Python Language Generators Searching


The next function is useful even without iterating. Passing a generator expression to next is a quick way to search for the first occurrence of an element matching some predicate. Procedural code like

def find_and_transform(sequence, predicate, func):
    for element in sequence:
        if predicate(element):
            return func(element)
    raise ValueError

item = find_and_transform(my_sequence, my_predicate, my_func)

can be replaced with:

item = next(my_func(x) for x in my_sequence if my_predicate(x))
# StopIteration will be raised if there are no matches; this exception can
# be caught and transformed, if desired.

For this purpose, it may be desirable to create an alias, such as first = next, or a wrapper function to convert the exception:

def first(generator):
        return next(generator)
    except StopIteration:
        raise ValueError