Extracts elements with a step of 2 (every other item) from a sequence.

Expresses the intent to downsample or sample alternate items from a list, string, or other sequence.

When you need to sample every other element (e.g., get even-indexed items) When downsampling a signal or time series When extracting alternating characters from a string

Working with lists, strings, tuples, or NumPy arrays Data preprocessing or feature extraction Implementing algorithms that require stride-2 access

[start:stop:step] where start and stop are omitted and step = 2

[start:stop:step] with start and stop omitted, step = 2

object: list|string|tuple|array start: int (optional) stop: int (optional) step: int (here 2)

• list string tuple numpy.ndarray range

• different step values (e.g.
• 1
• 3
• -1) – adjust stride specifying start or stop indices – change range using negative step for reverse stride – iterate backwards

Using step 0 raises ValueError Assuming start defaults to end when step negative Off-by-one errors when expecting odd/even indices Confusing step with slice length

t[::1] – every element t[::-1] – reversed sequence t[1::2] – odd-indexed items t[::3] – every third element t[::-2] – every second from end

Coming from MATLAB: using end-1 indexing instead of slice step — Python uses zero-based indexing and slice syntax Coming from R: using negative indices to drop elements — Python negative indices index from end, not delete Coming from C: assuming pointer arithmetic with stride — Python slicing creates new object

• slice syntax

Avoid when you need to modify original list in-place (slicing creates a copy); performance: creates new object, O(n) memory and time; boundary: with negative start/stop and step=2, selection starts from end unless start specified

Signals intent to downsample or select alternating elements, often for efficiency or pattern extraction.

Every other item

A step of zero raises ValueError; step of 2 with omitted start/stop yields indices 0, 2, 4, ...

Learn arbitrary step slicing, then negative steps, then combine with start/stop for advanced slicing.