feat: Add serde_skip_default field attribute

A shorthand of `skip_if=lambda v: v == <default_value>`.

Author: yukinarit

docs/features/conditional-skip.md

@@ -1,12 +1,14 @@
 # Conditional Skip
 
-If you conditionally skip some fields, you can pass function or lambda in `serde_skip_if`.
+## `skip_if`
+
+If you conditionally skip some fields, you can pass function or lambda in `skip_if`.
 
 ```python
 @serde
 class World:
     player: str
-    buddy: str = field(default='', metadata={'serde_skip_if': lambda v: v == 'Pikachu'})
+    buddy: str = field(default='', skip_if=lambda v: v == 'Pikachu')
 
 world = World('satoshi', 'Pikachu')
 print(to_json(world))
@@ -22,4 +24,12 @@ As you can see below, field is skipped in serialization if `buddy` is "Pikachu".
 {"player": "green", "buddy": "Charmander"}
 ```
 
+## `skip_if_false`
+
+`skip_if_false` is a shorthand of `skip_if=lambda v: not v`.
+
+## `skip_if_default`
+
+`skip_if_false` is a shorthand of `skip_if=lambda v: v == <default_value>`.
+
 For complete example, please see [examples/skip.py](https://github.com/yukinarit/pyserde/blob/master/examples/skip.py)

examples/skip.py

@@ -26,6 +26,7 @@ class World:
     player: str
     enemies: List[str] = field(default_factory=list, skip_if_false=True)
     buddy: str = field(default='', skip_if=lambda v: v == 'Pikachu')
+    town: str = field(default='Masara Town', skip_if_default=True)
 
 
 def main():
@@ -35,10 +36,12 @@ def main():
     ]
     print(to_json(resources))
 
+    # "buddy" and "town" field will be omitted
     world = World('satoshi', ['Rattata', 'Pidgey'], 'Pikachu')
     print(to_json(world))
 
-    world = World('green', [], 'Charmander')
+    # "enemies" field will be omitted
+    world = World('green', [], 'Charmander', 'Black City')
     print(to_json(world))
 
 

serde/core.py

@@ -4,6 +4,7 @@
 import dataclasses
 import datetime
 import decimal
+import functools
 import ipaddress
 import logging
 import pathlib
@@ -268,6 +269,10 @@ def skip_if_false(v):
     return not bool(v)
 
 
+def skip_if_default(v, default=None):
+    return v == default
+
+
 @dataclass
 class FlattenOpts:
     """
@@ -280,7 +285,8 @@ def field(
     rename: Optional[str] = None,
     skip: Optional[bool] = None,
     skip_if: Optional[Callable] = None,
-    skip_if_false: Optional[Callable] = None,
+    skip_if_false: Optional[bool] = None,
+    skip_if_default: Optional[bool] = None,
     serializer=None,
     deserializer=None,
     flatten: Optional[FlattenOpts] = None,
@@ -301,6 +307,8 @@ def field(
         metadata["serde_skip_if"] = skip_if
     if skip_if_false is not None:
         metadata["serde_skip_if_false"] = skip_if_false
+    if skip_if_default is not None:
+        metadata["serde_skip_if_default"] = skip_if_default
     if serializer:
         metadata["serde_serializer"] = serializer
     if deserializer:
@@ -355,6 +363,9 @@ class Foo:
     * `skip_if_false` (Attribute name: `serde_skip_if_false`) skips (de)serialization if the field value evaluates
     to `False`. For example, this code skip (de)serialize `v` if `v` is empty.
 
+    * `skip_if_default` (Attribute name: `serde_skip_if_default`) skips (de)serialization if the field value is equal
+    to the default value
+
     ```python
     @deserialize
     @serialize
@@ -387,6 +398,7 @@ class Foo:
     skip: Optional[bool] = None
     skip_if: Optional[Func] = None
     skip_if_false: Optional[bool] = None
+    skip_if_default: Optional[bool] = None
     serializer: Optional[Func] = None  # Custom field serializer.
     deserializer: Optional[Func] = None  # Custom field deserializer.
     flatten: Optional[FlattenOpts] = None
@@ -398,7 +410,12 @@ def from_dataclass(cls, f: dataclasses.Field) -> 'Field':
         """
         skip_if_false_func: Optional[Func] = None
         if f.metadata.get('serde_skip_if_false'):
-            skip_if_false_func = Func(skip_if_false, cls.mangle(f, 'skip_if'))
+            skip_if_false_func = Func(skip_if_false, cls.mangle(f, 'skip_if_false'))
+
+        skip_if_default_func: Optional[Func] = None
+        if f.metadata.get('serde_skip_if_default'):
+            skip_if_def = functools.partial(skip_if_default, default=f.default)
+            skip_if_default_func = Func(skip_if_def, cls.mangle(f, 'skip_if_default'))
 
         skip_if: Optional[Func] = None
         if f.metadata.get('serde_skip_if'):
@@ -432,7 +449,7 @@ def from_dataclass(cls, f: dataclasses.Field) -> 'Field':
             metadata=f.metadata,
             rename=f.metadata.get('serde_rename'),
             skip=f.metadata.get('serde_skip'),
-            skip_if=skip_if or skip_if_false_func,
+            skip_if=skip_if or skip_if_false_func or skip_if_default_func,
             serializer=serializer,
             deserializer=deserializer,
             flatten=flatten,

tests/test_basics.py

@@ -458,6 +458,21 @@ class Foo:
     assert ff.comments == []
 
 
+@pytest.mark.parametrize('se,de', all_formats)
+def test_skip_if_default(se, de):
+    @serde.serde
+    class Foo:
+        a: str = serde.field(default='foo', skip_if_default=True)
+
+    f = Foo()
+    assert f == de(Foo, se(f))
+
+    assert serde.to_dict(Foo()) == {}
+    assert serde.to_dict(Foo('bar')) == {'a': 'bar'}
+    assert serde.from_dict(Foo, {}) == Foo()
+    assert serde.from_dict(Foo, {'a': 'bar'}) == Foo('bar')
+
+
 @pytest.mark.parametrize('se,de', format_msgpack)
 def test_inheritance(se, de):
     @serde.serde