Project 3 — Calculator · Project 4 — Unit Converter · Project 5 — To-Do List App · Project 6 — Vocabulary Quiz

import re

def safe_eval(expr, ans):
    expr = expr.replace("ANS", str(ans))
    # Allow only numbers, operators, parens, spaces
    if not re.fullmatch(r'[\d.+\-*/%(). \*\*]+', expr):
        raise ValueError("Invalid characters in expression")
    try:
        return eval(compile(expr, "<string>", "eval"),
                    {"__builtins__": {}}, {})
    except ZeroDivisionError:
        raise ZeroDivisionError("division by zero")

history, ans = [], 0
print("Calculator (type 'quit' to exit)")
while True:
    line = input("> ").strip()
    if line.lower() in ("quit", "exit"): break
    if line.lower() == "history":
        for h in history: print(f"  {h}")
        continue
    if line.lower() == "clear":
        history.clear(); ans = 0; continue
    try:
        result = safe_eval(line, ans)
        print(f"= {result}")
        history.append(f"{line} = {result}")
        ans = result
    except Exception as e:
        print(f"Error: {e}")

# All conversions to SI base unit (metre, gram, kelvin)
LENGTH = {"mm":1e-3,"cm":1e-2,"m":1,"km":1e3,"in":0.0254,"ft":0.3048,"mi":1609.344}
WEIGHT = {"mg":1e-3,"g":1,"kg":1e3,"lb":453.592,"oz":28.3495}

def convert_linear(value, from_u, to_u, table):
    return value * table[from_u] / table[to_u]

def convert_temp(value, from_u, to_u):
    # Convert to Kelvin first
    if from_u == "C": k = value + 273.15
    elif from_u == "F": k = (value - 32) * 5/9 + 273.15
    else: k = value
    if to_u == "C": return k - 273.15
    if to_u == "F": return (k - 273.15) * 9/5 + 32
    return k

while True:
    print("\n1) Length  2) Weight  3) Temperature  4) Quit")
    c = input("Choice: ").strip()
    if c == "4": break
    elif c == "1": tbl = LENGTH; cat = "Length"
    elif c == "2": tbl = WEIGHT; cat = "Weight"
    elif c == "3":
        from_u = input(f"From (C/F/K): ").upper()
        to_u = input(f"To (C/F/K): ").upper()
        val = float(input("Value: "))
        print(f"{val}°{from_u} = {convert_temp(val,from_u,to_u):.4f}°{to_u}")
        continue
    else: print("Invalid"); continue
    units = list(tbl.keys())
    from_u = input(f"From unit ({"/".join(units)}): ").lower()
    to_u = input(f"To unit: ").lower()
    val = float(input("Value: "))
    result = convert_linear(val, from_u, to_u, tbl)
    print(f"{val} {from_u} = {result:.6g} {to_u}")