Is it safe to mix readline() and line iterators in python file processing?

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