Python Tutor中的对象不完整？

Question

I have this training exercise where I need to find the problems in this program. I can't find them and python tutor does not allow me to see the object created. It just says incomplete object. The program should create a kind of queue for strings. What is wrong with the code?

class A():
"""Eine Art Warteschlange für Strings"""

    def __init__(self, wait = [], now = ""):
        self.wait = wait
        self.now = now

    def new_string(self, x):
        """Fügt einen String zur Warteschlange hinzu"""
        self.wait.append(str(x))

    def next_string(self):
        """Holt den nächsten String aus der Warteschlange, speichert ihn als aktuellen String"""
        self.now = self.wait[0]
        self.wait.pop(0)

    def top(self):
        """Gibt den aktuellen String zurück"""
        return self.now

    def __str__(self):
        return self.top

Answer 1

Here are the few bugs I came across in your class.(There may be more):

• Under indentation of the class docs.
  Fix: Indent it by 4 spaces to the right.

• The __str__ magic method doesn't return a string .
  Fix: Replace self.top with self.top() . You would want to return the value returned by the method top .

• Not updating the attribute now whenever new_string is added.
  Since, you are using the object's now attribute in __str__ , it is advisable to update it whenever an empty object is initialised using new_string method. There are several ways to do. I would suggest you fix this as you see fit.

Example:

In []: a = A()
In []: a.new_string('foo')
In []: print(a)

In []: a.now
Out[]: ''
In []: a.wait
Out[]: ['foo']