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 cat3 

and you do file.readline() you will get cat1\n . The next file.readline() will return cat2\n .

Is there some functionality like file.some_function_here_nextline() to get cat1\n then you can later do file.readline() and get back cat1\n?

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