Sure, you can play with the default argument value:

class AB:    def __init__(self, data=None):        self.data = data if data is not None else []    def __add__(self, other):        return AB(self.data + [other.arg])

Other notes:

• list is a bad variable name as it is shadowing the built-inlist keyword
• remove redundant parentheses after the class name

Demo:

In [1]: ab = AB()In [2]: ab += A(1)In [3]: ab += A(2)In [4]: print(ab.data)[<__main__.A instance at 0x10afb14d0>, <__main__.A instance at 0x10afa0998>]

• \$\begingroup\$I was looking for other ways to build__add__, but I guess that changing__init__ works too. Thanks for the tips. Note that it is meant forAB(self.data + [other]) (without.arg) in order to maintain a list ofA objects.\$\endgroup\$

  dfernan

  – dfernan

  2017-02-07 21:36:01 +00:00

  CommentedFeb 7, 2017 at 21:36
• \$\begingroup\$@dfernan ah, gotcha, I'll remove the last suggestion. Thanks.\$\endgroup\$

  alecxe

  – alecxe

  2017-02-07 21:36:43 +00:00

  CommentedFeb 7, 2017 at 21:36
• \$\begingroup\$If you just usedata=[] directly instead of beNone indirection does that reuse a single list between__init__ calls? Otherwise that seems cleaner.\$\endgroup\$

  CAD97

  – CAD97

  2017-02-07 23:05:00 +00:00

  CommentedFeb 7, 2017 at 23:05
• 1
  \$\begingroup\$Just got an opportunity to test. Yes, doingdata=[] as a default argument leads to the same list being used for all invocations of__init__. However, this is only an issue if you implement__iadd__.\$\endgroup\$

  CAD97

  – CAD97

  2017-02-08 04:46:09 +00:00

  CommentedFeb 8, 2017 at 4:46

I agree that default arguments can (and should) be used for the backing list of your class.

In addition, consider inheriting fromcollections.abc.Sequence and delegating__getitem__ and__len__ to the backing list. (Add other list-like methods as necessary.) This will make sure your class acts as a well-behaved list-like.

In order to qualify as anIterable, you must define__iter__ or__getitem__. In order to qualify as aSequence you must be an Iterable that provides__getitem__ and__len__. It's up to you how much functionality you want to provide, but the Sequence ABC exists for a reason.

My two cents.

class A():    def __init__(self, arg):        self.arg = argclass AB():    def __init__(self):        pass    def __iadd__(self, other):        if not hasattr(self, '_list'):            setattr(self, '_list', [other])        else:            self._list.append(other)        return selfab = AB()ab += A(1)ab += A(3)print(ab._list[1].arg) # Prints 3

We completely avoid the creation of the_list parameter, unless we add anA() object.

Note that we are overriding the+= operator specifically.

• python