Variables, types, and reading input

A tracker that only prints fixed text isn't tracking anything. In this part the script starts asking questions and doing arithmetic with the answers — which is where Python's types stop being trivia and start mattering.

Variables hold values

A variable is a name pointing at a value. No declaration keyword, no type annotation required:

amount = 12.5
category = "food"
note = "lunch with the team"

Python infers the type from the value: 12.5 is a float, and the quoted values are str. You can confirm it any time with the built-in type():

print(type(amount))
print(type(category))

That prints <class 'float'> and <class 'str'>. Reaching for type() when something behaves unexpectedly saves a lot of guessing, because most confusing errors in Python come from a value being a different type than you pictured.

Reading what the user types

input() prints a prompt and waits for a line of text:

category = input("Category: ")
amount = input("Amount: ")

print("Recorded", amount, "for", category)

Run it and it works. Now try doing math with what you got back:

amount = input("Amount: ")
print(amount * 2)

Type 12.5 and you get 12.512.5, not 25.0. input() always returns a string, and multiplying a string by an integer repeats it. This is the single most common surprise for people starting out in Python.

Converting types

Convert the string into a number before doing arithmetic:

amount = float(input("Amount: "))
print(amount * 2)

float() parses the text into a number, so 12.5 becomes 12.5 and the multiplication does what you expect. Use int() when you want whole numbers only — but be aware int("12.5") raises an error rather than rounding, because it refuses to guess whether you meant 12 or 13.

Formatting output

String concatenation with + fails the moment a number is involved, because Python won't silently mix types:

print("Spent " + amount + " on " + category)

That raises TypeError: can only concatenate str (not "float") to str. An f-string handles it — prefix the string with f and put expressions in braces:

print(f"Spent {amount} on {category}")

Anything inside the braces is evaluated and converted to text, so the type mismatch disappears. You can format numbers inline too:

print(f"Spent ${amount:.2f} on {category}")

The :.2f part means "as a fixed-point number with two decimals", turning 12.5 into 12.50. For money that's not cosmetic — it stops 9.1 and 9.10 from looking like different amounts in a list.

Where the script stands

category = input("Category: ")
amount = float(input("Amount: "))
note = input("Note: ")

print(f"Recorded ${amount:.2f} for {category} ({note})")

One expense, held in three separate variables, forgotten the moment the script exits. In the next part you'll group those three values into a single object and keep a list of them.