如何在python 3中编写每一行？

Question

def every_second_line(report):
    """ (Open File for reading) -> list of str

    Return a list containing every second line (with leading and trailing
    whitespace removed) in report, starting with the first line.
    """
    list=[]
    for line in report:
        list.append(line.strip)
    report.readline()
    return list

I just came across this code while I was reviewing for my exam. Can someone please tell me why this code is not working? Thanks in advance!

Answer 1

The minimal change to your code is just to put report.readline() inside the for loop.

for line in report:
    lst.append(line.strip())
    report.readline()

You could also use next(report) (which works for all iterators, not just file objects) instead of report.readline() .

Note that if your file has an odd number of lines, that report.readline() might throw a StopIteration error (or is it EOFError ?) Best to catch that.

for line in report:
    lst.append(line.strip())
    try:
        next(report)
    except (StopIteration, EOFError): # because I don't remember which it throws
        break # leave the for loop

Or as JuniorCompressor notes in the comments, you can use the optional argument to next to ignore completely

for line in report:
    lst.append(line.strip())
    next(report, None)

Answer 2

You can slice using itertools.islice with a step of 2 using map to str.strip each line:

from itertools import islice

with open(fle) as f:
    print(list(map(str.strip,islice(f,0,None,2))))

If you want every second line from the second start from 1:

 print(list(map(str.strip,islice(f,1,None,2))))

According to your specification you are meant to open a file which I don't see you doing.

In your own code if you actually have a file object passed you just need to call next on the file object and actually call strip with parens:

    lst=[]
    for line in report:
        lst.append(line.strip()) # to call strip you need parens
        next(report,"")
    return lst

Put next before lst.append if you want every second line starting from the second. next takes a default value so we pass an empty string to avoid a stopIteration error in case we run out of lines before and still call next.

Answer 3

try this to get every 2nd element in list:

for line in report[::2]:

If you dont want 1st line

for line in report[1::2]:

Considering if report is generator object :

Simply call list

for line in list(report)[::2]:

IF report must act as a generator (huge file), then

try:
    for x in report:
        line = next(report)
        list.append(line.strip()) #dont use list as variable name
except StopIteration:
    pass

Answer 4

list=[] for line in report: list.append(line.strip()) report.readline() return list This is the right version. Thanks! Guys