以下是一个使用Python解析FASTA文件并计算最大ORF长度的示例代码：

def read_fasta(file_path):
    sequences = []
    with open(file_path, 'r') as file:
        sequence = ''
        for line in file:
            line = line.strip()
            if line.startswith('>'):
                if sequence:
                    sequences.append(sequence)
                sequence = ''
            else:
                sequence += line
        if sequence:
            sequences.append(sequence)
    return sequences

def find_orf(sequence):
    orfs = []
    start_codon = 'ATG'
    stop_codons = ['TAA', 'TAG', 'TGA']
    start_indices = [i for i in range(len(sequence)) if sequence[i:i+3] == start_codon]
    for start_index in start_indices:
        stop_index = None
        for i in range(start_index+3, len(sequence)-2, 3):
            codon = sequence[i:i+3]
            if codon in stop_codons:
                stop_index = i + 3
                break
        if stop_index:
            orfs.append(sequence[start_index:stop_index])
    return orfs

def get_max_orf_length(sequences):
    max_orf_length = 0
    for sequence in sequences:
        orfs = find_orf(sequence)
        for orf in orfs:
            orf_length = len(orf)
            if orf_length > max_orf_length:
                max_orf_length = orf_length
    return max_orf_length

fasta_file = 'sequences.fasta'
sequences = read_fasta(fasta_file)
max_orf_length = get_max_orf_length(sequences)
print("Maximum ORF length:", max_orf_length)

要运行此代码，您需要将FASTA文件的路径替换为fasta_file变量的值。这段代码首先定义了三个函数：read_fasta函数用于从FASTA文件中读取序列，find_orf函数用于在一个序列中查找ORF，get_max_orf_length函数用于计算最大ORF长度。最后，代码读取FASTA文件，调用相应函数，并打印出最大ORF长度。