10.7 List Methods

All eleven list methods, grouped by what they do. append/extend/insert and remove/pop/clear were already covered individually in 10.4 Modifying Lists and 10.5 Removing Elements — this is the complete lookup table.

Query Methods

index() returns the position of the first matching value and raises ValueError if absent — the list equivalent of str.index() (see 9.7 Common String Methods). count() returns how many times a value appears.

>>> [1, 2, 3, 2].index(2)
1
>>> [1, 2, 3, 2].count(2)
2

sort()

Sorts the list in place and returns None — a common gotcha is writing l = l.sort(), which discards the list entirely. See 10.12 Sorting and Searching for sort() vs. sorted().

>>> l = [3, 1, 2]
>>> l.sort()
>>> l
[1, 2, 3]

reverse()

Reverses the list in place — distinct from the l[::-1] slice idiom, which returns a new list instead.

>>> l = [1, 2, 3]
>>> l.reverse()
>>> l
[3, 2, 1]

copy()

Returns a shallow copy — equivalent to l[:] (see 10.3 List Indexing and Slicing and 10.11 Copying and Mutability).

>>> l2 = l.copy()
>>> l2 is l
False

Quick Interview Answer

“The eleven list methods split cleanly into four groups: add (append, extend, insert), remove (remove, pop, clear), query (index, count — read-only, no mutation), and reorder/copy (sort, reverse, copy). The one detail worth memorizing cold: sort() and reverse() both mutate in place and return None — assigning their result (l = l.sort()) silently throws the list away.”

Common Mistakes

• Writing l = l.sort() expecting the sorted list back — sort() returns None; the list is already sorted in place, so just use l directly afterward.
• Using index() when the value might not be present — it raises ValueError instead of returning a sentinel like -1; check in first or catch the exception.
• Confusing l.reverse() (in-place, mutates, returns None) with l[::-1] or reversed(l) (both return a new sequence, original untouched).