List specific moves from a pgn file using python chess.pgn

Question

Using python chess, I would like to be able to list specific moves from a pgn string. For instance, in this string:

1. c4 e5 2. g3 c6 ( 2... d5 3. cxd5 ) 3. Nf3 e4 4. Nd4 d5 5. cxd5 Qxd5 6. Nc2 ( 6. e3 ) 6... Nf6 7. Nc3 Qe5 8. Bg2 Na6 9. O-O Be7 10. Ne3 O-O 11. a3 Re8 ( 11... Nc7 ) 12. b4 Ng4 13. Bb2 Qh5 14. h4 Bf6 ( 14... Nxe3 15. dxe3 f5 16. Qb3+ Kh8 17. Na4 Qg6 18. Qc2 Nc7 19. Bd4 ) 15. Qc2 Nxe3 16. dxe3 Bf5 17. Na4 Bxb2 ( 17... Bxh4 18. gxh4 ) 18. Nxb2 ( 18. Qxb2 Nc7 ) 18... Nc7 19. Nc4 Re6 20. Rfd1 Nd5 21. Rd4 ( 21. b5 g5 ) 21... h6 ( 21... g5 ) 22. Qd2 ( 22. Rad1 g5 ) 22... Rae8 23. Kh2 ( 23. Rxd5 ) 23... Bg4 24. Na5 ( 24. Re1 g5 ) 24... Rf6 25. Kg1 g5 26. Nxb7 ( 26. Qb2 Ree6 27. Bxe4 gxh4 28. Bf3 Rg6 29. Rxg4 Rxg4 30. Bxg4 Qxg4 ) 26... gxh4 27. Nc5 h3 28. Rxe4 ( 28. Nxe4 Rfe6 ) 28... hxg2 29. Rxe8+ Kg7 30. f4 Qh1+ 31. Kf2 Qxa1 32. Kxg2 Bh3+ 0-1

I would like the first 5 moves and their variations (or a different set for example moves 6 to 10):

1. c4 e5 2. g3 c6 ( 2... d5 3. cxd5 ) 3. Nf3 e4 4. Nd4 d5 5. cxd5 Qxd5

The string is generated using a command like this:

exporter = chess.pgn.StringExporter(headers=False, variations=False, comments=False)

pgn_string = games1.accept(exporter)

print(pgn_string)

Answer 1

Here's a solution completely ignoring the chess library, only requiring the pgn string as input:

def m2m(pgn, m1, m2):

    sub_var_level = 0
    idx_1 = idx_2 = len(pgn)

    for idx, char in enumerate(pgn):
   
        if char == '(':
            sub_var_level += 1
        elif char == ')':
            sub_var_level -= 1
    
        elif not sub_var_level:
            if pgn[idx:].startswith(' %d. ' % m1):
                idx_1 = idx
            elif pgn[idx:].startswith(' %d. ' % (m2 + 1)):
                idx_2 = idx

    return pgn[idx_1:idx_2]

Now you can call the function, e.g.

>>> m2m(pgn, 6, 10)
' 6. Nc2 ( 6. e3 ) 6... Nf6 7. Nc3 Qe5 8. Bg2 Na6 9. O-O Be7 10. Ne3 O-O'

>>> m2m(pgn, 30, 35)
' 30. f4 Qh1+ 31. Kf2 Qxa1 32. Kxg2 Bh3+ 0-1'