Python3替换使用字典

Question

Could anyone please explain what is wrong here:

def get_complementary_sequence(string):
    dic = {'A':'T', 'C':'G', 'T':'A', 'G':'C'}
    for a, b in dic.items():
        string = string.replace(a, b)
    return string

I get proper results for 'T' and 'C', but 'A' and 'C' won't replace. Got really stuck.

String looks like 'ACGTACG'.

Answer 1

You are first replacing all A s with T s before then replacing all T s with A s again (including those you just replaced A s with!):

>>> string = 'ACGTACG'
>>> string.replace('A', 'T')
'TCGTTCG'
>>> string.replace('A', 'T').replace('T', 'A')
'ACGAACG'

Use a translation map instead, fed to str.translate() :

transmap = {ord('A'): 'T', ord('C'): 'G', ord('T'): 'A', ord('G'): 'C'}
return string.translate(transmap)

The str.translate() method requires a dictionary mapping codepoints (integers) to replacement characters (either a single character or a codepoint), or None (to delete the codepoint from the input string). The ord() function gives us those codepoints for the given 'from' letters.

This looks up characters in string , one by one in C code, in the translation map, instead of replacing all A s followed by all T s.

str.translate() has the added advantage of being much faster than a series of str.replace() calls.

Demo:

>>> string = 'ACGTACG'
>>> transmap = {ord('A'): 'T', ord('C'): 'G', ord('T'): 'A', ord('G'): 'C'}
>>> string.translate(transmap)
'TGCATGC'

Answer 2

Mutable data is your enemy :)

See, you first replace all A s with T s, then , in another iteration, replace all T s with A s again.

What works:

# for Creek and Watson's sake, name your variables sensibly
complements = {ord('A'):'T', ord('C'):'G', ord('T'):'A', ord('G'):'C'}
sequence = "AGCTTCAG"
print(sequence.translate(complements))

It prints TCGAAGTC .