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itertools for lazy iteration patterns

itertools gives you building blocks for lazy iteration. It is especially useful when you want to transform, combine, or slice streams of values without building extra lists.

Why it is useful​

A few high-value tools cover many cases:

  • chain() to combine iterables
  • islice() to take a slice from any iterable
  • product() for combinations
  • groupby() for grouped iteration

Example:

from itertools import chain

values = chain([1, 2], [3, 4])
print(list(values))

Output:

[1, 2, 3, 4]

Where it shines​

itertools works well for:

  • data pipelines
  • lazy processing
  • combinatorics
  • iterator-heavy code

When not to use it​

Some iterator chains become harder to read than a normal loop. Prefer the clearest version, especially when the logic is not naturally stream-shaped.

Rules of thumb​

  • Reach for itertools when you are shaping iterables lazily.
  • Use it to avoid unnecessary intermediate lists.
  • Stop when the pipeline becomes harder to read than a loop.