function and classes, usage of __str__

Question

I am having some issues concerning the interaction between functions, methods and have a uncertainty of how to utilize the use of the built-in function __str__.

To start this of, I have a class called vara, which looks like:

class vara(object):

    def __init__(self, kod, namn, pris, butikantal):
        self.kod = kod
        self.namn = namn
        self.pris = pris
        self.antal = butikantal

and i have a function that creats a list of object for the vara klass, that looks like:

def mina_varor():

  varor_fran_fil = open("varor.txt", "r")
  varulista = []
  for rad in varor_fran_fil:
      varje_vara1 = rad
      varje_vara2 = varje_vara1.split("/")
      varorna = vara(varje_vara2[0], varje_vara2[1], varje_vara2[2], varje_vara2[3])
      varulista.append(varorna)

  return(varulista)

Now i want to be able to access a singel object in this list by simply typing then "kod" for the object. But I cant find any "kod" in my list. It seemed strange, so i tried printning the list, and got that:

[<__main__.vara object at 0x00000000031503C8>, <__main__.vara object at 0x00000000031507F0>, <__main__.vara object at 0x0000000003150710>, <__main__.vara object at 0x00000000031502B0>, <__main__.vara object at 0x00000000031505F8>, <__main__.vara object at 0x00000000031504E0>]

i looked it up, and it seems that python cant decide how to interpret my list. I think i need an __str__ method to do so, but how should i make my __str__ method look like if i want it to print something like:

Name: Hat
Price: 150
Quantity: 100
Code: 223

?

Answer 1

Issue 1: __str__() not being called when printing a container

It's not that Python can't interpret your list, its that it doesn't know how you expect it to be displayed.

Implement the __str__ method if you want to do something like

print(varulista[0])

Something like the following would give you something like what you expect for that list entry:

def __str__(self):
    s  = "Name:    %s\n" % self.namn
    s += "Price:   %s\n" % self.pris
    s += "Quantity:%s\n" % self.antal
    s += "Code:    %s\n" % self.kod

But if you want print(varulista) to make any sense, you'll also have to implement the __repr__ method, since varulista is a list and when printing it Python looks for the __repr__ method.

But that being said, the idea of __repr__ is to be able to pass it back into eval() and create an equivalent object. So you could go one of two ways:

1. Ignore the __repr__()/eval() interaction

In this case, just implement __repr__() however you want. Remember that this method is going to be called when it is an item in a container you're printing, so keeping the output to a single line might be helpful. Just know that unless you unambiguously represent the state of the object in the __repr__() output, you won't be able to recreate an equivalent object "down the road" using eval().

2. (Preferred option, IMO) Implement __repr__() correctly and simply don't print containers if you don't like the way it looks.

If output like

[Item{Name:"Hat",Price:150,Quantity:100,Code:223},Item{Name:"Shirt",Price:450,Quantity:10,Code:225}]

is too unfriendly because of how you had to implement __repr()__, just don't print containers. In other words, don't use

print(varulista)

but instead use

for item in varulista: print item

which would call the __str__() method you defined, the human-friendly one.

EDIT: @bernie's links to a great answer by @AlexMartelli regarding the difference between __str__ and __repr__ that is worth linking twice.

---

Issue 2: being able to access a list element by it's code

You say

Now i want to be able to access a singel object in this list by simply typing then "kod" for the object. But I cant find any "kod" in my list. It seemed strange, so i tried printning the list, and got that:

You have two main options here. (1) Use a dictionary keyed on the item code. (2) Search through list for code using list comprehension. I'm only going to show (1), because I think it's a much better alternative.

Option 1: Use a dictionary keyed on the item code

Consider this code:

# Define a convenience function to add items to the dictionary
def add_item(d,i): d[i.kod] = i

# Create a new dictionary
items = {}

# Add items to dictionary
add_item(items, vara(223, "Hat", 150, 100))
add_item(items, vara(225, "Shirt", 450, 10))

You can now access any defined item from the items dictionary as follows:

items[223] #<-- This is the "Hat" item
items[225] #<-- This is the "Shirt" item

Answer 2

You could overide __repr__ like this:

class vara():
    def __init__(self, kod, namn, pris, butikantal):
        self.kod = kod
        self.namn = namn
        self.pris = pris
        self.antal = butikantal

    def __repr__(self):
        return 'Name: %s\nPrice: %s\nQuantity:%s\nCode: %s\n'\
               % (self.namn, self.pris, self.antal, self.kod)

output:

>>> print([vara('223', 'Hat', '150', '100'), vara('115', 'Bla', '154', '200')])
[Name: Hat
Price: 150
Quantity:100
Code: 223
, Name: Bla
Price: 154
Quantity:200
Code: 115
]

Edit:

But as explained by @jedwards, it is a better approach to override __str__ just the same way I did for __repr__ and print each object individually rather than printing the whole list.