🧮 Calculator

In your terminal, inside the calculator folder, create an empty file called calc.py. Open it in your editor.

You can do this any way you like — touch calc.py, code calc.py, "File → New" in your editor, whatever's natural for you.

In calc.py, write a single line that prints "calc.py is alive" (or whatever you want — the point is to confirm the file runs).

Then run it from your terminal: python3 calc.py.

calc.py is alive

Replace your print with code that asks the user for a number and prints back what they typed. Run it; type 42; you should see you typed: 42.

Enter a number: 42
you typed: 42

Now ask for two numbers and print their sum. Convert each input from a string to an integer with int(...) so + does math, not string concatenation.

First number: 7
Second number: 5
sum: 12

Extend the program: after reading the two numbers, ask for an operator (+, -, *, /) and print the right result.

First number: 12
Second number: 4
Operator (+, -, *, /): /
result: 3.0

The program runs once and exits. Wrap the whole "ask + compute + print" sequence in a while True: loop so you can do calculation after calculation without re-running.

(Don't worry about exiting the loop yet — just press Ctrl+C to kill it. We'll add a clean exit in the next step.)

First number: 1
Second number: 2
Operator (+, -, *, /): +
result: 3
First number: 10
Second number: 4
Operator (+, -, *, /): -
result: 6
^C

Right after reading the first number, check whether the user typed quit. If so, print a goodbye and break out of the loop.

First number: 5
Second number: 3
Operator (+, -, *, /): +
result: 8
First number: quit
goodbye!

Right now, typing seven instead of 7 crashes with ValueError. Wrap the integer parsing in a try/except block: catch ValueError, print "that wasn't a number, try again," and continue to the next loop iteration.

First number: hello
that wasn't a number, try again.
First number: 4
Second number: 2
Operator (+, -, *, /): /
result: 2.0

At the top of the file (above the loop), make an empty list called history. Each time you compute a result, append a description string to that list — something like "7 + 5 = 12".

When the user quits, before the goodbye, print the whole history.

First number: 1
Second number: 2
Operator (+, -, *, /): +
result: 3
First number: 4
Second number: 6
Operator (+, -, *, /): *
result: 24
First number: quit
history:
1 + 2 = 3
4 * 6 = 24
goodbye!

Move the four operators into a single function: def calc(a, b, op): ... that returns a number (or raises a clear error for unknown operators). Replace the if/elif chain in the loop with one call to calc(a, b, op).

Then, outside the loop, write three tiny assertions to convince yourself the function works:

assert calc(2, 3, "+") == 5
assert calc(10, 4, "-") == 6
assert calc(7, 6, "*") == 42

Run the file. If the assertions pass, you see no output for them. If one fails, you'll see AssertionError.

First number: 5
Second number: 3
Operator (+, -, *, /): +
result: 8
First number: quit
history:
5 + 3 = 8
goodbye!

Add a shebang line at the very top: #!/usr/bin/env python3. Then make the file executable: chmod +x calc.py. Now you can run it as ./calc.py instead of python3 calc.py.

First number: 9
Second number: 3
Operator (+, -, *, /): /
result: 3.0
First number: quit
history:
9 / 3 = 3.0
goodbye!

Two small polish moves:

1. At the top of the loop body (or above the loop), print a one-line help: "Type 'quit' to exit. Operators: + - * /".
2. Switch int(...) to float(...) so the calculator handles decimals.

Type 'quit' to exit. Operators: + - * /
First number: 1.5
Second number: 2.5
Operator (+, -, *, /): +
result: 4.0
First number: quit
history:
1.5 + 2.5 = 4.0
goodbye!