Serializer Hooks

Note

To customize the load or dump process for annotated types instead of individual fields, please see the Type Hooks section.

You can optionally add hooks that are run before a JSON string or a Python dict object is loaded to a dataclass instance, or before the dataclass instance is converted back to a Python dict object.

To customize the load process:

• To pre-process data before from_dict is called, simply implement a _pre_from_dict method which will be called whenever you invoke the from_dict or from_json methods. Please note that this will pass in the original dict object, so updating any values will affect data in the underlying dict (this might change in a future revision).

• To post-process data, after a dataclass instance is de-serialized, simply implement the __post_init__ method which will be run by the dataclass decorator.

To customize the dump process, simply implement a _pre_dict method which will be called whenever you invoke the to_dict or to_json methods. Please note that this will pass in the original dataclass instance, so updating any values will affect the fields of the underlying dataclass (this might change in a future revision).

A simple example to illustrate both approaches is shown below:

from dataclasses import dataclass
from dataclass_wizard import JSONWizard
from dataclass_wizard.type_def import JSONObject


@dataclass
class MyClass(JSONWizard):
    my_str: str
    my_int: int
    my_bool: bool = False

    def __post_init__(self):
        self.my_str = self.my_str.title()
        self.my_int *= 2

    @classmethod
    def _pre_from_dict(cls, o: JSONObject) -> JSONObject:
        # o = o.copy()  # Copying the `dict` object is optional
        o['my_bool'] = True  # Adds a new key/value pair
        return o

    def _pre_dict(self):
        self.my_str = self.my_str.swapcase()


data = {"my_str": "my string", "myInt": "10"}

c = MyClass.from_dict(data)
print(repr(c))
# prints:
#   MyClass(my_str='My String', my_int=20, my_bool=True)

string = c.to_json()
print(string)
# prints:
#   {"myStr": "mY sTRING", "myInt": 20, "myBool": true}