Variables Lab · Lecture 2.2

Memory boxes: run code one line at a time

Each variable is a labelled box in memory. Step through the code and watch which box changes, what it now holds, and what type of value it is.

Example

Press Run next line. The orange line is the one Python will run next.

Memory

Output

What to notice

= means store: the value on the right goes into the box named on the left.
x = x + 1 reads the old value first, then stores the new one.
Nothing updates by itself: share keeps its old value until a line recalculates it.

Try this

In Counting spam, predict what line 6 prints before you run it. Edit the code: set messages_today = 0. Which line fails, and what is the error? In Four types, remove str( ) around total. Read the error carefully. In Swapping two values, explain why temp is needed in the first method.

Operators: one pair of numbers, every operator

Change a and b and watch every result and its type update. Try negative numbers, decimals and zero.

a = b =

Type whole numbers (like 7) or decimals (like 7.0).

Quick pairs

What to notice

/ always gives a float, even for 10 / 5.
// rounds down, towards minus infinity: -7 // 2 is -4, not -3.
One float anywhere makes the whole result a float.

Try this

Find two whole numbers where a / b looks whole but is still a float. Set b = 0. Which operators fail, and do their error messages differ? Check: is (a // b) * b + (a % b) always equal to a? Try negatives.

Text tools: the first building blocks of a spam filter

Type any message. Every box shows exactly what Python returns for it, live.

message = word =

Positions (indexes): top row counts from the start, bottom row from the end

Your own expression

Use the names message and word, for example message.lower().count("e").

What to notice

len() counts every character, spaces included.
"free" in message is case-sensitive: FREE does not match until you use .lower().
Indexes start at 0, and -1 is always the last character.

Try this

Find a message where word in message is False but word in message.lower() is True. Make message[0:5] show exactly the first word. Keep word = "free" and pick Claim your FR33 g1ft. Why does even .lower() not catch it?

f-strings: put values into sentences

Move the sliders. Each {…} in the f-string is calculated and replaced. After a colon you can choose the format.

spam_found = 3 total = 12

Your f-string

Insert a format

Output

How each {…} was filled

What to notice

Forgetting the f prints the braces literally.
:.0% multiplies by 100 and adds a percent sign; :.2f keeps two decimals.
Rounding of exact halves goes to the even digit, as in round().

Try this

Show the share with one decimal place as a percentage. Delete the f at the start. What appears? Write a sentence that uses total - spam_found inside the braces.

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.