Determining the Emptiness of a Generator

In Python, generators are a powerful tool for lazy evaluation and iterating over large datasets. However, determining if a generator is empty presents a unique challenge due to its dynamic nature. Unlike lists or tuples, generators do not have a size attribute, and simply iterating over them would result in a StopIteration exception.

Peek vs. HasNext

Some may suggest using the peek() function to check if a generator has items. While peek() allows you to retrieve the first item of a generator without consuming it, it does not provide a way to determine if the generator is empty. This is because peek() raises a StopIteration exception when the generator is empty, which is indistinguishable from the exception that would be raised when trying to iterate over an empty generator.

Similarly, the hasNext() function, commonly found in Java, is not available in Python generators.

Checking for Emptiness from the Start

To determine if a generator is empty from the start, you can use a custom function like the one provided in the response:

def peek(iterable):
    try:
        first = next(iterable)
    except StopIteration:
        return None
    return first, itertools.chain([first], iterable)

This function retrieves the first element of the generator, returning None if the generator is empty. It also preserves the generator's state by returning a modified version that includes the first element.

Usage

To use this function, you can call it with the generator as an argument:

res = peek(mysequence)
if res is None:
    # sequence is empty.  Do stuff.
else:
    first, mysequence = res
    # Do something with first, maybe?
    # Then iterate over the sequence:
    for element in mysequence:
        # etc.

This check allows you to determine if the generator is empty and handle the empty case appropriately, before iterating over it.