Flask-RESTful - don't return object property instead of returning null

I would use the marshal function instead of the marshal_with decorator:

class ClientList(Resource):    def get(self):       clients = []       for client in model.Client.query.all():           if client.email:               clients.append(marshal(client_fields))           else:               clients.append(marshal(client_fields_no_email))       return clients

Or even better

class ClientList(Resource):    def get(self):       return [client_marshal(client) for client in model.Client.query.all()]

with

def client_marshal(client):    if client.email:        return {'id' : fields.String,                'name' : fields.String,                'email' : fields.String}    else:        return {'id' : fields.String,                'name' : fields.String}

There are two ways it can be done, pre-marshalling and post-marshalling modification. Pre-marshalling removes any default values given to field names in the client_fields dict but post-marshalling preserves them.

In pre-marshalling method, you have to pass a modified fields dict to marshal function if client's email is None.
For example;

import jsonfrom flask_restful import fields, marshal, marshal_withclass Client(object):    def __init__(self, id, name, email=None):        self.id = id        self.name = name        self.email = emailclient_fields = {    'id': fields.String,    'name': fields.String,    'email': fields.String}def get():    clients =[Client(1, 'Tom'), Client(2, 'John', 'john@example.com')]    return [marshal(client, client_fields if client.email else {k: v for k, v in client_fields.items() if k != 'email'}) for client in clients]print(json.dumps(get()))

Output;

[{"id": "1", "name": "Tom"}, {"email": "john@example.com", "id": "2", "name": "John"}]

In post-marshalling you have to remove the email field of the OrderedDict returned by marshal_with if it is None.
The de_none function by default removes all fields those are None or you have to explicitly pass the field names if that's not desired and you have to pass envelope argument too if marshal_with takes the same.

from functools import wrapsimport jsonfrom flask_restful import fields, marshal, marshal_withclient_fields = {    'id': fields.String,    'name': fields.String,    #'email': fields.String(default='user@example.com')    'email': fields.String}class Client(object):    def __init__(self, id, name, email=None):        self.id = id        self.name = name        self.email = emaildef de_none(envelope=None, *fields):    def decorator(func):        def dict_remove(d):            if fields:                for field in fields:                    if d[field] is None:                        d.pop(field)            else:                for k, v in d.items():                   if v is None:                       d.pop(k)        @wraps(func)        def decorated(*args, **kwargs):            data = result = func(*args, **kwargs)            if isinstance(result, tuple):                data = result[0]            if envelope:                data = data[envelope]            if isinstance(data, (list, tuple)):                for d in data:                    dict_remove(d)            else:                dict_remove(data)            return result        return decorated    return decorator@de_none()@marshal_with(client_fields)def get():    #return [Client(1, 'Tom'), Client(2, 'john', 'john@example.com')], 200, {'Etag': 'blah'}    #return [Client(1, 'Tom'), Client(2, 'john', 'john@example.com')]    #return Client(1, 'Tom'), 200, {'Etag': 'foo'}    return Client(1, 'Tom')print(json.dumps(get()))@de_none()@marshal_with(client_fields)def get():    return Client(2, 'John', 'john@example.com'), 201, {'Etag': 'ok'}print(json.dumps(get()))

Output;

{"id": "1", "name": "Tom"}{"email": "john@example.com", "id": "2", "name": "John"}

UPDATE Request hooks
The app.after_request decorator can be used to modify response object. Any default values given to fields are preserved.
The remove_none_fields request hook takes fields parameter which can be None to remove all fields with None value or a list of field names to selectively remove.

import jsonfrom flask import Flask, Responsefrom flask_restful import fields, marshal_with, Api, Resourceapp = Flask(__name__)api = Api(app)class Client(object):    def __init__(self, id, name, email=None):        self.id = id        self.name = name        self.email = emailclient_fields = {    'id': fields.String,    'name': fields.String,    'email': fields.String,    'age': fields.String}class ClientList(Resource):    @marshal_with(client_fields)    def get(self):        clients =[Client(1, 'Tom'), Client(2, 'John', 'john@example.com')]        return clients, 200@app.after_requestdef remove_none_fields(resp, fields=('email',)):    """    removes all None fields    """    if not 'application/json' in resp.content_type:        return resp    def dict_remove(d, fields):        if fields:            for field in fields:                if d[field] is None:                    d.pop(field)        else:            for k, v in tuple(d.items()):                if v is None:                    d.pop(k)    data = json.loads(resp.get_data())    if isinstance(data, list):        for obj in data:            dict_remove(obj, fields)    else:        dict_remove(data, fields)    resp.set_data(json.dumps(data, indent=1))    resp.content_length = resp.calculate_content_length()    return respapi.add_resource(ClientList, '/')if __name__ == '__main__':    app.run(debug=True)

Output;

[ {  "age": null,  "name": "Tom",  "id": "1" }, {  "age": null,  "email": "john@example.com",  "name": "John",  "id": "2" }]

update patching flask_restful.marshal
I filter out None values in a genexp inside marshal function and replace flask_restful.marshal with marshal defined here.

from collections import OrderedDictfrom flask import Flaskimport flask_restfulfrom flask_restful import fields, marshal_with, Api, Resourceapp = Flask(__name__)api = Api(app)class Client(object):    def __init__(self, id, name, email=None):        self.id = id        self.name = name        self.email = emailclient_fields = {    'id': fields.String,    'name': fields.String,    'email': fields.String,}def marshal(data, fields, envelope=None):    def make(cls):        if isinstance(cls, type):            return cls()        return cls    if isinstance(data, (list, tuple)):        return (OrderedDict([(envelope, [marshal(d, fields) for d in data])])                if envelope else [marshal(d, fields) for d in data])    items = ((k, marshal(data, v) if isinstance(v, dict)              else make(v).output(k, data))             for k, v in fields.items())    #filtering None    items = ((k,v) for k, v in items if v is not None)    return OrderedDict([(envelope, OrderedDict(items))]) if envelope else OrderedDict(items)flask_restful.marshal = marshalclass ClientList(Resource):    @marshal_with(client_fields)    def get(self):        clients =[Client(1, 'Tom'), Client(2, 'John', 'john@example.com')]        return clients, 200api.add_resource(ClientList, '/')if __name__ == '__main__':    app.run(debug=True)

Output;

[       {           "id": "1",        "name": "Tom"    },    {           "email": "john@example.com",        "id": "2",        "name": "John"    }]

You should use the skip_none property of the @marshal decorator. Its much more convenient than the approaches suggested in the other answers.

@marshal(some_model, skip_none=True)def get():   ...

The documentation can be found here: https://flask-restplus.readthedocs.io/en/stable/marshalling.html

This is also possible using Flask Restx :)