If ... else .... ignored when checking Json attribute existance

Question

I have the following Python3 code to check if a attribute is in a JSON (or has a value different than null)

def has_attribute(data, attribute):
    return (attribute in data) and (data[attribute] is not None)

This is the function which checks that the attribute is in the JSON and has a value different than null.

if has_attribute(json_request,"offer_id"):
        print("IT IS")
else:
        print("NO")

The thing is that when I pass a JSON, it doesn't print anything. Why?

This code is in a flask app.

I tested this code sending a json object (which is a simple json object like {"a":"a","b":"b"}) through Postman.
When the query is done it is received because I see the json that flask received but none of the prints from the if..else is printed.

Answer 1

JSON isn't a type in Python. It is either a string, or a dictionary.

For example, your code works fine when testing a dictionary

def has_attribute(data, attribute):
    return (attribute in data) and (data[attribute] is not None)

json_request = {'foo' : None, 'hello' : 'world'}

print("IT IS" if has_attribute(json_request, "foo") else "NO")  # NO
print("IT IS" if has_attribute(json_request, "bar") else "NO")  # NO
print("IT IS" if has_attribute(json_request, "hello") else "NO")  # IT IS

Likewise, it would also work if you did import json; json_request = json.loads(some_json_string) because that returns you a dictionary.

If you were testing a class, you would need to rewrite this using getattr() built-in function

class A:
  def __init__(self):
    self.x = None
    self.y = 'a'

  def has_attribute(self, attribute):
    return getattr(self, attribute, None) is not None

a = A()
print("IT IS" if a.has_attribute("foo") else "NO")  # NO
print("IT IS" if a.has_attribute("x") else "NO")  # NO
print("IT IS" if a.has_attribute("y") else "NO")  # IT IS

This code is in a flask app.

1. Print statements end up in the console / server logs, not the web page or in PostMan. You need to return a string or response object in Flask.
2. See How to get POSTed json in Flask?

Here's a sample Flask app that works.

from flask import Flask, request
app = Flask(__name__)

def has_attribute(data, attribute):
    return attribute in data and data[attribute] is not None

@app.route("/postit", methods=["POST"])
def postit():
    json_request = request.get_json()
    print(has_attribute(json_request, 'offer_id'))
    return str(has_attribute(json_request, 'offer_id'))

Logs

 $ FLASK_APP=app.py python -m flask run
 * Serving Flask app "app"
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
False
127.0.0.1 - - [21/Jan/2018 14:19:10] "POST /postit HTTP/1.1" 200 -
True
127.0.0.1 - - [21/Jan/2018 14:19:25] "POST /postit HTTP/1.1" 200 -

Request & Response

$ curl -XPOST -H 'Content-Type:application/json' -d'{"offer_id":null}' http://localhost:5000/postit
False
$ curl -XPOST -H 'Content-Type:application/json' -d'{"offer_id":"data"}' http://localhost:5000/postit
True

Answer 2

The way you should read from a JSON object should be object[property_name].

For example object[a] will ideally return a based on your example.