gistfile1.py

def is_operator(symbol):
    return isinstance(symbol, basestring)

def to_infix(postfix):
    stack = []
    
    for symbol in postfix:
        if is_operator(symbol):
            operand2 = stack.pop()
            operand1 = stack.pop()
            stack.append([operand1, symbol, operand2])
        else:
            stack.append(symbol)

    return stack[0]

print(to_infix([3, 4, 5, "*", "+"]))