def validate_base_sequence(base_sequence, RNAflag = False):      #判断序列是否只含有A、T、G、C、U

    seq = base_sequence.upper()

    return len(seq) == (seq.count('U' if RNAflag else 'T') +seq.count('C') +

               seq.count('A') +seq.count('G'))              

def gc_content(base_seq):                                #计算GC含量

    assert validate_base_sequence(base_seq), 'argument has invalid characters'

    seq = base_seq.upper()

    return (base_seq.count('G') +base_seq.count('C')) / len(base_seq)

def recognition_site(base_seq, recognition_seq):

    return base_seq.find(recognition_seq)

def test():

    assert validate_base_sequence('ACTG')

    assert validate_base_sequence('')

    assert not validate_base_sequence('ACUG')

    assert validate_base_sequence('ACUG', True)

    assert not validate_base_sequence('ACUG', False)

    assert validate_base_sequence('ACTG', False)

    assert .5 == gc_content('ACTG')

    assert 1.0 == gc_content('CCGG')

    assert .25 == gc_content('ACTT')

    print('All tests passed.')

test()

>>> DNABases, RNABases = 'TCAG', 'UCAG'

>>> DNABases

'TCAG'

>>> RNABases

'UCAG'

>>> bases = 'TCAG', 'UCAG' # a two-element tuple

>>> bases

('TCAG', 'UCAG')

def recognition_site(base_seq, recognition_seq):

    return base_seq.find(recognition_seq)

def restriction_cut(base_seq, recognition_seq, offset = 0):

    site = recognition_site(base_seq, recognition_seq)

    return base_seq[:site+offset], base_seq[site+offset:]

aseq1 = 'AAAAATCCCGAGGCGGCTATATAGGGCTCCGGAGGCGTAATATAAAA'

left, right = restriction_cut(aseq1, 'TCCGGA')

def read_FASTA(filename):

    with open(filename) as file:

        contents = file.read()

    entries = contents.split('>')[1:] # skip blank first entry

    partitioned_entries = [entry.partition('\n') for entry in entries]

    result = [(entry[0], entry[2].replace('\n', '')) for entry in partitioned_entries]

    return result

def longest_sequence(filename):

    longest_seq = ''

    for info, seq in read_FASTA(filename):

        longest_seq = max(longest_seq, seq, key=len)

    return longest_seq