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pythoniterable

Is file object in python an iterable

I have a file "test.txt":

this is 1st line
this is 2nd line
this is 3rd line

the following code

lines = open("test.txt", 'r')
for line in lines:
    print "loop 1:"+line
for line in lines:
    print "loop 2:"+line

only prints:

loop 1:this is 1st line

loop 1:this is 2nd line

loop 1:this is 3rd line

It doesn't print loop2 at all.

Two questions:

  1. the file object returned by open(), is it an iterable? that's why it can be used in a for loop?

  2. why loop2 doesn't get printed at all?

Solution

  • It is not only an iterable, it is an iterator [1], which is why it can only traverse the file once. You may reset the file cursor with .seek(0) as many have suggested but you should, in most cases, only iterate a file once.

    [1]: The "file" object is of class TextIOBase which is a subclass of IoBase, and IoBase supports the iterator protocol.