improve docs about snake case and camel case

A Marshmallow user I’m supporting writes:

I’m running into an issue with a marshmallow/swagger/javascript interaction. In javascript (and serialized json in general), field names are expected to be camelCase. In python they’re supposed to be snake_case. So to work around this, I need to dump/load in python to do snake_case to camelCase translation.

Here is what we came up with:

from marshmallow import Schema as OriginalSchema, fields

def camel_from_snake(s):
    parts = iter(s.split("_"))
    return next(parts) + "".join(i.title() for i in parts)

class Schema(OriginalSchema):
    """Override marshmallow.Schema for snake_case/camelCase marshalling."""

    def on_bind_field(self, field_name, field_obj):
        # field_obj.data_key = camel_from_snake(field_name)  # TODO: marshmallow v3
        field_obj.load_from = camel_from_snake(field_name)
        field_obj.dump_to = camel_from_snake(field_name)

Searching the docs for “camelCase” currently only gives a single result in the “upgrading” section (in which, to be nitpicky, the example confuses PascalCase with camelCase).

This seems like such a common need that it deserves at least its own dedicated section in the docs, if not actual dedicated API support. Would you be open to a PR addressing this?

Thanks for your work on marshmallow!

[sloria]

See https://marshmallow.readthedocs.io/en/latest/examples.html#inflection-camel-casing-keys

[deckar01]

https://marshmallow.readthedocs.io/en/3.0/extending.html#pre-processing-and-post-processing-methods

You can make a schema mixin that converts the data to snakecase before loading it and converts it back to camelcase after dumping it.

from marshmallow import Schema, fields, pre_load, post_dump
from stringcase import camelcase, snakecase

class JavaScriptMixin:
    @pre_load
    def to_snakecase(self, data, **kwargs):
        return {snakecase(key): value for key, value in data.items()}
    
    @post_dump
    def to_camelcase(self, data, **kwargs):
        return {camelcase(key): value for key, value in data.items()}

class TestSchema(Schema, JavaScriptMixin):
    foo_bar = fields.String()
    boo_baz = fields.String()

schema = TestSchema()
data = schema.load({'fooBar': 'test', 'booBaz': 'done'})
# {'foo_bar': 'test', 'boo_baz': 'done'}
schema.dump(data)
# {'fooBar': 'test', 'booBaz': 'done'}