Rename articles prefixed with python-

snippets/python/s/slice-notation.md

@@ -0,0 +1,78 @@
+---
+title: Understanding Python's slice notation
+shortTitle: Python slice notation
+type: story
+language: python
+tags: [list]
+cover: sliced-fruits
+excerpt: Learn everything you need to know about Python's slice notation with this handy guide.
+dateModified: 2021-06-12T19:30:41+03:00
+---
+
+#### Python slice notation
+
+- Understanding Python's slice notation (this blog post)
+- [Understanding Python's slice assignment](/blog/s/python-slice-assignment)
+
+### Basic syntax
+
+Python's slice notation is used to return a list or a portion of a list. The basic syntax is as follows:
+
+```py
+[start_at:stop_before:step]
+```
+
+Where `start_at` is the index of the first item to be returned (included), `stop_before` is the index of the element before which to stop (not included) and `step` is the stride between any two items.
+
+All three of the arguments are optional, meaning you can omit any of them. For example:
+
+```py
+nums = [1, 2, 3, 4, 5]
+
+nums[1:4]     # [2, 3, 4]   (start at 0, stop before 4)
+nums[2:]      # [3, 4, 5]   (start at 0, stop at end of list)
+nums[:3]      # [1, 2, 3]   (start at 0, stop before 3)
+nums[1:4:2]   # [2, 4]      (start at 1, stop before 4, every 2nd element)
+nums[2::2]    # [3, 5]      (start at 2, stop at end of list, every 2nd element)
+nums[:3:2]    # [1, 3]      (start at 0, stop before 3, every 2nd element)
+nums[::2]     # [1, 3, 5]   (start at 0, stop at end of list, every 2nd element)
+nums[::]      # [1, 2, 3, 4, 5] (start at 0, stop at end of list)
+```
+
+As you can probably tell from the examples above, the default values are `start_at = 0`, `stop_before = len(nums)`, `step = 1`.
+
+> An idiomatic way to shallow clone a list would be using `[:]` (e.g. `nums_clone = nums[:]`).
+
+### Negative values
+
+All three of the arguments also accept negative values. For `start_at` and `stop_before`, a negative value means counting from the end of the list instead of counting from the start. For example `-1` would represent the last element, `-2` the second last element etc. For example:
+
+```py
+nums = [1, 2, 3, 4, 5]
+
+nums[1:-2]    # [2, 3]      (start at 1, stop before 2nd to last)
+nums[-3:-1]   # [3, 4]      (start at 3rd to last, stop before last)
+```
+
+A negative `step` means that the list is sliced in reverse (from end to start). This also means that `start_at` should be greater than `stop_before` and that `stop_before` in the context of a reverse stride is more like `stop_after` if you are looking at the list non-reversed. For example:
+
+```py
+nums = [1, 2, 3, 4, 5]
+
+nums[::-1]    # [5, 4, 3, 2, 1]   (reversed)
+nums[4:1:-1]  # [5, 4, 3]   (reversed, start at 4, stop after 1)
+nums[-1:1:-2] # [5, 3]      (reversed, start at last, stop after 1, every 2nd)
+```
+
+### Empty slices
+
+Bear in mind that slice notation is very forgiving, so you'll get an empty list if the arguments' values are out of the list's range. For example:
+
+```py
+nums = [1, 2, 3, 4, 5]
+
+nums[6:8]     # []
+nums[:-10]    # []
+```
+
+[Continue on Understanding Python's slice assignment](/blog/s/python-slice-assignment)