如何遍历字符串列表以在条件中使用它们？

Question

source = open("file1")    
out = open("file2", "w")

days = ['Mon','Tue','Wed','Thu','Fri','Sat','Sun']

for line in source:
    out.write(line)
    if line.startswith('HERE IS WHERE I WOULD LIKE THE DAYS TO BE LOOPED'):
        break    
out.close()

Answer 1

Since all your strings are the same length, you could do this:

if line[:3] in days:
    break

If that's too much of an assumption:

if any(line.startswith(day) for day in days):
    break

Another tip (assuming you're on Python 2.7 or 3.2:

with open("file1") as source, open("file2", "w") as out:
    for line in source:
        out.write(line)
        if any(line.startswith(day) for day in days):
            break

and you don't have to think about closing your files manually, even if an exception occurs.

And if you're still on Python 2.4 or lower (why??) just build your own any() function:

def any(iterable):
    for element in iterable:
        if element:
            return True
    return False

Answer 2

Looking at help(str.startswith) , you can see that the method accepts a tuple of strings to search for, so you can do it all in one step:

>>> 'Mon is the first day'.startswith(('Mon','Tue','Wed','Thu','Fri','Sat','Sun'))
True

Here is a variant that runs on older versions of Python:

>>> import re
>>> days = ['Mon','Tue','Wed','Thu','Fri','Sat','Sun']
>>> pattern = '|'.join(days)
>>> if re.match(pattern, 'Tue is the first day'):
        print 'Found'

Found

Answer 3

if line.startswith(tuple(days)):

from the documentation:

S.startswith(prefix[, start[, end]]) -> bool

Return True if S starts with the specified prefix, False otherwise.
With optional start, test S beginning at that position.
With optional end, stop comparing S at that position.
prefix can also be a tuple of strings to try.

The last line is the answer to your question.

Answer 4

Try it like this:

if line[:3] in days:
    # True
    pass

Answer 5

#Hackish, but works
if min([line.find(x) for x in days]) == 0: