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pythonmonkeypatching

In Python can I change the structure of a class at runtime?

In Python you can add new instance variables to an instance of a class at runtime like this...

>> class Foo:
>>     def __init__(self, bar):
>>         self.bar = bar
>> 
>> f = Foo()
>> setattr(f, "baz", "spam")
>> f.baz
"spam"

...but it only has effect on the instance f not the class foo.

>> x = Foo()
>> x.baz
AttributeError: 'Foo' object has no attribute 'baz'

Is there a way to add the new instance variable baz to the class Foo, so that all new instances of that class have the new variable?

Solution

  • You could set the attribute of the class instead of the instance.

    setattr(Foo, "baz", "spam")

    Output

    >>> class Foo:
...     def __init__(self, bar):
...         self.bar = bar

>>> f = Foo('bar')
>>> setattr(Foo, "baz", "spam")
>>> f.baz
'spam'
>>> f2 = Foo('bar')
>>> f2.baz
'spam'