Mixing files and loops

Question

I\'m writing a script that logs errors from another program and restarts the program where it left off when it encounters an error. For whatever reasons, the developers of t

Answer 1

You get the ValueError because your code probably has for line in original: in addition to original.readline(). An easy solution which fixes the problem without making your program slower or consume more memory is changing

for line in original:
    ...

to

while True:
    line = original.readline()
    if not line: break
    ...

Answer 2

Assuming you need only one line, this could be of help

import itertools

def getline(fobj, line_no):
    "Return a (1-based) line from a file object"
    return itertools.islice(fobj, line_no-1, line_no).next() # 1-based!

>>> print getline(open("/etc/passwd", "r"), 4)
'adm:x:3:4:adm:/var/adm:/bin/false\n'

You might want to catch StopIteration errors (if the file has less lines).

Answer 3

Use for and enumerate.

Example:

for line_num, line in enumerate(file):
    if line_num < cut_off:
        print line

NOTE: This assumes you are already cleaning up your file handles, etc.

Also, the takewhile function could prove useful if you prefer a more functional flavor.

Answer 4

Here's a version without the ugly while True pattern and without other modules:

for line in iter(original.readline, ''):
    if …:   # to the beginning of the title or abstract
        for i in range(lineNumber):
            print original.readline(),
        break