Your First Python: Values, Names, and Output

2.1Python does exactly one line at a time

A Python program is a list of instructions, executed top to bottom, one at a time, with no imagination whatsoever. This is the mental model to hold for the rest of the course — and it is also why counting steps is possible at all.

print("first")
print("second")
print("third")

print(...) means "show this on the screen". The round brackets hold what to show. The quotes mark where the text starts and stops.

"One line at a time" is not a slogan — it is a promise you can lean on. When a program misbehaves, you can read it the way Python does: put your finger on line 1, work out what it leaves behind, move to line 2, and so on. Nine bugs in ten give themselves up to that slow finger. We will lean on the same habit in week 3 to count how many times a line runs.

2.2Values come in kinds

Two kinds matter this week:

• Numbers — 7, -3, 2.5. No quotes. You can do arithmetic with them.
• Text (Python calls it a string) — "quokka", "7". In quotes. It is a sequence of characters.

7 and "7" are not the same thing, in the same way that the number seven is not the same as the pencil mark you draw for it:

print(7 + 3)
print("7" + "3")

Mixing them is the single most common beginner error: print("7" + 3) stops with TypeError: can only concatenate str (not "int") to str. Translated: "you asked me to glue a number onto some text and I do not know how."

If you are ever unsure what kind of value you are holding, ask Python directly with type(...). It is a habit worth building early:

print(type(7))
print(type(2.5))
print(type("7"))

int is a whole number, float a decimal one, str a string. Those three names appear in every error message you will read this week, so it pays to recognise them on sight.

2.3Variables: giving a value a name

A variable is a name pointing at a value. The = sign is not "equals" in the school sense — read it as "gets" or "is now".

student_count = 32
course_name = "Algorithm Analysis"

print(course_name)
print(student_count)

student_count = student_count + 1   # one more student joined
print(student_count)

That fifth line looks like nonsense as mathematics and is perfectly ordinary as an instruction: take the current value of student_count, add one, and make the name point at the result.

2.4Arithmetic and f-strings

a = 17
b = 5

print(a + b)     # 22   addition
print(a - b)     # 12   subtraction
print(a * b)     # 85   multiplication
print(a / b)     # 3.4  division, always gives a decimal
print(a // b)    # 3    whole-number division ("how many times fits")
print(a % b)     # 2    remainder
print(a ** 2)    # 289  a to the power 2

To mix text and values in one message, put an f before the opening quote and wrap any value in curly braces. This is called an f-string and it is how every message in this course gets printed:

name = "Ada"
items = 1000
seconds = 0.0421

print(f"{name} searched {items} items in {seconds} seconds")
print(f"That is {items / seconds:.0f} items per second")

The :.0f means "show it as a decimal number with 0 digits after the point". :.3f gives three digits — you will use that constantly once we start timing things.

The // and % pair is worth a second look, because it is exactly how you split a total into groups. Suppose 100 students must be seated at tables of 7:

students = 100
per_table = 7

full_tables = students // per_table    # whole tables that fill up
leftover    = students % per_table      # students still standing

print(f"{full_tables} full tables, {leftover} students at a part-full one")

// answers "how many whole groups?" and % answers "how many left over?". That remainder trick — is a number even? is it a multiple of 3? — shows up again in week 3 the moment we start writing loops.

2.5A few everyday things you can do with text

Strings are not just something to print — you can measure and reshape them. Four moves cover most of what you need this term:

word = "Quokka"

print(len(word))          # 6      how many characters
print(word.upper())       # QUOKKA
print(word.lower())       # quokka
print(word + "s")         # Quokkas   joining
print("ha" * 3)           # hahaha    repeating

len(...) gives the number of characters — the same len you will use on lists in week 4. The .upper() and .lower() are called methods: little jobs a value knows how to do to itself, written as a dot after the value. Joining with + and repeating with * are the same operators as for numbers, doing the text-shaped thing instead.

first = "ada"
last  = "lovelace"

full = first.upper() + " " + last.upper()
print(f"Welcome, {full} — your name has {len(first) + len(last)} letters.")

Nothing here is deep, but it is the everyday plumbing of real programs, and it lets you make output that reads like a sentence rather than a pile of raw values.

2.6Comments, and asking the user

# Anything after a # is a note for humans. Python skips it.
answer = input("How many rows does your file have? ")
rows = int(answer)          # turn the text "500" into the number 500

print(f"At one row per second that takes {rows} seconds.")
print(f"That is about {rows / 60:.1f} minutes.")

input() always hands back text, even when the user typed digits. int(...) converts text to a whole number, float(...) to a decimal one, str(...) converts back to text. Forgetting int() is beginner error number two.

2.7Three error messages, decoded

2.8Common mistakes: kinds and conversion

Almost every error you will hit for the next month comes from mixing kinds of value. Here are the three that catch everyone, each shown broken and then fixed.

1. Adding text to a number. The classic, straight from input:

age = input("Your age: ")    # suppose the user types 20
# print(age + 1)            -> TypeError: age is the text "20"
print(int(age) + 1)          # 21   convert first, then add

2. Gluing a number into a message with +. Use an f-string instead:

score = 95
# print("You scored " + score)   -> TypeError
print("You scored " + str(score))   # works: convert the number to text
print(f"You scored {score}")        # nicer: f-string does it for you

3. Losing the decimals with int. int does not round, it chops:

print(int(3.9))        # 3   the .9 is thrown away, not rounded
print(round(3.9))      # 4   round() rounds to nearest
print(round(3.14159, 2))  # 3.14   round to 2 decimals

2.9A first step count

We can already do a tiny piece of analysis. Count the instructions Python performs in this program:

price = 250
tax = price * 0.20
total = price + tax
print(f"Total: {total}")

Four lines, four steps, every single time you run it. It does not matter whether price is 250 or 250 million: the work does not depend on the size of the input at all. Hold on to that idea — in week 8 it gets the name O(1), and it is the best kind of program there is.

2.10Try it yourself

item = input("Item: ")
price = float(input("Unit price: "))
qty = int(input("Quantity: "))

line_total = price * qty
with_tax = line_total * 1.20

print(f"{qty} x {item} = {line_total:.2f}")
print(f"With 20% tax:  {with_tax:.2f}")

temp = a
a = b
b = temp

# or, the Python shortcut:
a, b = b, a

print(10 / 4)
print(10 // 4)
print(10 % 4)
print("10" * 3)
print(10 * 3)

bill = 137
people = 4

each = bill // people
left = bill % people

print(f"{each} each, {left} left over")
print(f"Exact share: {bill / people:.2f}")

first = input("First name: ")
last = input("Surname: ")

full = first.upper() + " " + last.upper()
letters = len(first) + len(last)

print(f"[ {full} ]")
print(f"{letters} letters in your name")

n = input("A number: ")
print(n + 5)

n = int(input("A number: "))
print(n + 5)

2.11Self-check

2.12Homework

2.13Words from this week