Reading a Line From File Without Advancing [Pythonic Approach]

Question

What\'s a pythonic approach for reading a line from a file but not advancing where you are in the file?

For example, if you have a file of

cat1
cat2         


        

Answer 1

As far as I know, there's no builtin functionality for this, but such a function is easy to write, since most Python file objects support seek and tell methods for jumping around within a file. So, the process is very simple:

• Find the current position within the file using tell.
• Perform a read (or write) operation of some kind.
• seek back to the previous file pointer.

This allows you to do nice things like read a chunk of data from the file, analyze it, and then potentially overwrite it with different data. A simple wrapper for the functionality might look like:

def peek_line(f):
    pos = f.tell()
    line = f.readline()
    f.seek(pos)
    return line

print peek_line(f) # cat1
print peek_line(f) # cat1

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You could implement the same thing for other read methods just as easily. For instance, implementing the same thing for file.read:

def peek(f, length=1):
    pos = f.tell()
    data = f.read(length) # Might try/except this line, and finally: f.seek(pos)
    f.seek(pos)
    return data

print peek(f, 4) # cat1
print peek(f, 4) # cat1