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

Solutions with tell()/seek() will not work with stdin and other iterators. More generic implementation can be as simple as this:

class lookahead_iterator(object):
    __slots__ = ["_buffer", "_iterator", "_next"]
    def __init__(self, iterable):
        self._buffer = [] 
        self._iterator = iter(iterable)
        self._next = self._iterator.next
    def __iter__(self):
        return self 
    def _next_peeked(self):
        v = self._buffer.pop(0)
        if 0 == len(self._buffer):
            self._next = self._iterator.next
        return v
    def next(self):
        return self._next()
    def peek(self):
        v = next(self._iterator)
        self._buffer.append(v)
        self._next = self._next_peeked
        return v

Usage:

with open("source.txt", "r") as lines:
    lines = lookahead_iterator(lines)
    magic = lines.peek()
    if magic.startswith("#"):
        return parse_bash(lines)
    if magic.startswith("/*"):
        return parse_c(lines)
    if magic.startswith("//"):
        return parse_cpp(lines)
    raise ValueError("Unrecognized file")