CODEWITHSUNDEEP
pythonclassvariablesinstancedeclaration
DsL
DsL

Reputation: 13

Do instance variables declared outside of `__init__()` have any differences in python?

I'm wondering weather it is necessary to define class instance variable within class declarations.

I tried assigning a new instance variable after the object (class instance) was already created, and looks like there is no difference. Are there any caveats in this approach?

class Context():
    def __init__(self, extension):
        self.extension = extension

c = Context('extension+')

print(f"before: {c.__dict__}")

c.new_var = 'new_var_content'
print(c.extension + c.new_var)

print(f"after: {c.__dict__}")

printed:

before: {'extension': 'extension+'}
extension+new_var_content
after: {'extension': 'extension+', 'new_var': 'new_var_content'}

Upvotes: 1

Views: 143

Answers (2)

Prashanti
Prashanti

Reputation: 173

Speaking in terms of whether you can assign a value to the property or create a new property, there is no difference if you do it within init or anywhere else after the object is created as in both cases it gets added in dict of the object(unless you use slots)

However, if you want your class to be initialized with desired state (i.e, having some mandatory variables with default/preset values) , you should put it in init. Since init is called implicitly as soon as object is created, you object will be having desired state.

Upvotes: 0

Rob Truxal
Rob Truxal

Reputation: 6408

There is no difference between declaring self.foo within a def __init__(self, <arguments>): definition, and declaring it after an object has been instantiated.

Both assignments have instance-level scope.

Given -

class Context:
    i_am_a_class_variable = 'class_string'
    def __init__(self, bar):
        self.bar = bar

See -

  1. class attributes can be accessed without instantiating an object.
>>> Context.i_am_a_class_variable
'class_string'
  1. instance attributes can be assigned during instantiation using the __init__(self) function. 
>>> Context.bar
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-8be0704da5be> in <module>                                 
----> 1 Context.bar
>>> instance = Context('bar')
>>> instance.bar
'bar'
  1. instance attributes can be assigned after instantiation, directly
>>> instance = Context('bar')
>>> instance.foo = 'foo'
>>> instance.foo
'foo'

Upvotes: 1

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