From Scratch Programming Class

Session 2: Variables and types

Phase 3 — Python basics · Session 2 of 16

What we’re learning today

You’ve been using variables for a while now (Phase 1 Session 6, Phase 2 Session 5). Today we get specific about what kinds of values variables can hold — numbers, decimals, text, and what happens when you mix them up. By the end of class, you’ll have written a program that does math with answers from the user, and you’ll know why "5" + "3" is "53" instead of 8.

You’ll need to remember from last time

• print(...) — Python tells you something.
• input(...) — Python asks you something. Returns whatever the user typed.
• + — sticks two strings together (concatenation).
• Variables — name = "Sam".

Part A: Three kinds of values

Open Thonny and start a new file. Save it as types.py.

A type is a category of value. Python has lots of types, but three matter for almost everything you’ll do:

Notice that 5 and 5.0 look almost the same but are different types — one is an int, one is a float. And "42" (with quotes) is a string even though it looks like a number.

See the type with type()

Python has a built-in function called type() that tells you what type a value is. Try this:

print(type(5))
print(type(5.0))
print(type("Hello"))
print(type("5"))

Save. Run.

The shell shows:

<class 'int'>
<class 'float'>
<class 'str'>
<class 'str'>

Notice the last two: 5.0 is a float, but "5.0" (with quotes) is a string. The quotes change everything.

Why types matter

Try this:

print(5 + 3)
print("5" + "3")

Run. The shell shows:

8
53

Two completely different answers. Same-looking values; different types; different behavior.

• 5 + 3 is number addition. Five plus three is eight.
• "5" + "3" is string concatenation. “5” stuck together with “3” is “53”.

Python + does different things depending on what it’s between. With numbers, math. With strings, sticking together.

This matters because input() always returns a string — even if the user typed a number. Try:

age = input("How old are you? ")
print(type(age))
print(age + 1)

Save. Run. Type your age and press Enter.

You get an error:

TypeError: can only concatenate str (not "int") to str

Translation: “I can’t add a number to a string. They’re different types.”

The fix: convert the string to a number before doing math with it.

Type conversion

Python has functions for converting between types:

Fix the previous program:

age = int(input("How old are you? "))
print(type(age))
print(age + 1)

The int(input(...)) does two things: gets the user’s input (as a string), then converts it to an integer.

Save. Run. Type your age. Now you see:

<class 'int'>
[your age plus 1]

The error is gone because age is now an int, and you can add 1 to an int.

This is one of the most common patterns in Python: convert input to the type you actually want to work with.

One more conversion

You might also need str() to go the other direction. Try this without conversion:

score = 100
print("Your score is " + score + " points!")

Run. Error: TypeError: can only concatenate str (not "int").

Fix it with str():

score = 100
print("Your score is " + str(score) + " points!")

Now the int 100 becomes the string "100", which can be concatenated with the rest of the message.

(There’s a much nicer way to do this in Python called f-strings — coming next week.)

Checkpoint: You’ve used type() to check the type of at least three different values, you’ve seen the difference between 5 + 3 and "5" + "3", and you’ve used int() to convert input to a number for math. This is the natural stop point if class is cut short.

Part B: A real program with math

Now let’s build something useful with input + types.

Base goal — age in years

Build this:

print("Welcome to the Age Calculator!")
age = int(input("How old are you? "))
years = int(input("How many years from now? "))

future_age = age + years
print("In " + str(years) + " years, you'll be " + str(future_age) + ".")

Run. Type your age (e.g., 12). Type a number of years (e.g., 8). The shell tells you what age you’ll be then.

Walk through what’s new:

• Two int(input(...)) calls to get two numbers.
• Math on variables: age + years adds them.
• str() on the way out so we can concatenate the numbers into the output string.

Tedious but it works. You’ll see why f-strings are nicer next week.

Stretch — decimals

What if you want fractional answers? Use float:

print("Average calculator!")
a = float(input("First number: "))
b = float(input("Second number: "))
c = float(input("Third number: "))

average = (a + b + c) / 3
print("The average is " + str(average))

Now the user can type decimals (like 4.5 or 7.25) and the result handles fractional averages.

The parentheses in (a + b + c) / 3 matter — they make sure the addition happens first, then the division. Math order of operations applies in Python just like math class.

Extension — multi-step calculator

Build a tip calculator. Or a “how many minutes until…” program. Or “convert miles to feet.” Or anything that asks the user for numbers, does some math, and reports the result.

For example, a savings calculator:

print("Savings Calculator")
weekly_save = float(input("How much do you save each week? $"))
weeks = int(input("How many weeks? "))

total = weekly_save * weeks
print("In " + str(weeks) + " weeks, you'll save $" + str(total))

Or a unit converter:

miles = float(input("How many miles? "))
feet = miles * 5280
print(str(miles) + " miles is " + str(feet) + " feet.")

Pick something you’d want to know an answer to, and write the program to compute it.

Wrap-up

Before we leave, share with the room:

• What did your calculator compute?
• Did anyone hit a TypeError they had to fix?
• For the kids who built the savings or unit conversion — does the answer look right?

You learned today how Python tells different kinds of values apart, and how to convert between them. Types are one of the fundamental ideas in programming. Every value has a type. Operations behave differently depending on type. Errors happen when types don’t match.

In Phase 2 you got away with not thinking about types because turtle commands took numbers and you typed numbers. Now that input() is in the mix, type conversion becomes part of every interactive program.

If you missed this session

Open Thonny and start a new file. Save as types.py. Then:

1. Try print(type(5)), print(type(5.0)), print(type("hi")). See the three types: int, float, str.

2. Try print(5 + 3) and print("5" + "3"). Notice the different answers.

3. Build the age calculator from Part B base:

   age = int(input("How old are you? "))
   years = int(input("How many years from now? "))
   future_age = age + years
   print("In " + str(years) + " years, you'll be " + str(future_age) + ".")
   

4. Customize: build a calculator that asks for numbers and reports something useful (savings, conversion, average).

About 30 minutes. If you get stuck, ask your buddy at the start of next class.

Stretch and extension ideas

• What does int("5.7") do? What does int(5.7) do? Try both and see.
• What does float("hello") do? Run it and read the error message.
• What does str(5) + str(3) return? Why?
• What does int("5") + int("3") return? Why?
• Try 5 + 5.0 — what type is the result? (Hint: it’ll be a float. When you mix int and float, Python promotes to float.)

What’s next

Next week we dig deeper into strings — text is the most common kind of data in real programs (names, addresses, sentences, file contents). We’ll learn how to slice strings, search them, change their case, and — most importantly — use f-strings to format text without all the + str(...) + mess from today.