It should be assert req['status'] == 0, i. e. comparison (==) instead of assignment (=) and 0 as integer not "0" as string (not entirely sure about the latter).

Example Python script returns a boolean if a string is valid json:

import json

def is_json(myjson):
  try:
    json.loads(myjson)
  except ValueError as e:
    return False
  return True

Which prints:

print is_json("{}")                          #prints True
print is_json("{asdf}")                      #prints False
print is_json('{ "age":100}')                #prints True
print is_json("{'age':100 }")                #prints False
print is_json("{\"age\":100 }")              #prints True
print is_json('{"age":100 }')                #prints True
print is_json('{"foo":[5,6.8],"foo":"bar"}') #prints True

Convert a JSON string to a Python dictionary:

import json
mydict = json.loads('{"foo":"bar"}')
print(mydict['foo'])    #prints bar

mylist = json.loads("[5,6,7]")
print(mylist)
[5, 6, 7]

Convert a python object to JSON string:

foo = {}
foo['gummy'] = 'bear'
print(json.dumps(foo))           #prints {"gummy": "bear"}

If you want access to low-level parsing, don't roll your own, use an existing library: http://www.json.org/

Great tutorial on python JSON module: https://pymotw.com/2/json/

Is String JSON and show syntax errors and error messages:

sudo cpan JSON::XS
echo '{"foo":[5,6.8],"foo":"bar" bar}' > myjson.json
json_xs -t none < myjson.json

Prints:

, or } expected while parsing object/hash, at character offset 28 (before "bar}
at /usr/local/bin/json_xs line 183, <STDIN> line 1.

json_xs is capable of syntax checking, parsing, prittifying, encoding, decoding and more:

https://metacpan.org/pod/json_xs

self.assertContains(result, "abcd")

You can modify it to work with json.

Use self.assertContains only for HttpResponse objects. For other objects, use self.assertIn.