如果匹配项之间没有3个字符串，如何将一个空字符串插入列表

Question

How to insert an empty string to list if there are not 3 strings between matches. I want to find out if every 4th element is some kind of number (ie 12.1234., 1.12.13, etc.) and if not I want to insert an empty string before the second number so there are always 3 strings between each match.

  list =  ['1.1', 'ab','ac','','1.2','dd','','1.3','cb','dd','', '1.4', 'de','']
  wanted_list =['1.1', 'ab','ac','','1.2','dd','','', '1.3','cb','dd','', '1.4', 'de','','']

This is what Ii got so far, but the loop never ends and inserts way to many empty strings at the end (not just when there are not 3 strings between the matches).

list =  ['1.1', 'ab','ac','','1.2','dd','','1.3','cb','dd','', '1.4', 'de','']
start_rx = re.compile('|'.join(
    ['\d\d\.\d\d\.\d\d\.\d\d\d', '\d\d\.\d\d\.\d\d\.', '\d\d\.\d\d\d\d', '\d\.\d\.\d\.', '\d\.\d\.\d\.\d\d\.',
     '\d\.\d\.\d\.\d\d\d\.', 'A\d\d\d\d', '^\d\.', '^\d\.\d', '^\d\.\d\.\d', '^\d\.\d\.\d\d', '\d\d\.\d\d\.\d\d\d\d', '\d.\d']))
count = 1
for i, line in enumerate(list):
    count += 4
    if re.match(start_rx, line):
        pass
    else:
        i=count
        list.insert(i, '')
        print (list)

Answer 1

The following approach groups the list into nested lists of digit/decimal values and others and iterates over the non-digit/decimal groups (odd indexed groups) to determine whether they contain the required 3 elements and fill with empty strings if needed. You could use regex in the isfloat() function below in line with your initial attempt but it seemed easier just to test for digits after removing decimals.

from itertools import groupby

def isfloat(s):
    return s.replace('.','').isdigit()

items =  ['1.1', 'ab','ac','','1.2','dd','','1.3','cb','dd','', '1.4', 'de','']
groups = [list(g) for _,g in groupby(items, key=isfloat)]
for group in groups[1::2]:
    group += [''] * (3 - len(group))

result = [item for group in groups for item in group]
print(result)
# OUTPUT
# ['1.1', 'ab', 'ac', '', '1.2', 'dd', '', '', '1.3', 'cb', 'dd', '', '1.4', 'de', '', '']