Add Create A Range Of Descending Values as a Python TIL

python/create-a-range-of-descending-values.md

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+# Create A Range Of Descending Values
+
+A typical use of `range` looks something like this:
+
+```python
+>>> list(range(1, 5))
+[1, 2, 3, 4]
+```
+
+Which is equivalent to this one where we give a `step` value of `1`.
+
+```python
+>>> list(range(1, 5, 1))
+[1, 2, 3, 4]
+```
+
+If we try to create a _negative range_, that is, a range of values in decreasing
+order, we get an empty list.
+
+```python
+>>> list(range(0, -7))
+[]
+```
+
+That's because the `step` value still defaults to `1`. And there are no positive
+steps between `0` and `-7`. So, let's give `range` a `step` value of `-1`.
+
+```python
+>>> list(range(0,-7, -1))
+[0, -1, -2, -3, -4, -5, -6]
+```
+
+One practical use case of a negative range like this is using a list
+comprehension to transform it into a list of the _last seven days_.
+
+```python
+>>> from datetime import datetime, timedelta
+>>> [datetime.now().date() + timedelta(days=days) for days in range(0,-7, -1)]
+[datetime.date(2026, 3, 19), datetime.date(2026, 3, 18), datetime.date(2026, 3, 17), datetime.date(2026, 3, 16), datetime.date(2026, 3, 15), datetime.date(2026, 3, 14), datetime.date(2026, 3, 13)]
+```
+
+Of course this could have been written with a positive range and then
+subtracting the `timedelta`. I like that I have the option of doing this in
+whatever way makes the code most readable.