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Python: variables in the class scope are class, not instance

Posted by jpluimers on 2019/09/03

A very subtle thing that keeps biting me as my background is from languages where by default, identifiers on the class scope are instance level, not class level:

In Python, variables on class level are class variables.

If you need instance variables, initialise them in your constructor with a self.variable = value.

The example in the Python 3 docs [WayBackClasses – A First Look at Classes – Class and Instance Variables is the same as in the Python 2 docs [WayBackClasses – A First Look at Classes – Class and Instance Variables:

Generally speaking, instance variables are for data unique to each instance and class variables are for attributes and methods shared by all instances of the class:

class Dog:

    kind = 'canine'         # class variable shared by all instances

    def __init__(self, name):
        self.name = name    # instance variable unique to each instance

>>> d = Dog('Fido')
>>> e = Dog('Buddy')
>>> d.kind                  # shared by all dogs
'canine'
>>> e.kind                  # shared by all dogs
'canine'
>>> d.name                  # unique to d
'Fido'
>>> e.name                  # unique to e
'Buddy'

For people new at Python: the __init__ is a constructor; see these links for more explanation:

Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__(). For example,

>>> class Complex:
...     def __init__(self, realpart, imagpart):
...         self.r = realpart
...         self.i = imagpart
...
>>> x = Complex(3.0, -4.5)
>>> x.r, x.i
(3.0, -4.5)

–jeroen

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