Order Values Predictably

Return an ordered list without changing its source, then make value-first ordering and label tie-breaking explicit through a named key.

Measurements often arrive in collection order but need a different order for comparison or display. Python can return an ordered result while preserving the source sequence.

`sorted` Returns a New List

sorted(readings) returns a new list. It does not change readings. This is useful when both arrival order and numerical order matter.

A list's .sort() method behaves differently:

readings.sort()

It changes the existing list and returns None. Do not write readings = readings.sort(), because that replaces the list reference with None.

Use reverse=True when descending order is the actual requirement:

highest_first = sorted(readings, reverse=True)

Q1. Keep the source order

After this code runs, what are ordered and readings?

Choose one

ordered is [1, 2, 3]; readings is [3, 1, 2]

both names refer to [1, 2, 3]

ordered is None; readings is [1, 2, 3]

Select one choice, then check.

HintFollow the returned value

sorted returns a new list; this code never calls readings.sort().

SolutionThe two orders remain available

ordered receives [1, 2, 3], while readings keeps [3, 1, 2].

Name the Value Used for Ordering

Labelled readings can be represented as small pairs:

To order by reading and then by label, define a key function:

Python calls the key function for each report. The returned pair supplies the order: compare the reading first; if readings tie, compare the label. The result is:

The key names the rule. It does not change the report.

Q2. Read a named sort key

What key does reading_then_label(("sensor-b", 20)) return?

Choose one

(20, "sensor-b")

("sensor-b", 20)

20

Select one choice, then check.

HintRead the return expression

The function returns reading, label.

SolutionThe key reverses the input roles

The report unpacks to label = "sensor-b" and reading = 20, then the key returns (20, "sensor-b").

Read the Equivalent Small `lambda`

You may see the same simple rule written without a separate function:

ordered = sorted(reports, key=lambda report: (report[1], report[0]))

Read lambda report: ... as a small unnamed function with one parameter and one returned expression. Here it returns the reading at index 1 and the label at index 0. The named function is clearer when the rule needs explanation, testing, or reuse.

Q3. Order labelled readings

Complete the named key so the program orders by reading and then label without changing reports.

Editable Python

HintReturn the comparison order

Use return reading, label, then assign ordered = sorted(reports, key=reading_then_label).

SolutionUse a named two-part key

sorted can produce a predictable order without changing the source. A named key makes the comparison rule visible, including tie-breaking. The next chapter uses meaningful keys for lookup and counting, then examines uniqueness and shared or copied collections.

sorted Returns a New List