How to create HTTP-request data with {single key:multi values}

Question

I need to send HTTP POST request with data as follow:

data = {'id[]': '1', 'id[]': '2', 'id[]': '3'}

Values list is actually unknown, but let it be values_list = ['1', '2', '3']

Of course if to try

for value in values_list:
    data["id[]"] = value

I get {'id[]': '3'} as key-value pair will be overwritten on each iteration...

I used this solution:

data = {}

class data_keys(object):
    def __init__(self, data_key):
        self.data_key = data_key

for value in values_list:
    data[data_keys('id[]')] = value

But my data looks like

{<__main__.data_keys object at 0x0000000004BAE518>: '2',
 <__main__.data_keys object at 0x0000000004BAED30>: '1',
 <__main__.data_keys object at 0x0000000004B9C748>: '3'}

What is wrong with my code? How else can I simply create dict with single key?

UPDATED

This how my HTTP request looks like:

requests.post(url, data={"id[]": '1', "id[]": '2', "id[]": '3'}, auth=HTTPBasicAuth(user_name, user_passw))

Title updated

Answer 1

While you can hack dictionary keys in order to allow seemingly “equal” keys, this is probably not a good idea, as this relies on the implementation detail on how the key is transformed into a string. Furthermore, it will definitely cause confusion if you ever need to debug this situation.

A much easier and supported solution is actually built into the form data encode mechanism: You can simply pass a list of values:

data = {
    'id[]': ['1', '2', '3']
}

req = requests.get(url='http://www.example.com', params=data)
print(req.url) # 'http://www.example.com/?id%5B%5D=1&id%5B%5D=2&id%5B%5D=3'

So you can just pass your values_list directly into the dictionary and everything will work properly without having to hack anything.

---

And if you find yourself in a situation where you think such a dictionary does not work, you can also supply an iterable of two-tuples (first value being the key, second the value):

data = [
    ('id[]', '1'),
    ('id[]', '2'),
    ('id[]', '3')
]

req = requests.get(url='http://www.example.com', params=data)
print(req.url) # 'http://www.example.com/?id%5B%5D=1&id%5B%5D=2&id%5B%5D=3'

Answer 2

In order to make your data_keys class do what you want you need to give it an appropriate __repr__ or __str__ method. This will allow the class instances to be displayed in the desired fashion when you print the dict, or when some other code tries to serialize it.

Here's a short demo that uses the 3rd party requests module to build a URL. I've changed the class name to make it conform to the usual Python class naming convention. This code was tested on Python 2.6 and Python 3.6

from __future__ import print_function
import requests

class DataKey(object):
    def __init__(self, data_key):
        self.data_key = data_key

    def __repr__(self):
        return str(self.data_key)

data = {}
values_list = ['1', '2', '3']

for value in values_list:
    data[DataKey('id[]')] = value

print(data)

req = requests.get(url='http://www.example.com', params=data)
print(req.url)

output

{id[]: '1', id[]: '2', id[]: '3'}
http://www.example.com/?id%5B%5D=1&id%5B%5D=2&id%5B%5D=3

---

Here's a more robust version inspired by Rogalski's answer that is acceptable to the json module, .

from __future__ import print_function
import requests
import json

class DataKey(str):
    def __init__(self, data_key):
        self.data_key = data_key

    def __repr__(self):
        return str(self.data_key)

    def __eq__(self, other):
        return self is other

    def __hash__(self):
        return id(self)

data = {}
values_list = ['1', '2', '3']

for value in values_list:
    data[DataKey('id[]')] = value

print(data)

req = requests.get(url='http://www.example.com', params=data)
print(req.url)

print(json.dumps(data))

output

{id[]: '3', id[]: '1', id[]: '2'}
http://www.example.com/?id%5B%5D=3&id%5B%5D=1&id%5B%5D=2
{"id[]": "3", "id[]": "1", "id[]": "2"}

As I mentioned in my comment on the question, this is weird, and creating dictionaries with multiple (pseudo)identical keys is really not a very useful thing to do. There will almost always be a far better approach, eg using a dict of lists or tuples, or as in this case, an alternative way of supplying the data, as shown in Poke's answer.

However, I should mention that multiple identical keys are not prohibited in JSON objects, and so it may occasionally be necessary to deal with such JSON data. I'm not claiming that using one of these crazy dictionaries is a good way to do that but it is a possibility...

Answer 3

Don't do it in real code, but basic structure would be something like:

data = {}
values_list = [1,2,3]

class data_keys(str):
    unique = 0
    def __init__(self, val):
        super(data_keys, self).__init__(val)
        self.unique += 1
        self.my_hash = self.unique
    def __eq__(self, other):
        return False
    def __hash__(self):
        return self.my_hash


for value in values_list:
    data[data_keys('id[]')] = value

print json.dumps(data)
# '{"id[]": 1, "id[]": 3, "id[]": 2}'

As you can see, key objects has to inherit from basestring class. I've used str. Also, there is a dirty hack (class variable) to ensure unique hashes and inequality between any of pair of data_keys.

Only proof of concept, it certainly may be done better.