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