Build and Change Lists

Begin with an empty result, append accepted readings in order, and distinguish a changed list from the None returned by a mutating method.

The measurement program can inspect existing readings. It may also need to collect a new list containing only the readings it accepts.

readings = [18, None, 21, 20, None, 23]

Begin with an Empty Result

An empty pair of square brackets creates a list with no items:

accepted_readings = []

The list is the result being built. Visit each source reading and append only an available one:

The output is:

[18, 21, 20, 23]

append adds one item to the end of the existing list. The order of the accepted readings remains the order in which the loop visited them. Repeated values would remain repeated; this loop filters unavailable readings but does not remove duplicates.

Trace the visible change after each accepted item:

Source reading	Action	`accepted_readings` after the action
18	append	`[18]`
`None`	skip	`[18]`
21	append	`[18, 21]`
20	append	`[18, 21, 20]`
`None`	skip	`[18, 21, 20]`
23	append	`[18, 21, 20, 23]`

Q1. Trace a growing result list

What is kept after this loop ends?

Choose one

[12, 15, 12]

[12, 15]

[12, None, 15, 12]

Select one choice, then check.

HintDo not remove duplicates

The condition filters only None.

SolutionKeep three readings

The loop appends 12, then 15, then the second 12, producing [12, 15, 12].

Mutation Changes the Existing List

append is a mutating method: it changes the list on which it is called. The name continues to refer to that changed list:

The method performs a change rather than calculating a replacement list. Its return value is None:

This distinction matters when reading an assignment:

accepted_readings = accepted_readings.append(23)

The method first changes the list. It then returns None, and the assignment makes accepted_readings refer to that None result. The name no longer gives access to the changed list. Call append on its own line instead.

Q2. Separate a change from its return value

What does this program display on its second line?

Choose one

[18, 21]

21

None

Select one choice, then check.

HintFollow both effects

values changes before the result of append is assigned to returned.

SolutionThe method returns None

The first line is [18, 21]. The second line is None because append reports no replacement value.

Build Derived Labels in the Same Way

A result list can contain values calculated during the loop. Here each accepted reading becomes a short label:

This displays:

['18 °C', '21 °C', '20 °C', '23 °C']

The source readings list remains as it was. The loop builds a separate result named labels. Chapter 6 will examine what happens when two names refer to the same changeable object; this lesson needs only the visible mutation of one named result list.

Text behaves differently. A string is ordered and supports indexing and slicing, but an individual character cannot be replaced:

Instead, string operations return new text. The next lesson will keep each intermediate result visible.

Q3. Repair a lost result list

The program should display [18, 22]. Repair the line that collects each available reading without replacing the list name with None.

Editable Python

HintLet the list name remain in place

Remove accepted = from the append line.

SolutionAppend without reassignment

An ordinary loop can build a result list one item at a time. append changes that list and returns None; it does not produce a replacement list. Text cannot be changed character by character, so the next lesson will transform it through explicit returned values.