How to treat the first line of a file differently in Python?

Question

I often need to process large text files containing headers in the first line. The headers are often treated differently to the body of the file, or my processing of the body is dependent on the headers. Either way I need to treat the first line as a special case. I could use simple line iteration and set a flag:

headerProcessed = false
for line in f:
    if headerProcessed:
        processBody(line)
    else:
        processHeader(line)
        headerProcessed = true

but I dislike a test in the loop that is redundant for all but one of the millions of times it executes. Is there a better way? Could I treat the first line differently then get the iteration to start on the second line? Should I be bothered?

Answer 1

You could:

processHeader(f.readline())
for line in f:
    processBody(line)

Answer 2

Use iter()

it_f = iter(f)
header = it_f.next()
processHeader(header)

for line in it_f:
    processBody(line)

It works with any iterable object.

Answer 3

f = file("test")
processHeader(f.next()) #or next(f) for py3
for line in f:
    processBody(line)

This works.

Edit:

Changed .__next__ to next (they are equivalent, but I suppose next is more concise).

Regaring file vs open, file just seems more clear to me, therefore I will continue to prefer it over open.

Answer 4

Large text files with headers in the first line? So it's tabular data.

Just to make sure: Have you looked at the csv module? It should handle all tabular data except such where the fields are not delimited but defined by position. And it does the header stuff too.