TheAlgorithms/Python · error · ValueError

invalid expression

Error message

invalid expression

What it means

Raised at the end of infix_to_postfix() in data_structures/stacks/infix_to_prefix_conversion.py:114 when, after consuming the whole expression, an unclosed '(' remains on the operator stack. It is the mirror of the empty-stack ')' error: this one detects an open bracket that never got closed.

Source

Thrown at data_structures/stacks/infix_to_prefix_conversion.py:114

                post_fix.append(stack.pop())  # Pop stack & add the content to Postfix
            stack.pop()
        elif len(stack) == 0:
            stack.append(x)  # If stack is empty, push x to stack
        else:  # while priority of x is not > priority of element in the stack
            while stack and stack[-1] != "(" and priority[x] <= priority[stack[-1]]:
                post_fix.append(stack.pop())  # pop stack & add to Postfix
            stack.append(x)  # push x to stack

        print(
            x.center(8),
            ("".join(stack)).ljust(print_width),
            ("".join(post_fix)).ljust(print_width),
            sep=" | ",
        )  # Output in tabular format

    while len(stack) > 0:  # while stack is not empty
        if stack[-1] == "(":  # open bracket with no close bracket
            raise ValueError("invalid expression")

        post_fix.append(stack.pop())  # pop stack & add to Postfix
        print(
            " ".center(8),
            ("".join(stack)).ljust(print_width),
            ("".join(post_fix)).ljust(print_width),
            sep=" | ",
        )  # Output in tabular format

    return "".join(post_fix)  # return Postfix as str


def infix_2_prefix(infix: str) -> str:
    """
    >>> infix_2_prefix("a+b^c")  # doctest: +NORMALIZE_WHITESPACE
     Symbol  |  Stack  | Postfix
    ----------------------------
       c     |         | c

View on GitHub (pinned to f5988cc097)

Solutions

1. Pre-check with balanced_parentheses(expr) and reject early
2. Fix the expression so every '(' has a closing ')'
3. Catch ValueError around the call when input cannot be trusted

Example fix

// before
result = infix_to_postfix('(1+2')  # ValueError: invalid expression

# after
assert expr.count('(') == expr.count(')') and balanced_parentheses(expr)
result = infix_to_postfix(expr)

if expr.count('(') != expr.count(')') or not balanced_parentheses(expr):
    raise ValueError('unbalanced expression')
result = infix_to_postfix(expr)

try:
    result = infix_to_postfix(expr)
except ValueError as e:
    if str(e) != 'invalid expression':
        raise
    # unclosed '(' — reject input

When it happens

Trigger: Expressions like '(1+2' or '((3+4)*5' — any input where a '(' is pushed and never popped by a matching ')'. All remaining operators are popped in the final while loop, but a '(' left on stack aborts with this error.

Common situations: Truncated expressions from files or user input; like the sibling IndexError, this converter performs no up-front balance check.

Related errors

• Mismatched parentheses
• list index out of range
• {token} is neither a number nor a valid operator
• number must be positive
• The value of input must be non-negative

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/6470001f2328e4e8. Report an issue: GitHub.