python parse prints only first line from list

Question

I have a list 'a',where I need to print all the matching letters of the list with the line of a text file 'hello.txt'.But it only prints the first word from the list and line instead of all the list and lines

a=['comp','graphics','card','part']

with open('hello.txt', 'r') as f:
    for key in a:
        for line in f:
            if key in line:
                print line, key

It results as:

comp and python
comp

Desired output:

comp and python
comp
graphics and pixel
graphics
micro sd card
card
python part
part

Please help me to get desires output.Answers willbe appreciated!

Community · Accepted Answer · 2017-05-23 12:13:09Z

7

The file-object f is an iterator. Once you've iterated it, it's exhausted, thus your for line in f: loop will only work for the first key. Store the lines in a list, then it should work.

a=['comp','graphics','card','part']
with open('hello.txt', 'r') as f:
    lines = f.readlines()  # loop the file once and store contents in list
    for key in a:
        for line in lines:
            if key in line:
                print line, key

Alternatively, you could also swap the loops, so you iterate the file only once. This could be better if the file is really big, as you won't have to load all it's contents into memory at once. Of course, this way your output could be slights different (in a different order).

a=['comp','graphics','card','part']
with open('hello.txt', 'r') as f:
    for line in f:     # now the file is only done once...
        for key in a:  # ... and the key loop is done multiple times
            if key in line:
                print line, key

Or, as suggested by Lukas in the comments, use your original loop and 'reset' the file-iterator by calling f.seek(0) in each iteration of the outer key loop.

edited May 23, 2017 at 12:13

CommunityBot

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answered Jul 20, 2014 at 10:31

tobias_k

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2 Comments

Lukas Graf Over a year ago

Or alternatively, use f.seek(0) to reset the cursor to the starting position for every term in a.

Lukas Graf Over a year ago

@user3830379 readlines isn't really the important aspect of tobias' answer. "You will exhaust iterators as you consume them" is the important part to take home. For most other use cases, your for line in f: is absolutely correct and the best way to iterate over files line by line, and should be preferred over readlines()!

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