How to remove blank elements at the beginning and end of a list Python

Question

I have a list of strings and need to remove the sequences of empty strings only at the beginning and end of the list. I need to keep any empty strings that are between non-empty strings.

For example,

my_list = ['', '', 'Sam sits', '', 'Thinking of you.', '', 'All ideas bad.', '', '', '']

The output should be;

['Sam sits', '', 'Thinking of you.', '', 'All ideas bad.']

Most methods i have tried to use get rid of the blank lines in the middle too. Any advice would be greatly appreciated.

user3483203 · Accepted Answer · 2018-07-10 16:43:37Z

6

There is a more efficient way, but if you choose an element that isn't contained in the elements of the list, you can join and strip and split which only removes the element from the front and back, preserving the empty elements in the middle.

>>> '-'.join(my_list).strip('-').split('-')
['Sam sits', '', 'Thinking of you.', '', 'All ideas bad.']

Extending this approach to concatenate longer runs of empty strings in the middle of a list to a single empty string:

import re

def remove_join(arr, el):
    return re.split(r'\{}+()'.format(el), el.join(arr).strip(el))

>>> my_list = ['', '', 'test', '', '', '', 'test2', '', '']
>>> remove_join(my_list, '-')
['test', '', 'test2']

edited Jul 10, 2018 at 16:43

answered Jul 10, 2018 at 16:14

user3483203

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6 Comments

PM 2Ring Over a year ago

What if you can't specify a char in advance that isn't in the strings? I guess you could use '\0', since that's pretty unlikely to be used in normal text strings. Failing that, you'd need to scan all the strings first to choose an unused char.

user3483203 Over a year ago

Yea, there are plenty of things wrong with doing it this way, just thought it was clever.

p seth Over a year ago

Thank you this worked out well. Although I forgot to add something, if there is multiple blank lines in the middle, is there a way to still have it all be represented by one single empty string ' ' in the output?

user3483203 Over a year ago

So you want to concatenate multiple empty strings in the middle to a single empty string?

p seth Over a year ago

Yes exactly just for the ones in the middle.

|

Prune · Accepted Answer · 2018-07-10 16:31:48Z

5

Look in the various list methods. You can use searches from left and right, and check to see whether they're the first and last elements. Conversely, simply remove the left-most element as long as it's an undesirable one. For instance:

while my_list[0] == '':
    my_list.pop(0)

while my_list[-1] == '':
    my_list.pop(-1)

For better efficiency (make a new list, but only one list alteration):

# First, form a Boolean list that identifies non-empty elements
has_content = [len(s) > 0 for s in my_list]
# Then find the left and right non-empty elements.
left  = has_content.find(True)    # find the left  non-empty string
right = has_content.rfind(True)   # find the right non-empty string
new_list = my_list[left:right+1]

This doesn't check edge cases, but gives the general idea.

edited Jul 10, 2018 at 16:31

answered Jul 10, 2018 at 16:15

Prune

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7 Comments

Anatolii Over a year ago

If my_list is empty or gets emptied this will crash.

Rory Daulton Over a year ago

This is the best answer--it is direct and clear (though you should handle the edge case of the empty list and other such cases.). Programmers should avoid tricky code unless it serves some very strong purpose. The only disadvantage of this code is that it can be slow if the list is long and there are many empty strings at the list's beginning, since removing from the beginning of a string is somewhat slow.

PM 2Ring Over a year ago

Each .pop(0) call requires all remaining list items to be moved down to fill the gap. So it's not a good idea to call it in a loop.

Prune Over a year ago

It's not at all efficient -- but it's easy to understand. Yes, it would be more efficient to find the first non-empty element on each end, and then simply slice the list in one operation.

PM 2Ring Over a year ago

The new version is much better, and with appropriate tests to handle the edge cases it's probably the best way to do this.

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jpp · Accepted Answer · 2020-08-20 21:37:50Z

4

You can calculate your indices first with next and enumerate. Then slice your list.

my_list = ['', '', 'Sam sits', '', 'Thinking of you.', '', 'All ideas bad.', '', '', '']

idx1 = next(i for i, j in enumerate(my_list) if j)
idx2 = -next(i for i, j in enumerate(reversed(my_list)) if j)

res = my_list[idx1: idx2 if idx2 !=0 else None]

print(res)

['Sam sits', '', 'Thinking of you.', '', 'All ideas bad.']

edited Aug 20, 2020 at 21:37

answered Jul 10, 2018 at 16:21

jpp

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