阅读特定行并使用python忽略其他行

Question

I have a text file like this:

input file
yuorvsdsd
dfdsfsd
?dsfsdfsd
sdfsdfs
?dfd
ds

I am trying to make it so that it only reads the lines start with ? . The readlines function reads all lines, so I put a condition on it, but I am not satisfied with it, as it's not showing me the accurate results. I want output to be:

?dsfsdfsd
?dfd

Program

fp = open("file.txt")
z = fp.readlines
if z=='?':
    print z
fp.close() 

This is the code which I tried.

Answer 1

I would use something like the following:

with open("file.txt") as fp:
    for line in fp:
        if line.startswith('?'):
            print line

By using a context manager ( with ... as ) the closing of the file will happen automatically after you are done with it.

Answer 2

列表理解使这一过程变得简单：

filtered = [line for line in fp.readlines() if line.startswith("?")]

Answer 3

I think I'd go with a generator expression:

(line for line in open('file.txt') if line.startswith('?'))

A complete program looks like:

lines = (line for line in open('file.txt') if line.startswith('?'))
print ''.join(lines)

Answer 4

Does this not work?

with open("file.txt") as fin:
    for line in fin:
        if not line.startswith('>'):
            print line

Alternatively, could you not just pipe this through grep first?

Answer 5

Alternatively, you can use line[0] to get the first character of each line.

with open("file.txt") as fp:
    for line in fp:
        if line[0] == '?':
            print line

This is possible because String in Python can be indexed. Python's String, like C, starts with the index 0 . line[0] will give you a character at index 0, which is the first character.