List Methods, Step by Step
Interactive lab
Try it: List Methods, Step by Step
What append, extend, insert, remove, pop, del, index, count, sort and reverse really do to a Python list — which items shift, what each returns, CPython's exact error messages — and why lst = lst + other makes a new list while += changes it in place.
How it works
- append/extend add at the end; nothing else moves.
- insert(i, x) resolves i (negative counts from the end; out of range is clamped, never an error), shifts the tail right, then places x.
- remove(x) scans for the FIRST equal item (ValueError if none); pop(i) and del lst[i] take one position (IndexError if out of range); the tail shifts left.
- index/count scan and return a number; sort/reverse rearrange in place and return None.
- lst + other builds a new list object and rebinds the name; lst += other extends the same object.
Default run (5 steps): lst = [30, 10, 20, 10] — list object #1, 4 items. Choose a method. Example: lst.insert(1, 99). … lst.insert(1, 99) returned None. lst is now [30, 99, 10, 20, 10].
Simplified: A list of up to 12 ints; sort() is shown as its result, not step by step.
Educational simulation
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