fix: Add dataclass decorator for all example code

@serde decorator works without @DataClass decorator but there is
no way to prevent mypy from producing type error. Adding @DataClass
decorator back to the all the example code.

Find related mypy page
https://mypy.readthedocs.io/en/stable/additional_features.html#caveats-known-issues

Author: yukinarit

Diff for: README.md

@@ -15,6 +15,7 @@ Declare a class with pyserde's `@serde` decorator.
 
 ```python
 @serde
+@dataclass
 class Foo:
     i: int
     s: str

Diff for: docs/features/attributes.md

@@ -15,9 +15,11 @@ Field attributes are options to customize (de)serialization behaviour specific t
 You can specify field attributes as keyword argument in `serde.field`. See [serde.core.Field](https://yukinarit.github.io/pyserde/api/serde/core.html#Field) for available field attributes. This is recommended way since pyserde v0.6. Use `dataclasses.field` below if you are using pyserde < v0.6.
 
 ```python
+from dataclasses import dataclass
 from serde import field, serde
 
 @serde
+@dataclass
 class Foo:
     class_name: str = field(rename='class')
 ```

Diff for: docs/features/case-conversion.md

@@ -4,6 +4,7 @@ Converting `snake_case` fields into supported case styles e.g. `camelCase` and `
 
 ```python
 @serde(rename_all = 'camelcase')
+@dataclass
 class Foo:
     int_field: int
     str_field: str

Diff for: docs/features/conditional-skip.md

@@ -6,6 +6,7 @@ If you conditionally skip some fields, you can pass function or lambda in `skip_
 
 ```python
 @serde
+@dataclass
 class World:
     player: str
     buddy: str = field(default='', skip_if=lambda v: v == 'Pikachu')

Diff for: docs/features/custom-class-serializer.md

@@ -16,6 +16,7 @@ def deserializer(cls, o):
         raise SerdeSkip()
 
 @serde(serializer=serializer, deserializer=deserializer)
+@dataclass
 class Foo:
     i: int
     dt1: datetime

Diff for: docs/features/custom-field-serializer.md

@@ -3,15 +3,17 @@
 If you want to provide a custom function to override the default (de)serialization behaviour of a field, you can pass your functions to `serializer` and `deserializer` in `serde.field`.
 
 ```python
+from dataclasses import dataclass
 from serde import serde, field
 
 @serde
+@dataclass
 class Foo:
     dt1: datetime
     dt2: datetime = field(
         serializer=lambda x: x.strftime('%d/%m/%y'), deserializer=lambda x: datetime.strptime(x, '%d/%m/%y')
     )
 ```
-`dt1` in the example will serialized into `2021-01-01T00:00:00` because the pyserde's default (de)serializier for datetime is ISO 8601. `dt2` field in the example will be serialized into `01/01/21` by the custom field serializer.
+`dt1` in the example will serialized into `2021-01-01T00:00:00` because the pyserde's default (de)serializer for datetime is ISO 8601. `dt2` field in the example will be serialized into `01/01/21` by the custom field serializer.
 
 For complete example, please see [examples/custom_field_serializer.py](https://github.com/yukinarit/pyserde/blob/master/examples/custom_field_serializer.py)

Diff for: docs/features/decorators.md

@@ -5,6 +5,7 @@
 @serde is a shortcut of @serialize and @deserialize decorators. This code
 ```python
 @serde
+@dataclass
 class Foo:
     ...
 ```
@@ -14,6 +15,7 @@ is equivalent to the following code.
 ```python
 @deserialize
 @serialize
+@dataclass
 class Foo:
     ...
 ```
@@ -26,18 +28,21 @@ class Foo:
 
 ```python
 @serde(serializer=serializer, deserializer=deserializer)
+@dataclass
 class Foo:
     ...
 ```
 
-* If you want to have extra attributes to the dataclass decorator, you can have both `@dataclass` and `@serde` decorators
+> **Note:** `@serde` actually works without @dataclass decorator, because it detects and add @dataclass to the declared class automatically. However, mypy will produce `Too many arguments` or `Unexpected keyword argument` error. This is due to the current mypy limitation. See the following documentation for more information.
+https://mypy.readthedocs.io/en/stable/additional_features.html#caveats-known-issues
+>
+>
+> ```python
+> @serde
+> class Foo:
+>     ...
+> ```
 
-```python
-@serde
-@dataclass(unsafe_hash=True)
-class Foo:
-    ...
-```
 
 ## `@serialize`/`@deserialize`
 

Diff for: docs/features/flatten.md

@@ -4,11 +4,13 @@ You can flatten the fields of the nested structure.
 
 ```python
 @serde
+@dataclass
 class Bar:
     c: float
     d: bool
 
 @serde
+@dataclass
 class Foo:
     a: int
     b: str

Diff for: docs/features/forward-reference.md

@@ -3,12 +3,14 @@
 You can use a forward reference in annotations.
 
 ```python
+@dataclass
 class Foo:
     i: int
     s: str
     bar: 'Bar'  # Specify type annotation in string.
 
 @serde
+@dataclass
 class Bar:
     f: float
     b: bool

Diff for: docs/features/postponed-evaluation-of-type-annotation.md

@@ -5,8 +5,10 @@
 ```python
 from __future__ import annotations
 from serde import serde
+from dataclasses import dataclass
 
 @serde
+@dataclass
 class Foo:
     i: int
     s: str

Diff for: docs/features/python3.9-type-hinting.md

@@ -4,6 +4,7 @@ For python >= 3.9, you can use [PEP585](https://www.python.org/dev/peps/pep-0585
 
 ```python
 @serde
+@dataclass
 class Foo:
     i: int
     l: list[str]

Diff for: docs/features/rename.md

@@ -4,6 +4,7 @@ In case you want to use a keyword as field such as `class`, you can use `serde_r
 
 ```python
 @serde
+@dataclass
 class Foo:
     class_name: str = field(metadata={'serde_rename': 'class'})
 

Diff for: docs/features/skip.md

@@ -4,6 +4,7 @@ You can skip serialization for a certain field, you can use `serde_skip`.
 
 ```python
 @serde
+@dataclass
 class Resource:
     name: str
     hash: str

Diff for: docs/features/union-operator.md

@@ -3,21 +3,25 @@
 For python >= 3.10, you can use [PEP604](https://www.python.org/dev/peps/pep-0604/) Union operator `X | Y`.
 
 ```python
+from dataclasses import dataclass
 from serde import serde
 from serde.json import from_json, to_json
 
 
 @serde
+@dataclass
 class Bar:
     v: int
 
 
 @serde
+@dataclass
 class Baz:
     v: float
 
 
 @serde
+@dataclass
 class Foo:
     a: int | str
     b: dict[str, int] | list[int]

Diff for: docs/features/union.md

@@ -8,14 +8,17 @@ This is the default Union representation for pyserde<0.7. Given these dataclasse
 
 ```python
 @serde
+@dataclass
 class Bar:
     b: int
 
 @serde
+@dataclass
 class Baz:
     b: int
 
 @serde(tagging=Untagged)
+@dataclass
 class Foo:
     a: Union[Bar, Baz]
 ```
@@ -28,6 +31,7 @@ This is the default Union representation since 0.7. A class declaration with `Ex
 
 ```
 @serde(tagging=ExternalTagging)
+@dataclass
 class Foo:
     a: Union[Bar, Baz]
 ```
@@ -36,6 +40,7 @@ class Foo:
 >
 > ```python
 > @serde(tagging=ExternalTagging)
+> @dataclass
 > class Foo:
 >    a: Union[List[int], Set[int]]
 > ```
@@ -46,6 +51,7 @@ A class declaration with `InternalTagging` looks like below. If you serialize `F
 
 ```python
 @serde(tagging=InternalTagging("type"))
+@dataclass
 class Foo:
     a: Union[Bar, Baz]
 ```
@@ -56,6 +62,7 @@ A class declaration with `AdjacentTagging` looks like below. If you serialize `F
 
 ```python
 @serde(tagging=AdjacentTagging("type", "content"))
+@dataclass
 class Foo:
     a: Union[Bar, Baz]
 ```

Diff for: docs/getting-started.md

@@ -18,12 +18,14 @@ Or all at once:
 pip install "pyserde[all]"
 ```
 
-Define your class with pyserde's `@serialize` and `@deserialize` decorators. Be careful that module name is `serde`, not `pyserde`. `pyserde` depends on `dataclasses` module. If you are new to dataclass, I would recommend to read [dataclasses documentation](https://docs.python.org/3/library/dataclasses.html) first.
+Define your class with pyserde's `@serde` decorators. Be careful that module name is `serde`, not `pyserde`. `pyserde` depends on `dataclasses` module. If you are new to dataclass, I would recommend to read [dataclasses documentation](https://docs.python.org/3/library/dataclasses.html) first.
 
 ```python
+from dataclasses import dataclass
 from serde import serde
 
 @serde
+@dataclass
 class Foo:
     i: int
     s: str
@@ -37,9 +39,11 @@ pyserde generates methods necessary for (de)serialization by `@serde` when a cla
 >
 > e.g. If you don't need deserialization functionality, you can add `@serialize` decorator only. But, calling deserialize API e.g. `from_json` for `Foo` will raise an error.
 > ```python
+> from dataclasses import dataclass
 > from serde import serialize
 >
 > @serialize
+> @dataclass
 > class Foo:
 >     i: int
 >     s: str

Diff for: docs/supported-data-formats.md

@@ -5,6 +5,7 @@ Currently `dict, `tuple`, `JSON`, `Yaml`, `Toml` and `MsgPack` are supported.
 ```python
 
 @serde
+@dataclass
 class Foo:
     i: int
     s: str

Diff for: docs/supported-types.md

@@ -19,10 +19,12 @@
 You can write pretty complex class like this:
 ```python
 @serde
+@dataclass
 class bar:
     i: int
 
 @serde
+@dataclass
 class Foo:
     i: int
     l: List[str]

Diff for: examples/any.py

@@ -1,15 +1,18 @@
+from dataclasses import dataclass
 from typing import Any
 
 from serde import serde
 from serde.json import from_json, to_json
 
 
 @serde
+@dataclass
 class Bar:
     v: float
 
 
 @serde
+@dataclass
 class Foo:
     a: Any
     b: Any

Diff for: examples/collection.py

@@ -1,4 +1,5 @@
 import sys
+from dataclasses import dataclass
 from typing import Dict, List, Tuple
 
 from serde import serde
@@ -8,6 +9,7 @@
 
 
 @serde
+@dataclass
 class Foo:
     l: List[str]
     t: Tuple[str, bool]
@@ -19,6 +21,7 @@ class Foo:
 if PY39:
 
     @serde
+    @dataclass
     class FooPy39:
         l: list[str]
         t: tuple[str, bool]

Diff for: examples/custom_class_serializer.py

@@ -1,3 +1,4 @@
+from dataclasses import dataclass
 from datetime import datetime
 
 from serde import SerdeSkip, default_deserializer, default_serializer, field, serde
@@ -28,6 +29,7 @@ def deserializer(cls, o):
 
 
 @serde(serializer=serializer, deserializer=deserializer)
+@dataclass
 class Foo:
     i: int
     dt1: datetime

Diff for: examples/custom_field_serializer.py

@@ -1,10 +1,12 @@
+from dataclasses import dataclass
 from datetime import datetime
 
 from serde import field, serde
 from serde.json import from_json, to_json
 
 
 @serde
+@dataclass
 class Foo:
     dt1: datetime
     dt2: datetime = field(

Diff for: examples/default.py

@@ -1,6 +1,7 @@
+from dataclasses import field
 from typing import Dict
 
-from serde import field, serde
+from serde import serde
 from serde.json import from_json, to_json
 
 

Diff for: examples/enum34.py

@@ -1,4 +1,5 @@
 import enum
+from dataclasses import dataclass
 
 import imported
 
@@ -24,6 +25,7 @@ class IE(enum.IntEnum):
 
 
 @serde
+@dataclass
 class Foo:
     v0: IE
     v1: IE = IE.V1  # Default enum value.