Understanding python for loop, with object used as index?

Question

Here is some test code that I wrote.

class myclass:
    def __init__(self, aty):
        self.number = 0;
        self.anitype = aty;

l = [];
l.append(myclass("tiger"))
l.append(myclass("lion"))
l.append(myclass("puma"))
l.append(myclass("jagur"))

# Method 1 : of course this doesn't work, but I saw a case where it works
#for idx in l: l[idx].number = idx

# Method 2 : of course this works
for idx, lst in enumerate(l):
   l[idx].number = idx

for idx, lst in enumerate(l):
    print("%d : %s" % (lst.number, lst.anitype))

The result :

0 : tiger
1 : lion
2 : puma
3 : jagur

Of course, Method 1 should not work, but today I saw some code that I received from another company and it has a line below.(only the snippets shown)

for idx in ls: ls[idx].Oi = idx

Here ls is a list of class objects. The class has an instance member variable Oi. Thinking this line strange, I ran the code with a debugger, and it works! The Oi values of the list member objects are being assigned 0,1,2,3,..

How does this work? By the way, this line of code is seen in a function which is not a member function of a class (it's a global function).

Answer 1

In Python there's a mapping type dict. When iterating over a dictionary, you're iterating over its keys. To get the values, you still need to use the keys as indices:

d = {'a': 3, 'b': 5, 'c': 4}
for key in d:
    print(key, d[key])

Output (order may vary):

a 3
b 5
c 4

It's very likely the code you see is doing this (iterating over a dict, or another mapping object, instead of a list). If you see something initialized like

d = {}  # An empty dict

Then it's a dict object. (Empty sets are initialized as set())