Lazy evaluation defers producing values until a program needs them. Python's range(), reversed(), map(), and filter() return lazy results rather than immediately building complete lists. section 10.1.3 section 14.5
Use a consumer when you need to force or traverse a lazy result:
- Call list() in the REPL to materialize values, such as list(range(0, 44)). section 10.1.3
- Iterate directly with a for loop; range() supplies indexes as the loop requests them. section 10.1.3
- Wrap reversed(range(1, 5)) in list() to inspect [4, 3, 2, 1]. section 11.1.1
- Consume map() or filter() with join(), a loop, or list(); filter() returns only elements accepted by its function. section 14.3.1 section 14.5
Inspecting deferred values
>>> range(len('The quick brown fox jumps over the lazy dog.')) range(0, 44) >>> list(range(len('The quick brown fox jumps over the lazy dog.'))) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43] section 10.1.3
Filtering without an eager list
>>> cars = ['blue Honda', 'red Chevy', 'blue Ford'] >>> list(filter(lambda car: car.startswith('blue '), cars)) ['blue Honda', 'blue Ford'] The filter() computation remains lazy until list() requests its results; a list comprehension expresses the same selection more directly. section 14.3.1
>>> list(reversed(range(1, 5))) [4, 3, 2, 1]
Shows a concrete reversed() result after list() forces the lazy sequence. section 11.1.1
