Build a Stateful Calculator

Stateful Calculator Implementation

Your task is to implement the core logic for a stateful calculator that supports basic arithmetic operations (+, -, *, /). Unlike a simple calculator, this one must handle operator chaining and state transitions effectively.

Requirements

1. Input Handling: The calculator should accept numeric digits, operators, and a 'clear' command.
2. State Management: Keep track of the current value being entered, the previous operand, and the pending operator.
3. Operator Chaining: If a user enters '5 + 5 +', the calculator should immediately resolve the first addition (displaying 10) before waiting for the next input.
4. Interface: Implement a class or function that exposes press(key) and returns the current display value as a string.

Example Flow

• press('5') -> display "5"
• press('+') -> display "5"
• press('5') -> display "5"
• press('+') -> display "10" (resolves 5+5, prepares for next operation)
• press('2') -> display "2"
• press('=') -> display "12" (resolves 10+2)

Constraints

• Handle division by zero by returning "Error".
• The input is provided as a sequence of strings (digits or operators).
• Ensure that multiple operator presses (e.g., '5 + + 5') do not crash the application (overwrite the operator).

class Calculator:
    def __init__(self):
        pass

    def press(self, key: str) -> str:
        # Implementation here
        pass