Trivial

Range

It will not generate all of number immediately, but generate when it is used. And it will not return a list unless you convert it to.

>>> range(3, 6)
range(3, 6) # not [3, 4, 5]
 
>>> r = range(3, 6)
>>> [r[0], r[2]]
[3, 5]
 
>>> range(4)[-1]
3

In Python, a for loop’s iteration count is determined when the loop starts by the initial sequence (e.g., range(n)), and does not change dynamically if you modify variables afterward. Key points:

1. range(n) creates a fixed sequence at the start; changing n during the loop won’t affect iterations.
2. Modifying a list (e.g., inserting/deleting elements) while using for i in range(len(list)) won’t change the loop count, but can cause index mismatches (elements may be skipped or revisited).
3. For dynamic scenarios (e.g., inserting into a list while iterating), use a while loop to manually manage the index and check the list’s current length.

Conclusion: Use for loops for fixed iterations; use while loops when you need to adjust the index dynamically (e.g., when modifying the iterated container).