There are a number of possible options! And remember you can always pipe pyp to pyp. Here are some ideas, in order of what came to mind, where the operation is incrementing.

  pyp 'lines[lines.index("start") + 1:]' | pyp 'lines[:lines.index("end")]' | pyp 'int(x) + 1'
  
  pyp 'start = lines.index("start"); end = lines.index("end", start); map(lambda x: int(x) + 1, lines[start + 1:end])'
  
  pyp 'dropwhile(lambda x: x != "start", lines)' | pyp 'lines[1:]' | pyp 'takewhile(lambda x: x != "end", lines)' | pyp 'int(x) + 1'
  
  pyp 'map(lambda x: int(x) + 1, takewhile(lambda x: x != "end", islice(dropwhile(lambda x: x != "start", lines), 1, None)))'
  
  pyp -b 'cond = False' 'if cond and x == "end": break' 'if cond: print(int(x) + 1)' 'if x == "start": cond = True'

Having moved from Perl to Python many years ago, this is one of those things that Perl has magic to handle, for example:

    perl -lne '$sum += $_ if /start:/ .. /end:/; END{print $sum}' file.txt

With the -n option, perl runs your code for every line in the input (stdin, or filenames). $_ holds the content of the line. And the .. operator is a stateful (!) operator that becomes true when its left part becomes true, until the right part becomes true.

So I write 95% of my code in Python, but for command line magic, it's hard to beat Perl.