I am a complete Python novice so my question might seem to be dumb. I've seen there are two ways of assigning values to object attributes in Python:
Using __dict__:
class A(object):
def __init__(self,a,b):
self.__dict__['a'] = a
self.__dict__['b'] = b
Without __dict__:
class A(object):
def __init__(self,a,b):
self.a = a
self.b = b
Can anyone explain where is the difference?
Unless you need set attributes dynamically and bypass descriptors or the .__setattr__() hook, do not assign attributes directly to .__dict__.
Not all instances have a .__dict__ attribute even, not if the class defined a .__slots__ attribute to save memory.
If you do need to set attributes dynamically but don't need to bypass a descriptor or a .__setattr__() hook, you'd use the setattr() function normally.
From the Python documentation:
Both class types (new-style classes) and class objects (old-style/classic classes) are typically created by class definitions (see section Class definitions). A class has a namespace implemented by a dictionary object. Class attribute references are translated to lookups in this dictionary, e.g.,
C.xis translated toC.__dict__["x"](although for new-style classes in particular there are a number of hooks which allow for other means of locating attributes).
The much more typical way of assigning a new element to a class is your second example, which allows for a number of customizations (__seattr__, __getattr__, etc.).
__dict__approach is that it'll also allow you to create attributes like'a b', but then you can't access such attributes without using__dict__becausea bis an invalid variable name in python.__dict__.