{"record":{"id":"ed8ebbb06fb8239c","repo":"TheAlgorithms/Python","slug":"input-is-not-a-valid-postfix-expression","errorCode":null,"errorMessage":"Input is not a valid postfix expression","messagePattern":"Input is not a valid postfix expression","errorType":"exception","errorClass":"ArithmeticError","httpStatus":null,"severity":"error","filePath":"data_structures/stacks/postfix_evaluation.py","lineNumber":186,"sourceCode":"                \"\".rjust(8),\n                f\"pop({a})\".ljust(12),\n                stack,\n                sep=\" | \",\n            )\n        # evaluate the 2 values popped from stack & push result to stack\n        stack.append(OPERATORS[x](a, b))  # type: ignore[index]\n        if verbose:\n            # output in tabular format\n            print(\n                f\"{x}\".rjust(8),\n                f\"push({a}{x}{b})\".ljust(12),\n                stack,\n                sep=\" | \",\n            )\n    # If everything is executed correctly, the stack will contain\n    # only one element which is the result\n    if len(stack) != 1:\n        raise ArithmeticError(\"Input is not a valid postfix expression\")\n    return float(stack[0])\n\n\nif __name__ == \"__main__\":\n    # Create a loop so that the user can evaluate postfix expressions multiple times\n    while True:\n        expression = input(\"Enter a Postfix Expression (space separated): \").split(\" \")\n        prompt = \"Do you want to see stack contents while evaluating? [y/N]: \"\n        verbose = input(prompt).strip().lower() == \"y\"\n        output = evaluate(expression, verbose)\n        print(\"Result = \", output)\n        prompt = \"Do you want to enter another expression? [y/N]: \"\n        if input(prompt).strip().lower() != \"y\":\n            break\n","sourceCodeStart":168,"sourceCodeEnd":201,"githubUrl":"https://github.com/TheAlgorithms/Python/blob/f5988cc09713315817df6a7e327e258013a94440/data_structures/stacks/postfix_evaluation.py#L168-L201","documentation":"Raised by evaluate() in data_structures/stacks/postfix_evaluation.py:186 as a final sanity check: after processing every token, the operand stack must hold exactly one value (the result). Any other size means the token sequence was not a well-formed postfix expression — typically too many operands (e.g. ['1','2','3','+']) or consecutive operators with insufficient operands would have already failed during popping.","triggerScenarios":"evaluate(['1','2']) leaves two items on the stack; evaluate(['1','2','3','+']) leaves two; an operator-heavy list like ['+','1'] fails earlier on stack.pop(). Any operand count != operator count + 1 in valid arrangement triggers it.","commonSituations":"Hand-written postfix strings, conversion bugs in an infix->postfix step feeding evaluate(), or user input that omitted an operator.","solutions":["Validate the shape before calling: len(tokens) >= 1 and operand count == operator count + 1, and every running prefix has operands > operators","Fix the upstream expression/converter so it emits balanced postfix","Catch ArithmeticError to reject malformed expressions at the boundary"],"exampleFix":"// before\nresult = evaluate(['1', '2', '3', '+'])  # ArithmeticError: stack ends with [1, 5]\n\n# after\ndef is_valid_postfix(tokens):\n    depth = 0\n    for t in tokens:\n        depth += -1 if t in OPERATORS else 1\n        if depth <= 0 and t in OPERATORS: return False\n    return depth == 1\nresult = evaluate(tokens) if is_valid_postfix(tokens) else None","handlingStrategy":"validation","validationCode":"from data_structures.stacks.postfix_evaluation import OPERATORS\n\ndef is_valid_postfix(tokens) -> bool:\n    depth = 0\n    for t in tokens:\n        if t in OPERATORS:\n            if depth < 2:\n                return False  # operator without two operands on stack\n            depth -= 1\n        else:\n            depth += 1\n    return depth == 1","typeGuard":null,"tryCatchPattern":"try:\n    result = evaluate(tokens)\nexcept ArithmeticError as e:\n    if str(e) != 'Input is not a valid postfix expression':\n        raise\n    result = None  # malformed: operand/operator counts mismatched","preventionTips":["A valid postfix list needs operands == operators + 1","Validate the output of any infix->postfix converter before feeding evaluate()"],"tags":["stack","expression-evaluation","arithmetic-error","python"],"backgroundTag":null,"analyzedSha":"f5988cc09713315817df6a7e327e258013a94440","analyzedAt":"2026-08-14T17:30:07.041Z","schemaVersion":2},"datasetVersion":"2026-08-15T22:17:37.221Z"}