How to protect againt typos when setting value for class members?

Question

Hi,

consider following example:

class A():
    def __init__(self):
        self.veryImportantSession = 1

a = A()
a.veryImportantSession = None # ok

# 200 lines below
a.veryImportantSessssionnnn = 2 # I wanna exception here!! It is typo!

How could I make it so, that exception will be raised it case of I will try to set member that is not set in init?

Code above won't fail when it will be executed, but gives me fun time to debug problems.

Like with str:

>>> s = "lol"
>>> s.a = 1
>>> s.a = 1
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: 'str' object has no attribute 'a'

Thanks!

Answer 1

You could override _setattr_ to only allow attribute names from a defined list.

class A(object):
    def __setattr__(self, name, value):
        allowed = ('x',)
        if name in allowed:
            self.__dict__[name]  = value
        else:
            raise AttributeError('No attribute: %s' % name) 

In operation:

>>> a = A()
>>> a.x = 5
>>> a.other = 5
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "myc.py", line 7, in __setattr__
    raise AttributeError('No attribute: %s' % name)
AttributeError: No attribute: other   

However, as msw has commented, attempts to make Python behave more like Java or C++ are usually a bad idea and will lead to losing lots of the benefits that Python provides. If you are concerned about making typos that might be missed then you are much better spending time writing unit tests for your code than trying to lock down the usage of your classes.

Answer 2

You can define a class variable called __slots__. See the Language Reference for more information.

__slots__ only work in new-style classes, so you need class A(object) instead of class A in this example.

class A(object):
    __slots__ = ['x']
    def __init__(self):
        self.x = 1

>>> a = A()
>>> a.x = 2
>>> a.y = 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'A' object has no attribute 'y'