Understanding enumerate in python

Question

I'm solving one python exercise and can't understand enumerate operation: Can anyone tell me how this will work:

>>> a = [[('A', {}), 1, None, None, 0], [('B', {}), 1, None, None, 0]]
>>> b = [('A', {}), ('B', {})]
>>> for each in b:
...    my_idx = [idx for idx, val in enumerate(a) if a[idx][0] == each][0]
...    print my_idx
...

and the output it produces is:

0
1

To understand this, I did following changes:

>>> a = [[('A', {}), 1, None, None, 0], [('B', {}), 1, None, None, 0], [('A', {}), 1, None, None, 0], [('B', {}), 1, None, None, 0]]
>>> b = [('A', {}), ('B', {}), ('A', {}), ('B', {})]
>>> for each in b:
...    my_idx = [idx for idx, val in enumerate(a) if a[idx][0] == each][0]
...    print my_idx
...

and thought it should print:

0
1
2
3

but it produced:

0
1
0
1

Where I'm going wrong? How should I modify it to produce:

0
1
2
3

Thanks.

Answer 1

To get the output you want you need to change your logic:

[idx for idx, val in enumerate(a) if any(val[0] == x for x in b)]

When printed outputs:

print("\n".join([str(idx) for idx, val in enumerate(a) if any(val[0] == x for x in b)]))
0
1
2
3

You can also just test for membership using in :

print("\n".join([str(idx) for idx, val in enumerate(a) if val[0] in b]))

0
1
2
3

If val[0] which corresponds to each tuple in the sublists of a is in b add the idx which is the index of the element.

Answer 2

You need to understand what enumerate does. It is just a special function, which will give you number of evelemt in array you are using in for.

for book in books:
    print book
for author in authors:
    print author

Will give you

Book A
Book B
Book C
Author A 
Author B
Author C

But if you need to count it, to make a handy list with numbers, you can use enumerate

for i, book in enumerate(books):
    print i, book
for i, author in enumerate(authors):
    print i, author

Will give you

1 Book A
2 Book B
3 Book C
1 Author A 
2 Author B
3 Author C

This is what enumerate does. It gives a nubmer to each element in array you are using. Nothing more. If you want to create global counter, you should consider using global variable.