check blunder move in pgn $4 or ??

[sugizo]

objective
want to know if there is a blunder move in pgn $4 or ??,
and
then delete the move before the blunder and add the fen tag for blunder move

code

pgn_book = 'book.pgn'

f = open(pgn_book, "w")
f.write("""
[White "b"]
[Black "?"]
[Result "*"]
[FEN "8/1R3pk1/1P4p1/7p/7P/1r4P1/5PK1/8 w - - 0 1"]
[SourceVersionDate "2021.05.20"]
[SetUp "1"]
[Annotator "GM Petar G. Arnaudov"]
[PlyCount "25"]

1. Kf1 Rb2 2. Ke1 Kf6 3. f3 $1 Rb3 4. Kd2 Rxf3 $4 { this move loses } 5.
Rc7 $1 { [%cal Rc7c3] } (5. Kc2 Re3 $3 { everything else loses } 6. Rc7 Re8
7. b7 Rb8 $11) 5... Rb3 6. b7 Kf5 7. Kc2 Rb6 8. Rxf7+ Kg4 9. Rg7 $3 Kh3 10.
Kc3 Rb1 11. Rd7 $1 Kxg3 (11... Rb6 { is not saving the game too. } 12. Kc4
Kxg3 13. Rd3+ Kxh4 14. Rb3 Rxb7 15. Rxb7 $18) 12. Rd3+ Kxh4 (12... Kh2 13.
Rd2+ { [%cal Rd2b2] }) 13. Rd4+ $18 { [%cal Rd4b4] } *
""")
f.close()

pgn = open(pgn_book)

first_game = chess.pgn.read_game(pgn)

board = first_game.board()

for move in first_game.mainline_moves():
    board.push(move)
    if chess.pgn.NAG_BLUNDER:
        print("blunder")

result
no error occured but the result is empty, not print anything

question
how to achieve it using python chess

thanks and best regards

[MarkZH]

After reading the game with first_game = chess.pgn.read_game(pgn), you'll want to iterate through first_game.mainline(). This will iterate through ChildNodes that contain all of the move information. For example:

import chess
import chess.pgn

pgn_book = 'book.pgn'

with open(pgn_book, "w") as f:
    f.write("""
[White "b"]
[Black "?"]
[Result "*"]
[FEN "8/1R3pk1/1P4p1/7p/7P/1r4P1/5PK1/8 w - - 0 1"]
[SourceVersionDate "2021.05.20"]
[SetUp "1"]
[Annotator "GM Petar G. Arnaudov"]
[PlyCount "25"]

1. Kf1 Rb2 2. Ke1 Kf6 3. f3 $1 Rb3 4. Kd2 Rxf3 $4 { this move loses } 5.
Rc7 $1 { [%cal Rc7c3] } (5. Kc2 Re3 $3 { everything else loses } 6. Rc7 Re8
7. b7 Rb8 $11) 5... Rb3 6. b7 Kf5 7. Kc2 Rb6 8. Rxf7+ Kg4 9. Rg7 $3 Kh3 10.
Kc3 Rb1 11. Rd7 $1 Kxg3 (11... Rb6 { is not saving the game too. } 12. Kc4
Kxg3 13. Rd3+ Kxh4 14. Rb3 Rxb7 15. Rxb7 $18) 12. Rd3+ Kxh4 (12... Kh2 13.
Rd2+ { [%cal Rd2b2] }) 13. Rd4+ $18 { [%cal Rd4b4] } *
""")

with open(pgn_book) as pgn:
    first_game = chess.pgn.read_game(pgn)
    print(first_game)
    print("")

    for node in first_game.mainline():
        print(node.san())
        if chess.pgn.NAG_BLUNDER in node.nags:
            print("    blunder")