nltk.collections.LazyZip

A lazy sequence whose elements are tuples, each containing the i-th element from each of the argument sequences. The returned list is truncated in length to the length of the shortest argument sequence. The tuples are constructed lazily -- i.e., when you read a value from the list, LazyZip will calculate that value by forming a tuple from the i-th element of each of the argument sequences.

LazyZip is essentially a lazy version of the Python primitive function zip. In particular, an evaluated LazyZip is equivalent to a zip:

>>> from nltk.collections import LazyZip
>>> sequence1, sequence2 = [1, 2, 3], ['a', 'b', 'c']
>>> zip(sequence1, sequence2) # doctest: +SKIP
[(1, 'a'), (2, 'b'), (3, 'c')]
>>> list(LazyZip(sequence1, sequence2))
[(1, 'a'), (2, 'b'), (3, 'c')]
>>> sequences = [sequence1, sequence2, [6,7,8,9]]
>>> list(zip(*sequences)) == list(LazyZip(*sequences))
True

Lazy zips can be useful for conserving memory in cases where the argument sequences are particularly long.

A typical example of a use case for this class is combining long sequences of gold standard and predicted values in a classification or tagging task in order to calculate accuracy. By constructing tuples lazily and avoiding the creation of an additional long sequence, memory usage can be significantly reduced.