.readline（）在Python中返回什么？

Question

EDIT of entire post, in order to be more clear of the problem:

s = "GATATATGCATATACTT"
t = "ATAT"

for i in range(len(s)):
    if t == s[i:i+len(t)]:
        print i+1,

So the purpose of the program above is to scan through the long line of DNA (s) with the short line of DNA (t), in order to find at which positions on s, that t matches. The output of the above code is:

2 4 10 #This are basically the index numbers of string s that string t matches. but as can be seen in the code above, it's i+1 to give a 1-based numbering output.

The problem I'm having is that when i try to change the code, in order to make it receive the values for s and t through a file, the readline() function is not working for me. The motif.txt file contains two strings of DNA, one on each line.

with open('txt/motif.txt', 'r') as f:

    s = f.readline()
    t = f.readline()

    for i in range(len(s)):
        if t == s[i:i+len(t)]:
            print i+1,

So this code, on the other hand will output nothing at all. But when I change t to:

t = f.readline().strip()

Then the program outputs the same result as the first example did.

So i hope this has made things more clear. My question is thus, if readline() returns a string, why isn't my program in example 2 working in the same way as in the very first example?

Answer 1

your problem statement is wrong, there's no way s or t has more content (and len(s) > 0 or len(t) > 0 ) in the first example than in the second.

basically with:

s = f.readline()

then s will contain a string like "foobar \\n" , and thus len(s) will be 9 .

Then with:

s = f.readline().strip()

with the same string, len(s) will be 6 because the stripped string is "foobar" .

so if you line is full of spaces like s = " \\n" , s.strip() will be the empty string "" , with len(s) == 0 .

Then in that case your loop won't start and will never print anything.

in almost all the other cases I can think of, you should get an execption raised, not silent exit.

But to be honest, your code is bad because nobody can understand what you want to do from reading it (including you in six months).