Question

Is it safe to read some lines with readline() and also use for line in file, and is it guaranteed to use the same file position?

Usually, I want to disregard the first line (headers), so I do this:

FI = open("myfile.txt")
FI.readline()             # disregard the first line
for line in FI:
    my_process(line)
FI.close()

Is this safe, i.e., is it guaranteed that the same file position variable is used while iterating lines?

Answer 1

No, it isn’t safe:

As a consequence of using a read-ahead
buffer, combining next() with other
file methods (like readline()) does
not work right.

You could use next() to skip the first line here. You should also test for StopIteration, which will be raised if the file is empty.

with open('myfile.txt') as f:
    try:
        header = next(f)
    except StopIteration as e:
        print "File is empty"
    for line in f:
        # do stuff with line