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Level 0 / Project 02 - Calculator Basics

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Before You Start

Recall these prerequisites before diving in: - Can you write a function that takes two arguments and returns their sum? - Can you convert a string to a number using int() or float()?

Estimated time: 15 minutes

Learn Your Way

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Concept This project — Quiz Flashcards Diagram Browser

Focus

  • numeric input, arithmetic, and safe casting

Why this project exists

Build a four-operation calculator that reads expressions from a file. You will practise arithmetic operators, float/int conversion, and dispatching operations with a dictionary.

Run (copy/paste)

Use <repo-root> as the folder containing this repository's README.md.

cd <repo-root>/projects/level-0/02-calculator-basics
python project.py
pytest -q

The program asks you to type expressions interactively. Type quit to exit.

Expected terminal output

=== Calculator ===
Type an expression like '10 + 5' or 'quit' to exit.

Supported operators: +  -  *  /

Enter expression: 10 + 5
  10 + 5 = 15.0

Enter expression: 6 * 7
  6 * 7 = 42.0

Enter expression: quit
Goodbye!
5 passed

Expected artifacts

  • Passing tests
  • Updated notes.md

Worked Example

Here is a similar (but different) problem, solved step by step.

Problem: Write a function that takes a string like "5 squared" and returns the result.

Step 1: Parse the input. Split the string into parts and figure out what operation to do.

def parse_power(text):
    parts = text.strip().split()
    base = float(parts[0])
    keyword = parts[1].lower()
    powers = {"squared": 2, "cubed": 3}
    if keyword not in powers:
        raise ValueError(f"Unknown power: {keyword}")
    return base ** powers[keyword]

Step 2: Test it mentally. "5 squared" splits into ["5", "squared"]. float("5") gives 5.0. powers["squared"] gives 2. 5.0 ** 2 gives 25.0. Looks right.

Step 3: Think about what could go wrong. What if the user types "hello squared"? float("hello") will raise ValueError. What if they type just "5"? parts[1] will raise IndexError. We should catch these.

def parse_power(text):
    parts = text.strip().split()
    if len(parts) != 2:
        raise ValueError(f"Expected 'number power', got: {text}")
    base = float(parts[0])  # raises ValueError if not a number
    keyword = parts[1].lower()
    powers = {"squared": 2, "cubed": 3}
    if keyword not in powers:
        raise ValueError(f"Unknown power: {keyword}")
    return base ** powers[keyword]

The thought process: Parse first, validate, then compute. This is the same pattern the calculator project uses with its operations dict.

Checkpoint: Baseline code runs and all tests pass. Commit your work before continuing.

Alter it (required) — Extension

  1. Add support for modulo (%) and exponentiation (**) operations.
  2. After each calculation, ask the user "Round to how many decimal places? (Enter to skip): " and round accordingly.
  3. Re-run script and tests.

Break it (required) — Core

  1. Type 10 / 0 -- does it raise ValueError or crash with ZeroDivisionError?
  2. Type just one number like 5 -- what happens when there is no operator or second operand?
  3. Type text instead of numbers like hello + world -- does float() fail gracefully?

Fix it (required) — Core

  1. Ensure divide() raises ValueError with a clear message for zero divisors.
  2. Add validation in calculate() that checks for exactly two numeric operands.
  3. Add a test for the malformed-expression case.

Checkpoint: All modifications done, tests still pass. Good time to review your changes.

Explain it (teach-back)

  1. Why does the operations dict map strings to functions, instead of using if/elif chains?
  2. What does float() do and when does it raise ValueError?
  3. Why is division by zero handled in divide() rather than in calculate()?
  4. Where would expression parsers be used in real software (spreadsheets, query engines)?

Mastery check

You can move on when you can: - run baseline without docs, - explain one core function line-by-line, - break and recover in one session, - keep tests passing after your change.

Stuck? Ask AI

If you are stuck after trying for 20 minutes, use one of these prompts:

  • "I am working on Calculator Basics. I got this error: [paste error]. Can you explain what this error means without giving me the fix?"
  • "I am trying to map operator strings to functions using a dictionary. Can you show me a simple example of using a dict to dispatch functions, using a different topic than calculators?"
  • "Can you explain what float() does and when it raises an error, with examples?"
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