TopolearnSlicing is the idiomatic way to take parts of sequences without index arithmetic loops.
A slice seq[start:stop:step] selects a range of a list, tuple, string or any sequence. It includes start and excludes stop, just like range. Every part is optional:
s[:3]the first three items,s[3:]everything from index 3s[-2:]the last two,s[:-1]all but the lasts[::2]every second item,s[::-1]the sequence reverseds[:]a shallow copy of the whole thing
Slices are forgiving: unlike indexing, out-of-range bounds are clipped rather than raising, so "abc"[1:100] is just "bc" and [][:5] is []. That makes "the first n items" safe on short inputs.
A slice always returns a new object of the same type (for built-in sequences). For lists that means a copy of the references — modifying the slice doesn't change the original.
On mutable sequences you can also assign to a slice, which replaces that range with the items of an iterable, even of a different length: nums[1:3] = [7, 8, 9]. del nums[::2] deletes every second item. Extended slices with a step require the replacement to have exactly the same length.
When the same slice is used in several places, name it: HEADER = slice(0, 4) then record[HEADER]. The slice object is exactly what the colon syntax creates under the hood, and your own classes receive it in __getitem__.
For very large data, remember each slice copies; NumPy arrays and memoryview slices are views instead.
Example
letters = "abcdefgh"
print(letters[2:5]) # cde
print(letters[:3], letters[-3:]) # abc fgh
print(letters[::2]) # aceg
print(letters[::-1]) # hgfedcba
print(letters[5:100]) # fgh (bounds are clipped)
nums = [0, 1, 2, 3, 4, 5]
copy = nums[:]
copy[0] = 99
print(nums[0]) # 0
nums[1:3] = [7, 8, 9] # replace 2 items with 3
print(nums) # [0, 7, 8, 9, 3, 4, 5]
del nums[::2]
print(nums) # [7, 9, 4]
YEAR = slice(0, 4)
print("2026-10-16"[YEAR]) # 2026Remember
s[start:stop:step]includes start, excludes stop- Slices clip out-of-range bounds instead of raising
s[::-1]reverses,s[:]shallow-copies
Watch out for
- Assuming a list slice is a view; changes to it don't affect the original.