Passing self into a constructor in python

Question

I recently was working on a little python project and came to a situation where I wanted to pass self into the constructor of another object. I'm not sure why, but I had to look up whether this was legal in python. I've done this many times in C++ and Java but I don't remember ever having to do this with python.

Is passing references to self to new objects something that isn't considered pythonic? I don't think I've seen any python programs explicitly passing self references around. Have I just happen to not have a need for it until now? Or am I fighting python style?

Um, few things are unpythonic per se, it's just that there are usually better approaches (whether on a design or implementation level). Your questions doesn't name the problem, so we can't judge whether your solution is any good. — user395760
– user395760, Commented Oct 15, 2010 at 20:38
Now that I think about it, I have to agree, this doesn't get done as much in Python as in e.g. Java. No idea why, though. — David Z
– David Z, Commented Oct 15, 2010 at 20:39
I don't know why people insist on saying "pythonic" when they simply mean "clean", but there's nothing inherently wrong with this. Just watch out for circular references and __del__. — Glenn Maynard
– Glenn Maynard, Commented Oct 15, 2010 at 21:07

Nick Craig-Wood · Accepted Answer · 2010-10-15 20:36:46Z

18

Yes it is legal, and yes it is pythonic.

I find myself using this pattern when you have an object and a container object where the contained objects need to know about their parent.

answered Oct 15, 2010 at 20:36

Nick Craig-Wood

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4 Comments

Falmarri Over a year ago

Yeah I understand why you need to do it, but I'm just not sure why I haven't seen it done very much in python.

Nick Craig-Wood Over a year ago

Possibly because python has a wicked set of polymorphic container types which in general are sufficient for all your needs?

Ashish Over a year ago

Can adapter pattern be considered one of the uses, I mean if we don't consider parameter to be passed as self but another object instance which has to be contained.

Jinglesting Over a year ago

@Nick - I’m pretty late to the party here, but please could you explain what you mean by polymorphic container types, and why they are relevant?

Mike DeSimone · Accepted Answer · 2010-10-15 20:39:49Z

4

Just pass it like a parameter. Of course, it won't be called self in the other initializer...

class A:
    def __init__(self, num, target):
        self.num = num
        self.target = target

class B:
    def __init__(self, num):
        self.a = A(num, self)

a = A(1)
b = B(2)
print b.a.num # prints 2

answered Oct 15, 2010 at 20:39

Mike DeSimone

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2 Comments

alternative Over a year ago

I think this is more or less asking whether its pythonic, not how.

user10217630 Over a year ago

super confusing.

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