Counting jump(no of lines) between first two 'String' occurrences in a file

Question

I have a huge data file with a specific string being repeated after a defined number of lines.

counting jump between first two \'Rank\' occurrences. For example the file

Answer 1

I assume you want to find the number of lines in a block where each block starts with a line that contains 'Rank' e.g., there are 3 blocks in your sample: 1st has 4 lines, 2nd has 4 lines, 3rd has 1 line:

from itertools import groupby

def block_start(line, start=[None]):
    if 'Rank' in line:
       start[0] = not start[0]
    return start[0]

with open(filename) as file:
     block_sizes = [sum(1 for line in block) # find number of lines in a block
                    for _, block in groupby(file, key=block_start)] # group
print(block_sizes)
# -> [4, 4, 1]

If all blocks have the same number of lines or you just want to find number of lines in the first block that starts with 'Rank':

count = None
with open(filename) as file:
     for line in file:
         if 'Rank' in line:
             if count is None: # found the start of the 1st block
                count = 1
             else: # found the start of the 2nd block
                break
         elif count is not None: # inside the 1st block
             count += 1
print(count) # -> 4