Preferred way to check if a line starts with a string from list?

I'm trying to to sort through a file line by line, comparing the beginning with a string from a list, like so:

for line in lines:
    skip_line = True
    for tag in tags:
        if line.startswith(tag) is False:
            skip_line = False
    if skip_line is False:
        #do stuff

While the code works just fine, I'm wondering if there's a neater way to check for this condition. I have looked at any(), but it seems to just give me the possibility to check if any of my lines start with a fixed tag (not eliminating the for loop needed to loop through my list.

So, essentially I'm asking this: Is there a better, sleeker option than using a for loop to iterate over my tags list to check if the current line starts with one of its elements?

As Paradox pointed out in his answer: Using a dictionary to lookup if the string exists has O(1) complexity and actually makes the entire code look a lot cleaner, while being faster than looping through a list. Like so:

tags = {'ticker':0, 'orderBook':0, 'tradeHistory':0}
for line in lines:
    if line.split('\t')[0] in tags:
        #do stuff


Instead of iterating over your tags list, you can put all your tags inside a HashMap and do a simple lookup like myMap.exists("word"). This would be much faster that iterating through your tags list and works in O(1) complexity. In python its actually a dictionary data structure.

If you're determined to pull this down into a one-liner, you can use a generator:

tagged_lines = (line for line in lines if any(line.startswith(tag) for tag in tags))
for line in tagged_lines:
    # Do something with line here 

Of course, how readable this is is a different question.

You've probably seen syntax like [x*x for x in range(10)] before, but by swapping the [] for (), we instead generate each item only when it's asked for.

This has been asked before. Take a look at this post for more solutions. I would flag this post as a duplicate but I still do not have the reputation.

You'll need to modify the regular expression so that it looks at the start of the line. Something like this should work for you '^tag' .

How about a combination off any() and filter() like in this example:

# use your data here ...
mytags = ('hello', 'world')
mylines = ('hello friend', 'you are great', 'world is cruel')

result = filter(lambda line: any(map(lambda tag: line.startswith(tag), mytags)), mylines)
print result

In fact any() will do the job

Looping each line

for line in lines:
     tagged = any(lambda: line.startswith(y), tags)

Any list start with any tag

any(lambda x: any(lambda y: x.startswith(y), tags), lines)

Filter tagged lines

filter(lambda x: any(lambda y: x.startswith(y), tags), lines)

Need Your Help

Eclipse failing to run libGDX iOS app

ios eclipse libgdx robovm

I'm trying to run my libGDX roboVM project on an actual device but I'm getting an error:

Ruby require directive causes an error

ruby-on-rails flickr

Trying to create a simple flickr application using Ruby on Rails 3.2.3, I faced a problem. First I installed a gem called flickraw