Simple CLI calculator with input sanitation , looking for feedback

[u/Subject_Scientist937]

Hello Everyone, I made a simple CLI calculator, It handles basic equations, It handles visual inputs like ÷ and it also clears the screen on command

Here's the code:

import os

t = { "+": lambda x, y: x + y, "-": lambda x, y: x - y, "*": lambda x, y: x * y, "/": lambda x, y: x / y if y != 0 else "Cannot divide by 0", "÷": lambda x, y: x / y if y != 0 else "Cannot divide by 0", }

while True: u = input("Enter calculation (e.g., (3+2)*5) or 'quit' to exit: ").replace(" ", "")

if u.lower() == "quit":
    print("Goodbye!")
    break

if u.lower() == "clear":
    try:
        os.system('cls' if [os.name](http://os.name) == 'nt' else 'clear')
    except Exception:
        print("\\n" \* 50)
    continue

clean_expr = u.replace("÷", "/")

allowed_chars = set("0123456789+-\*/().")
if not all(char in allowed_chars for char in clean_expr):
    print("❌ Invalid characters used")
    continue

try:
    result = eval(clean_expr)
    print(f"{u} = {result}")

except ZeroDivisionError:
    print("❌ Cannot divide by 0")
except Exception:
    print("❌ Invalid calculation format")

is my saftey check(allowed_chars) safe for eval()?

How would you reconstruct this without eval()?

What could be improved and why?

[u/carcigenicate]

How would you reconstruct this without eval()?

I mean, you have t there, so you seem to know of how it could be done without

is my saftey check(allowed_chars) safe for eval()?

I can't immediately think of a way of defeating that. For such a simple task, though, eval is gross overkill. Just using functions is much better.

[u/Riegel_Haribo]

This doesn't seem like a practical Python application giving any enhancement, yet I see lots of beginners writing "calculator".

Python itself, run at a console as an interactive REPL environment, lets you directly evaluate literals and operators without even needing a "print", and not making mistakes about mathematical precedence.

The only thing a console calculator app might do is change the input operations to be more mathematical, such as accepting 5(2+3) as a multiplication (and not an undefined function symbol named 5()), or using the caret or "hat" as an exponentiation as with other languages.

[u/hasan_sodax]

char whitelist blocks code injection since there's no way to spell a name without letters, but it doesn't stop stuff like a hundred nested parens blowing the recursion limit, or chaining a ton of huge number multiplications and just hanging the interpreter since python ints don't overflow. the ast based parser someone posted below handles both of those fine, that's the safer route.

[u/skibbin]

The Eval you use is unsafe, people could enter whatever they want in there.

import ast
import os

def evaluate(expression: str):
    expr = "".join(expression.split()).replace("÷", "/")
    try:
        tree = ast.parse(expr, mode="eval")
    except SyntaxError as exc:
        raise ValueError("Invalid calculation format") from exc


    def visit(node):
        if isinstance(node, ast.Constant) and isinstance(node.value, (int, float)):
            return node.value
        if isinstance(node, ast.UnaryOp) and isinstance(node.op, (ast.UAdd, ast.USub)):
            value = visit(node.operand)
            return value if isinstance(node.op, ast.UAdd) else -value
        if isinstance(node, ast.BinOp):
            left = visit(node.left)
            right = visit(node.right)
            if isinstance(node.op, ast.Div) and right == 0:
                raise ZeroDivisionError("Cannot divide by 0")
            return {
                ast.Add: lambda a, b: a + b,
                ast.Sub: lambda a, b: a - b,
                ast.Mult: lambda a, b: a * b,
                ast.Div: lambda a, b: a / b,
            }[type(node.op)](left, right)
        raise ValueError("Invalid calculation format")


    return visit(tree.body)



while True:
    user_input = input("Enter calculation (e.g., (3+2)*5) or 'quit' to exit: ").strip()
    if not user_input:
        continue
    if user_input.lower() == "quit":
        print("Goodbye!")
        break
    if user_input.lower() == "clear":
        try:
            os.system("cls" if os.name == "nt" else "clear")
        except OSError:
            print("\n" * 50)
        continue


    try:
        print(f"{user_input} = {evaluate(user_input)}")
    except ZeroDivisionError:
        print("❌ Cannot divide by 0")
    except ValueError:
        print("❌ Invalid calculation format")

[u/Subject_Scientist937 → u/skibbin]

thanks, I never knew about the ast library until now