Problem removing leading zeros using a list comprehension best expression

Question

result is a list of numbers:

result = [\'0\',\'5\',\'0\',\'1\']

I want to remove the leading zero, using the following expression:

Answer 1

The expression looks for the first non-zero in the list, returning the remaining list. More detail at the bottom.

ANALYSIS

However, you have characters: there is no integer 0 in your list. Therefore, the expression merely returns the original list.

SOLUTION

If you want it to work on characters, simply include the quotation marks required:

result[next((i for i, x in enumerate(result) if x != '0'), len(result)):] or ['0']

This give you:

Original code:

>>> result = ['0','5','0','1']
>>> result[next((i for i, x in enumerate(result) if x != 0), len(result)):] or [0]
['0', '5', '0', '1']

Adapted to strings / characters:

>>> result[next((i for i, x in enumerate(result) if x != '0'), len(result)):] or ['0']
['5', '0', '1']
>>> result = ['0','0','5','0','1']
>>> result[next((i for i, x in enumerate(result) if x != '0'), len(result)):] or ['0']
['5', '0', '1']
>>> result = ['7', '0','0','5','0','1']
>>> result[next((i for i, x in enumerate(result) if x != '0'), len(result)):] or ['0']
['7', '0', '0', '5', '0', '1']

EXPRESSION LOGIC

The critical clauses are in the deepest part of the comprehension:

(i for
    i, x in enumerate(result)
        if x != '0')

In the enumerate clause, i, x iterates through the list indices and its contents: (0, '0'), (1, '5'), etc.

The if checks for zero characters; it filters out all zeroes. The i for part doesn't get any i value until the inner clause finds a non-zero.

At that point, the outer next operator takes over: it takes the first index of a non-zero and returns that to the slice operator (note that colon at the end), which returns the remainder of the list.

As pault already mentioned, the len at the end is the default value: if your zero filter disqualifies the entire list (all zeroes), then the comprehension returns len(result) to next. This mean that next will return the list length, and your result slicing returns an empty list. Now your or expression has a "Falsy" value in the first operand; it returns the second operand, the list ['0'].