一次打印输出后如何脱离python综合列表

Question

Here is my code to print the reverse of DNA sequence, only if the string

contain character ["A","T","G","C"]

dna=input("Enter DNA sequnce")
[print(dna[::-1]) for x in dna if x in["A","T","G","C"]]

input: ATGC

the output:
CGTA
CGTA
CGTA
CGTA

How do I prevent the code from looping the number of characters present in the string. Can I get the output once and break the loop?

Answer 1

basically your code is checking one character at time , and if it is matched its printing the whole list in reverse ,try using

dna=input("Enter DNA sequnce")
result=[ x for x in dna if x in["A","T","G","C"]]
print "".join(result[::-1])

if you don't want to use list comprehensions then you can use this as well

for i in allowed_char:
    if i in dna:
        print dna[::-1]
        break

Answer 2

You can try sth along the lines of:

dna = input("Enter DNA sequnce") 
print(''.join(x for x in dna[::-1] if x in "ATGC"))
# CTGA -> AGTC
# ABCD -> CA

Answer 3

How about trying something functional programming tricks? Here I recommend the PyFunctional lib to do DNA stuff, cause PyFunctional seems to be born with handling sequence problems, hoping you can benefit from it.

to install functional

pip install PyFunctional

For your case, the code looks like:

from functional import seq
from operator import add

data = 'ATGC'

result = (seq(list(data))
          .filter(lambda x: x in {"A", "T", "G", "C"})
          .reduce(add))[::-1]

# CGTA
print(result)

From above, we can see we avoid the for-loop, and use functional programming teches like filter, but we go further by combining the method into a pipline chain, to avoid nested functional methods(something like map(list(filter())) )

BTW: using x in["A","T","G","C"] is quite slow, instead you should use x in {"A","T","G","C"} which can take advantage of the hash set.

For more info, check PyFunctional on PyPi , or the Github