Python+Flask: init() takes 2 positional arguments but 3 were given

Question

I am from Java world and new to python and so to Flask. Here is my views.py

from app import app

class Employee:
    def __init__(id,name): self.id = id; self.name = name;

@app.route('/')
@app.route('/index')
def index():
    return Employee(12345,"MyName")

In the function index(), when i tried to return "Hello World" without class Employee in the file and run the server, Hello World string was written to the webpage.

I then thought of outputting a json,

{"id":"12345","name":"MyName"}

So I started to introduce a class Employee and created a constructor to pass id and name.

Unfortunately it started showing this error.

File "C:\python\microblog\app\views.py", line 9, in index return Employee(12345,"MyName")

By the way, here i assume that if I can get rid of the error, flask would be able to convert the object to json by taking it as default mimetype for objects.

I had look at these answers, but didn't help with my knowledge.

__init__ should have self as it's first argument.

Łukasz Rogalski
– Łukasz Rogalski

2015-04-22 11:08:48 +00:00
Commented Apr 22, 2015 at 11:08 — Łukasz Rogalski
– Łukasz Rogalski, Commented Apr 22, 2015 at 11:08

bruno desthuilliers · Accepted Answer · 2015-04-22 11:25:31Z

Python "methods" are really functions and no pointer to the current instance is magically inserted in the function's scope, so you have to declare your first argument as a reference to the current instance (by convention this argument is named self) - or as a reference to the current class if it's a classmethod (and then the first argument is - by convention - named cls).

When the function name is looked up on an instance, it's turned into a (callable) method object that keeps a reference to both the function and the current instance and inserts the instance as the first argument to the function call, as explained here : https://wiki.python.org/moin/FromFunctionToMethod - so you dont have (and must not) explicitely pass the current instance when calling the method.

NB : I understand this might seems weird (and worse) coming from the Java world, but there are actually quite a few good reasons for this peculiar design, as you'll probably find out by yourself when you'll start exploring the dynamic programming / metaprogramming part of Python.

oxana · Accepted Answer · 2015-04-22 11:16:08Z

3

In first argument on __init__ you must pass self:

class Employee:
    def __init__(self, id,name): 
       self.id = id; self.name = name;

Because self it's ref ob object Employee. Every Python object has two special method: __new__ and __init__. The first method for create object, it hasn't ref on object (self) and the second for initialise created object so it has ref named self.

edited Apr 22, 2015 at 11:16

answered Apr 22, 2015 at 11:09

oxana

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