在两个特定行之间提取时

Question

practising using DropWhile in Python, and have hit a bump.

For example if this is line in a file:

Test1
Test2
Test3
Test4
Test5
Test6
Test7
Test8
Test9
Test10

And I want to pull out the lines between Test5 and Test8.

I know how to do this another way (for line in file...get the last number of line...if line > 5....if line < 8...print); but I specifically want to practise using DropWhile.

I tried this a few different ways but I can't seem to get it to work:

eg

dataset = open(sys.argv[1]).readlines()
def print_out(line):
    if int(line.strip()[-1]) > 5:
        if int(line.strip()[-1]) < 8:
            return True
        else:
            return False

for line in dropwhile(lambda line: print_out(line) == True, dataset):
    print line.strip()

This doesn't work, all lines are printed out.

Another way I tried to use a long lambda expression in the dropwhile line instead of using a separate function, but when I did something like this:

for line in dropwhile(lambda line: 5 < int(line.strip()[-1]) < 8, dataset):

This code works if I only have one expression (ie int(line.strip()[-1]) > 5 or int(line.strip()[-1]) < 8, but not both).

I'm wondering if someone could show me a pythonic way, using DropWhile, to pull out the lines between Test5 and Test8 in my test data set?

Answer 1

DropWhile is not what you need, from a functional aproach you have to use filter:

filter(lambda line: 5 < int(line.strip()[-1]) < 8, dataset)

DropWhile will stop once the condition is reached one time, so the list will continue to have the rest of the values once it reach Test6

Answer 2

If you plan to use dropwhile() on your dataset, then you need to also make use of takewhile() to grab the required lines as follows:

from itertools import takewhile, dropwhile


for line in takewhile(lambda x:  int(x.strip()[-1]) < 8, dropwhile(lambda x:  int(x.strip()[-1]) <= 5, dataset)):
    print line.strip()

This would give you:

Test6
Test7

So it works in two steps, first dropping each line until the required start point, and then only taking lines until the required end point, at which point it completes.