Sometimes the order is not a tuple. Then you write a comparison that says which item comes first, and make sure it is a consistent order.
About 2 min of reading
Before this new lesson, one small question about the lesson you just finished. Recall it, do not look it up.
Order items by a custom rule.
Sort by a rank map.
Before you run it, say out loud what it will print. Then open the answer.
Sort with a comparator.
Before you run it, say out loud what it will print. Then open the answer.
Reverse one field only.
Before you run it, say out loud what it will print. Then open the answer.
Compare comparator calls.
Before you run it, say out loud what it will print. Then open the answer.
Before you write anything, put these lines back in the right order. Click a line to move it up one step. This support step is easier than writing code from scratch, and it is here to make the last example stick.
Read the first example one line at a time. A line that prints shows what appears on screen; the rest set things up quietly.
Express the order as a value when you can. The key runs once per item; a comparator runs once per comparison, about n log n times.
If the comparison is not consistent, the sorted result is not predictable. Test the three rules: reflexive, antisymmetric, transitive.
from functools import cmp_to_key
def cmp(a, b):
return -1 if a + b > b + a else 1
nums = [3, 30, 34]
print(sorted(nums, key=cmp_to_key(cmp)))The comparator decides order by rule. Here it puts the concatenation that reads bigger first.
rows = [1, 2, 3]
print(sorted(rows, key=lambda x: x))A comparator must agree with itself. If a beats b and b beats c, a must beat c, or the sort can misbehave.
rows = [(1, 2), (1, 2)]
print(sorted(rows))Equal items must compare equal, not one before the other. A partial order leaves some pairs unordered.
order = {"low": 0, "mid": 1, "high": 2}
rows = ["high", "low", "mid"]
print(sorted(rows, key=order.get))A dictionary maps each label to its rank. Sorting by the rank gives the order you chose.
rows = [(5, "a"), (5, "b"), (5, "c")]
rows.sort(key=lambda r: r[0])
print(rows)Stable sort keeps input order on ties. If that is not the order you want, add the tiebreaker field.
rows = [(1, 3), (1, 1)]
rows.sort(key=lambda r: (r[0], -r[1]))
print(rows)Negate the field that should descend. The other fields stay ascending.
words = ["Owlww", "apple", "Bee"]
print(sorted(words, key=str.lower))Lower-casing in the key gives dictionary order. The default puts all capitals before all lowercase.
import math
print(round(1000 * math.log2(1000)))A comparator runs about n log n times. A key function runs n times, so a key is cheaper when it can express the order.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.