如何使用Python转换字符串中的每个字符

Question

I have a sequence of characters that I want to convert them into specified charter.

Seq = "AA" #This the sequence of characters

def complement (Seq):
    for nuc in Seq: # converting the sequence of characters into the desirable character 
        if nuc == 'A':
           comp = 'T'
    return comp

print "The complement of the Sequence AA is", complement(Seq)

When I tried to run the code above, the code is not recognizing the whole sequence of characters and convert them at a time into "T"s; however, it is only executing "T" once for the input sequence "AA"

Any idea how to make the code converts each "A" character in the sequence into "T"?

I appreciate your help!

Answer 1

Your script have a few errors. First, the variable comp doesn't have a initial value and returns only one "T"(last one) because the other get replaced every time they find a "A" in the loop. For what I understand you are trying to substitute some values in the same string. I find it easier to build a new string with the characters you want to replace.

Here is my script:

Seq = "AA" #This the sequence of characters

def complement (Seq):
    comp = ''
    for nuc in Seq: # converting the sequence of characters into the desirable character 
         if nuc == 'A':
             comp += 'T'
         else:
             comp += nuc
    return comp

print "The complement of the Sequence AA is", complement(Seq)

Answer 2

First, use string.maketrans to build a translation table. That function takes two strings, and builds a table that maps each character of the first string to the corresponding character in the second string. Now, you can pass that table to the translate method of the string you want to translate.

>>> import string
>>> table = string.maketrans("ATCG", "TAGC")
>>> 'AAAGTC'.translate(table)
'TTTCAG'

Answer 3

It's because of that after the condition you just put the 'T' on comp and at last returned it:

    if nuc == 'A':
       comp = 'T'
return comp

But as a more pythonic way you can use str.replace() :

>>> Seq = "AA"
>>> Seq.replace('A','T')
'TT'

And if you want to convert each character to a special character based on a condition you can use a list comprehension and join :

>>> Seq = "AA"
>>> ''.join(['T' for i in Seq if i=='A'])
'TT'

Also based on your task you can have another choices like using regex.Which in that case python comes with re.sub() function to replace a string based on a regex.

Answer 4

Ofcourse it is going to be only T . Because your loop is just appending T to comp instead of concatenating.

Try this:

Seq = "AA" #This the sequence of characters

def complement (Seq):
    comp = ""    <- declare empty string first
    for nuc in Seq:
        if nuc == 'A':
            comp += 'T'   <-- notice the +
    return comp

print "The complement of the Sequence AA is", complement(Seq)

Here is an basic tutorial for string concatenation and formatting.

Answer 5

if, you want get complement of sequence of DNA

complement = {"A":"T", "C":"G", "G":"C", "T":"A"}
seq = "ACGT"
complement_seq = "".join([complement[b] for b in seq])
complement_seq

this is similar to,

complement = {"A":"T", "C":"G", "G":"C", "T":"A"}
seq = "ACGT"
complement_seq = ""
for base in seq:
  complement_seq += complement[base]

complement_seq

you get

TGCA

better, using biopython library

from Bio.Seq import Seq
my_seq = Seq("ACGT")
my_seq.complement()

you get

Seq('TGCA', Alphabet())