Instance and undefined parameter in python

Question

I'm not very familiar with Python, so when I met this code, there are a few things not quite clear to me. Here is the code:

class TypeVariable(object):
    #A type variable standing for an arbitrary type.
    #All type variables have a unique id, but names are only assigned lazily, 
    #when required.

    next_variable_id = 0

    def __init__(self):
        self.id = TypeVariable.next_variable_id
        TypeVariable.next_variable_id += 1
        self.instance = None
        self.__name = None

    next_variable_name = 'a'

    @property
    def name(self):
        #Names are allocated to TypeVariables lazily, so that only TypeVariables present
        if self.__name is None:
            self.__name = TypeVariable.next_variable_name
            TypeVariable.next_variable_name = chr(ord(TypeVariable.next_variable_name) + 1)
        return self.__name

Why does def __init__(self): not have name as a parameter but in the above code there is self.__name = None?

Also, what does self.instance mean, because I don't recall that there is an attribute called instance.

Can someone please help me understand what this code does, and if a java version can be provided will be much appreciated since I'm more familiar with Java. Thanks

ShadowRanger · Accepted Answer · 2016-03-30 20:24:51Z

3

The code has no context to say what self.instance might be. As for self.__name, the property definition makes clear that it is lazily assigning sequential names to instances. It sets __name to None so it knows it hasn't been initialized; when self.name is accessed, it will recognize the placeholder, replace it with a new value, and return the new value.

Note that providing some assignment to all instance attributes in __init__ is considered good form in Python, since otherwise it can be very hard to figure out what attributes an instance might have. And in modern Python (3.3 and up), doing so gets you a memory savings; as long as all attributes are initialized in __init__ and no new attributes are added after __init__, the __dict__ for all instances is a key-sharing dictionary which reduces the cost of each instance significantly (by roughly two-thirds, though that's an implementation detail). So if some other method/property would set instance later, it's best to initialize it to something in __init__ both for maintainers to know what it's supposed to have, and reduced memory consumption in Py3.3+.

answered Mar 30, 2016 at 20:24

ShadowRanger

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2 Comments

Anda Zhao Over a year ago

Hi, can I consider the def __init__(self) as a constructor in Java, if so then how come it can do add operation in the constructor

ShadowRanger Over a year ago

@AndaZhao: Technically, __init__ is an initializer; the raw object is created before it is called (that's how you receive self). Python has a __new__ that constructs (and can initialize too), but it's typically only used for logically immutable objects (__init__ can be called more than once, which would violate immutability) or cached/singleton objects (__new__ can return an existing object, not just make new ones). But in any event, Python, unlike Java, doesn't define instance attribute names ahead of time in general; you can define any attribute, any time.

xiº · Accepted Answer · 2016-03-30 20:24:21Z

0

self.instance = None
self.__name = None

There are just object attributes with default value. You don't need to put them into __init__() parameters if they are not required for building class instance.

answered Mar 30, 2016 at 20:24

xiº

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1 Comment

ShadowRanger Over a year ago

self.__name is needed at the least since the @property assumes it exists. The implementation of name would have to change if it wasn't guaranteed to be initialized in __init__.

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