Maintain count in python list comprehension

Question

In Python, is there any counter available during the list comprehension as it would be in case of a for loop?

It would be more clear why I need a counter, with this example:

I wish to achieve the following:

Initial List: ['p', 'q', 'r', 's']

Desired List: [(1, 'P'), (2, 'Q'), (3, 'R'), (4, 'S')]

In the desired list, first element of every tuple are ordinal numbers. If it were just flat list, I could have used zip to achieve this. But however, the list I am dealing with is nested, three level deep (think of hierarchical data), and it is generated through list comprehension.

So, I was wondering is there any way to introduce those ordinal numbers during list comprehension. If not, what would be the best possible solution.

P.S. : Here the lower case letters are converted to uppercase, but that is not a part of problem, think of it as just a data conversion.

Code:

allObj = Category.objects.all()  tree =[(_, l1.name, [(__, l2.name, [(___, l3.name) for l3 in allObj if l3.parentid == l2.categoryid]) for l2 in allObj if l2.parentid == l1.categoryid]) for l1 in allObj if l1.parentid == None] 

allObj contains data from table category, which in turn contains hierarchical data represented in the form of Adjacency List.

I have put _ where I need ordinal numbers to be. Notice that the list is nested, so there will be a separate counter at each level represented by 1, 2 & 3 _s.

Answer 1

The most basic case

[(i, x) for i, x in enumerate(some_list, 1)] 

Apply a filter with an if-statements

[(i, x) for i, x in enumerate(some_list, 1) if i > 2] 

or like this

[(i, x) for i, x in enumerate(some_list, 1) if x != 'p'] 

A word of advice

Most often you don't need to do this. Instead you just call enumerate(some_list, 1) where the enumeration is needed, in a for loop for example.

Answer 2

I was looking to something slightly different when I stumbled upon this answer.

My case was to keep a count based on a condition inside the list comprehension. Just in case it's useful to someone else, this is how I solved it:

import itertools counter = itertools.count(0) [(next(counter), x) for x in some_list if x != 'p']

In this way the counter will only be incremented when the condition is met and not at every iteration.