Why are my Class member variables going missing in Python?

Question

I made a class. I think that I create a member variable named "myTup" in the initializer/constructor. I try to make sure that I actually did create myTup by making the following call right at the very end of the initializer:

assert(hasattr(self, 'myTup'))

However, later, the interpreter says that my instance/object doesn't have a member variable named "myTup."

Where did myTup go? Come back to me... please Q_Q

The following script is only long due to all of the little debug statements. Without the debugging, it's actually quite short and simple.

Code:

import inspect
def lineno():
    # Returns the current line number in our program.
    return inspect.currentframe().f_back.f_lineno

class DemoClass:

    def __init__(self, initial_data):

        went_in = False # did not enter the if-elif-else block

        if isinstance(initial_data, str):
            print('line number:', lineno())
            t = tuple(initial_data)
            self = DemoClass(t)
            went_in = True        
        elif isinstance(initial_data, DemoClass):
            print('line number:', lineno())
            self = initial_data
            went_in = True
        elif isinstance(initial_data, tuple):
            print('line number:', lineno())
            self.myTup = initial_data
            went_in = True
        else:
            print('line number:', lineno())
            went_in = True
            assert(False)

        assert(hasattr(self, 'myTup'))
        print('line number:', lineno())
        print('went_in:', went_in)
        print('myTup:', self.myTup)

    def __str__(self):
        return "DemoClass" + str(self.myTup)

obj = DemoClass("s")
print(obj)

Output:

line number: 13
line number: 22
line number: 31
went_in: True
myTup: ('s',)
line number: 31
went_in: True
myTup: ('s',)
Traceback (most recent call last):

  [... truncated / abridged ...]

  File "E:/awesome_folder_name/missing_member_var.py", line 36, in __str__
    return "DemoClass" + str(self.myTup)

AttributeError: 'DemoClass' object has no attribute 'myTup'

Answer 1

You can't replace the instance created in __init__. The following replaces only the self reference in the __init__ method, not the previous instance that self was bound to:

if isinstance(initial_data, str):
    # ...
    t = tuple(initial_data)
    self = DemoClass(t)

When __init__ returns, the obj = DemoClass("s") line still bound the first instance, not the one you created with your recursive call. That first instance has no myTup attribute.

You'd have to call self.__init__(t) instead to operate on the same instance:

if isinstance(initial_data, str):
    # ...
    t = tuple(initial_data)
    self.__init__(t)

This then calls __init__ again, with the same instance bound, and thus will set myTup on the right object.

Note that I'd avoid using recursion here. There is no need, just set a tuple in the different branches and then assign myTup at the end:

def __init__(self, initial_data):
    if isinstance(initial_data, str):
        initial_data = tuple(initial_data)
    elif isinstance(initial_data, DemoClass):
        initial_data = initial_data.myTup
    elif not isinstance(initial_data, tuple):
        raise ValueError('Invalid data')

    self.myTup = initial_data

If you must control what instance is returned when trying to create a new one with DemoClass(), then you can only do so with the __new__ method; it is responsible for producing the instance in the first place (after which the default implementation would call __init__ on that new instance).