F1.1 · LESSON 1 OF 6

Values and names

Write a small program that transforms data using conditions, loops and functions.

THE IDEA

A program changes state, one line at a time

A program is a list of instructions that Python carries out from top to bottom. The thing that changes as it goes is state: which values exist and which names refer to them. To read a program you do not guess what the code "sounds like". You track the state after each line, the way you will do in the board below.

The line x = 4 is an assignment. It is not a maths equation. It means: make the number 4, then attach the name x to it. A name is a label, and a value is the thing the label is stuck on.

Program (line 0 of 3)

x = 4y = xx = 7

Names

no names yet

Objects in memory

no objects yet

Nothing has happened yet. No names, no objects.

Step through the program with Run next line. Each coloured object is a value; each name points at exactly one object.

Reassigning moves the label, it does not edit the value

After y = x, both names point at the same object. When you later write x = 7, Python does not reach into the 4 and change it. It makes (or finds) a 7 and moves the label x over. The name y never moved, so it still sees 4.

Predict first. After x = 4, y = x and x = 7, what is y?

Values have types, and types decide what an operation does

Every value has a type. The four you meet first are int (whole numbers), float (numbers with a fraction), str (text) and bool (True or False). The same symbol can mean different things for different types, so always ask what type you are holding.

3 + 4 gives 7

Type of the result: int

Whole numbers add like you expect.

Click an expression. Notice that + glues text but adds numbers, and that / always gives a float.

Predict first. A form gives you price = "20" (text). What does price * 3 give?

The classic mistake: mixing text and numbers

Fails, then fixed

age = 30
message = "I am " + age + " years old"
# TypeError: can only concatenate str (not "int") to str

message = "I am " + str(age) + " years old"   # works: I am 30 years old

Convert on purpose: int(...), float(...), str(...).

Don't

Expect Python to guess whether "3" + 4 means 7 or "34". It refuses.

Chapter context

A program is a precise sequence of state changes. Python names refer to objects; reading a program means tracking the values those names refer to, rather than guessing from how the code sounds.

TRY IT YOURSELF

Exercise 1: swap two names

a and b point at two numbers. Make them trade places, so a ends up pointing at 3 and b at 10.
Writing a = b first loses the old value of a. Park it under a third name, temp, before you overwrite anything.
Press Run and check. Your program is also run again with different starting numbers, so typing 3 and 10 directly will not pass.

a = 10
b = 3

# TODO: swap the two names so that a is 3 and b is 10.
# Step 1: temp = a   (keep the old value somewhere)
# Step 2: a = b
# Step 3: b = temp

Ctrl/⌘ + Enter runs it

Exercise 2: text is not a number

price came out of a web form, so it is the text "20", not the number 20.
Fill in the three TODO lines. Convert with int(...), divide with /, and turn a number back into text with str(...).

price = "20"     # text typed into a form: this is a str
quantity = 3

# TODO 1: total = the price times the quantity as a NUMBER (convert price with int())
total = None

# TODO 2: half = total divided by 2 with / (division with / always gives a float)
half = None

# TODO 3: label = the text "total: 60", built from "total: " and the number total.
# You cannot add a str and an int, so convert with str(total) first.
label = None

print(label)

Ctrl/⌘ + Enter runs it

Both exercises must be solved to finish this lesson.

Stuck, or something not working? You can come back to it any time.

CHECK YOUR UNDERSTANDING

Predict x after x = 4; y = x; x = 7.

Ax is 7 and y is 11, because the earlier values are added together.Bx is 7 and y is 4, because y = x attached y to the object 4 and the later x = 7 only moved the name x to a new object.Cx is 7 and y is 7, because y = x makes y follow x whenever x changes.

Chapter reference: math, worked example & code

This section belongs to the whole chapter, so it is the same on every lesson in it and may cover a different lesson's topic. The code for the experiment you just ran is inside the experiment itself.

Math and notation

For an accumulator over numbers $x_1,\ldots,x_n$ :

s_0=0,\qquad s_i=s_{i-1}+x_i.

The subscript denotes a step, not a Python index. After the last step, $s_n$ is the sum. For a mean, divide by the number of items, and define what should happen for an empty list.

Worked example

Trace a robot's forward distances [2, 1, 3]. Position starts at zero and changes to 2, then 3, then 6. Replacing three copy-pasted moves with a loop preserves the order and final state. A mean distance is 6/3 = 2; the count is 3, not the last list index 2.

Reference code

The following example uses the Python standard library and does not call a remote service.

moves = [2, 1, 3]
position = 0
for distance in moves:
    position += distance
assert position == 6
assert sum(moves) / len(moves) == 2

Python examples are provided for study and local execution; this page does not execute Python.

Common mistakes

Assignment does not copy a mutable object. A loop stop value is exclusive. A function can return None even when its print output looks correct.