Python not equal 'for' iteration

Question

During the iteration over the text file and over the list I've found the unexpected behavior of for loop. Text file test.txt consists of only two strings: 1) He said: and 2) We said:. First for+for loop

file_ = open('c:\\Python27\\test.txt', 'r')
list_ = ['Alpha','Bravo','Charlie']
try:
    for string in file_:
        for element in list_:
            print string, element
except StandardError:
    print 'Ooops'

returns absolutely expectable result:

He said: Alpha
He said: Bravo
He said: Charlie
We said: Alpha
We said: Bravo
We said: Charlie

But if the for order was changed to

file_ = open('c:\\Python27\\test.txt', 'r')
list_ = ['Alpha','Bravo','Charlie']
try:
    for element in list_:
        for string in file_:
            print string, element
except StandardError:
    print 'Ooops'

the result is totally different:

He said: Alpha
We said: Alpha

Looks like the first for became uniterable. Why?

`lines = file.readlines()` then just do your iteration over the `lines` list. — David Zemens, Oct 13 '15 at 19:41

score 3 · Answer 1 · answered Oct 13 '15 at 19:28

3

When you complete the for loop over the file, you "consume" it. You would have to go back to its beginning for each iteration of the outer loop with file_.seek(0). Or use the original loop ordering.

You can replicate the behaviour with this simplified example:

    for string in file_:
        print string

    for string in file_:
        print string

where you'll see the second loop does nothing.

answered Oct 13 '15 at 19:28

juanchopanza

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Thanks, it's very forceful example – heavy_drinker Oct 13 '15 at 20:14

Dimitris Fasarakis Hilliard · Answer 2 · 2015-10-13T19:36:18.513

2

You need to use file_.seek(0) to return to the beginning of the file in order to re-iterate through its contents.

You have iterated through it via : for string in file_: and so the current position, obtained by calling file_.tell(), will be the size of the file in bytes (if I am not mistaken), signifying the end.

edited Oct 13 '15 at 19:36

answered Oct 13 '15 at 19:29

Dimitris Fasarakis Hilliard

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Python not equal 'for' iteration

2 Answers2