT,
+
     # Encoding
     'UNICODE_JSON': True,
     'COMPACT_JSON': True,

tests/test_fields.py

@@ -1772,9 +1772,64 @@ class TestDurationField(FieldValues):
     }
     field = serializers.DurationField()
 
+    def test_invalid_format(self):
+        with pytest.raises(ValueError) as exc_info:
+            serializers.DurationField(format='unknown')
+        assert str(exc_info.value) == (
+            "Unknown duration format provided, got 'unknown'"
+            " while expecting 'django', 'iso-8601' or `None`."
+        )
+        with pytest.raises(ValueError) as exc_info:
+            serializers.DurationField(format=123)
+        assert str(exc_info.value) == (
+            "Unknown duration format provided, got '123'"
+            " while expecting 'django', 'iso-8601' or `None`."
+        )
 
-# Choice types...
+    @override_settings(REST_FRAMEWORK={'DURATION_FORMAT': 'unknown'})
+    def test_invalid_format_in_config(self):
+        field = serializers.DurationField()
+
+        with pytest.raises(ValueError) as exc_info:
+            field.to_representation(datetime.timedelta(days=1))
+
+        assert str(exc_info.value) == (
+            "Unknown duration format provided, got 'unknown'"
+            " while expecting 'django', 'iso-8601' or `None`."
+        )
+
+
+class TestNoOutputFormatDurationField(FieldValues):
+    """
+    Values for `DurationField` with a no output format.
+    """
+    valid_inputs = {}
+    invalid_inputs = {}
+    outputs = {
+        datetime.timedelta(1): datetime.timedelta(1)
+    }
+    field = serializers.DurationField(format=None)
+
+
+class TestISOOutputFormatDurationField(FieldValues):
+    """
+    Values for `DurationField` with a custom output format.
+    """
+    valid_inputs = {
+        '13': datetime.timedelta(seconds=13),
+        'P3DT08H32M01.000123S': datetime.timedelta(days=3, hours=8, minutes=32, seconds=1, microseconds=123),
+        'PT8H1M': datetime.timedelta(hours=8, minutes=1),
+        '-P999999999D': datetime.timedelta(days=-999999999),
+        'P999999999D': datetime.timedelta(days=999999999)
+    }
+    invalid_inputs = {}
+    outputs = {
+        datetime.timedelta(days=3, hours=8, minutes=32, seconds=1, microseconds=123): 'P3DT08H32M01.000123S'
+    }
+    field = serializers.DurationField(format='iso-8601')
 
+
+# Choice types...
 class TestChoiceField(FieldValues):
     """
     Valid and invalid values for `ChoiceField`.