python looping through input file

Question

My question is related to file-input in Python, using open(). I have a text file mytext.txt with 3 lines. I am trying to do two things with this fi

Answer 1

Is there some reason you could not use the following:

input_file = open('mytext.txt', 'r')
count_lines = 0
for line in input_file:
    print line
    count_lines += 1
print 'number of lines:', count_lines

The thing returned by open is a file object. File objects keep track of their own internal position as you loop over them, so in order to do what you tried first, you would have to rewind it to the beginning manually, it won't do it by itself.

Answer 2

Try adding a input_file.seek(0) between the two loops. This will rewind the file back to the beginning, so you can loop over it again.

Answer 3

The file handle is an iterator. After iterating over the file, the pointer will be positioned at EOF (end of file) and the iterator will raise StopIteration which exits the loop. If you try to use an iterator for a file where the pointer is at EOF it will just raise StopIteration and exit: that is why it counts zero in the second loop. You can rewind the file pointer with input_file.seek(0) without reopening it.

That said, counting lines in the same loop is more I/O efficient, otherwise you have to read the whole file from disk a second time just to count the lines. This is a very common pattern:

with open('filename.ext') as input_file:
    for i, line in enumerate(input_file):
        print line,
print "{0} line(s) printed".format(i+1)

In Python 2.5, the file object has been equipped with __enter__ and __exit__ to address the with statement interface. This is syntactic sugar for something like:

input_file = open('filename.txt')
try:
    for i, line in enumerate(input_file):
        print line,
finally:
    input_file.close()
print "{0} line(s) printed".format(i+1)

I think cPython will close file handles when they get garbage collected, but I'm not sure this holds true for every implementation - IMHO it is better practice to explicitly close resource handles.

Answer 4

I thin the module fileinput is you want.

Here is the link

if __name__ == "__main__":
for line in fileinput.input():
    if fileinput.isfirstline():
        print("current file: %s" % fileinput.filename())

    print("line number: %d, current file number: %d" % 
          (fileinput.lineno(), fileinput.filelineno()))