Add reverse_range() (#615)

Author: gahjelle

python-range/README.md

@@ -2,6 +2,22 @@
 
 This repository holds the code for Real Python's [Python `range()`: Represent Numerical Ranges](https://realpython.com/python-range/) tutorial.
 
+## reverse_range()
+
+In [`reverse_range.py`](reverse_range.py), you can find an explicit implementation of a function that can reverse a general range.
+
+```python
+>>> from reverse_range import reverse_range
+
+>>> reverse_range(range(1, 20, 4))
+range(17, 0, -4)
+
+>>> list(reverse_range(range(1, 20, 4)))
+[17, 13, 9, 5, 1]
+```
+
+In practical applications, you should use `reversed(range(1, 20, 4))` or `range(1, 20, 4)[::-1]` instead.
+
 ## PiDigits
 
 The file [`pi_digits.py`](pi_digits.py) shows the implementation of `PiDigits` which is an integer-like type that can be used as arguments to `range()`:

python-range/reverse_range.py

@@ -0,0 +1,27 @@
+def reverse_range(rng):
+    """Explicitly calculate necessary parameters to reverse a general range.
+
+    In practice, you should use reversed() or [::-1] instead.
+    """
+    adj = 1 if rng.step > 0 else -1
+    return range(
+        (rng.stop - adj) - (rng.stop - rng.start - adj) % rng.step,
+        rng.start - adj,
+        -rng.step,
+    )
+
+
+if __name__ == "__main__":
+    numbers = range(1, 20, 4)
+
+    print("\nOriginal:")
+    print(numbers)
+    print(list(numbers))
+
+    print("\nReversed:")
+    print(reverse_range(numbers))
+    print(list(reverse_range(numbers)))
+
+    print("\nTwice reversed, has the same elements as the original:")
+    print(reverse_range(reverse_range(numbers)))
+    print(list(reverse_range(reverse_range(numbers))))