Lists Lab · Lecture 3.2

List playground: change a list one operation at a time

Each button runs one line of real Python on the list. Watch which box changes, what the method gives back, and the error you get when an operation is impossible.

Start with

x = i = x is any Python value: text needs quotes.

Change the list

Ask a question (the list stays the same)

>>>

Pick an operation. Each one runs a single line of Python on tasks.

Your code so far (ready to paste into Jupyter)

Trace: the list after each line

What to notice

append, insert, remove, sort change the list and give back None.
pop() is different: it removes the item and hands it back.
After insert or remove, the positions of later items shift by one.

Try this

Rebuild the slide example: start from ["read", "code"] and end with ['eat', 'study']. Use Empty list and press pop(). Read the error. Set i = 10 and press insert(i, x), then tasks[i] = x. Why does one work and the other fail? Type tasks = tasks.sort() in the >>> box. Where did your list go?

Slicing: take any part of a list or string

A slice [start:stop:step] starts at start, goes up to but not including stop, jumping step positions each time. Leave a part blank to use its default.

Sequence

start use

stop use

step use

Python code

Result

One item:

What to notice

The stop position is never included: [1:3] gives positions 1 and 2.
Slices never raise IndexError: positions past the end are just cut off. A single index past the end does fail.
A negative step walks backwards, so [::-1] reverses.

Try this

Before moving anything, predict nums[-3:], then check. Find two different slices that both give the last two items. Choose the string "PYTHON" and get "NOHTYP". Set step to 0. What does Python say?

Ask the list: statistics without a loop

Edit the marks. Every expression on the right is recalculated, and the chart marks the highest, the lowest and the average.

marks = [ … ]

x =

sorted() or .sort()?

Your own expression (use marks and x)

What to notice

sum(marks) / len(marks) is the average, and it is always a float.
sorted(marks) makes a new list; marks itself does not change.
sorted(marks)[-2] is "second largest" only when the top mark is not repeated.

Try this

Make sorted(marks)[-2] give the wrong answer for "second largest". Set x to a mark that is not in the list. Which row turns into an error? Empty the list completely. Which expressions still work?

Two names, one list: what `=` really does

A name is a label with an arrow to a value. b = a copies the arrow, not the list. Step through and watch the arrows.

Example

Output

Names and the values they point to

What to notice

Two arrows to one list means one list: a change through either name shows through both.
.copy() builds a second, separate list.
Tuples and strings cannot be changed in place, so sharing them is always safe.

Try this

In Two names, one list, predict what line 4 prints before running it. Edit the code: replace b = a with b = a.copy(). Which outputs change? In The sort trap, explain why nums ends up as None.

Works offline: no internet needed. The Python shown here runs in your browser with the same rules as Python 3, so results and error messages match what you will see in Jupyter.

List playground: change a list one operation at a time

Change the list

Ask a question (the list stays the same)

Your code so far (ready to paste into Jupyter)

Trace: the list after each line

What to notice

Try this

Slicing: take any part of a list or string

One item:

What to notice

Try this

Ask the list: statistics without a loop

sorted() or .sort()?

Your own expression (use marks and x)

What to notice

Try this

Two names, one list: what = really does

Output

Names and the values they point to

What to notice

Try this

Two names, one list: what `=` really does