![]() Note that, in this case, names will only be accessed as a class variable, so the mutable default is acceptable. One approach might be to iterate over the garbage collector’s list of objects, but it’s simpler to use class variables. We want to keep track of all the names that have been used. ![]() To make the scenario more concrete, let’s say we have a Person class, and every person has a name. This is sort of specific, but I could see a scenario in which you might want to access a piece of data related to every existing instance of a given class. Tracking all data across all instances of a given class. (Remember, though: take care when using mutable values as your defaults.) This only makes sense if you will want your typical instance of M圜lass to hold just 10 elements or fewer-if you’re giving all of your instances different limits, then limit should be an instance variable. # foo can now hold 50 elements-other instances can hold 10 We could then create instances with their own specific limits, too, by assigning to the instance’s limit attribute. Note that all instances of the class have access to class_var, and that it can also be accessed as a property of the class itself: foo = M圜lass(2) Here, class_var is a class attribute, and i_var is an instance attribute: class M圜lass(object): Let’s use a Python class example to illustrate the difference. Instance Variable: What’s the Difference?Ī Python class attribute is an attribute of the class (circular, I know), rather than an attribute of an instance of a class. In my experience, Python class attributes are a topic that many people know something about, but few understand completely. Instead, it’s defining data as a class attribute with value. I too was wrong in that it isn’t setting a “default value” for the instance attribute. My interviewer was wrong in that the above code is syntactically valid. Note: if you have an expert handle on class attributes, you can skip ahead to use cases. ![]() The real answer lay in understanding the distinction between Python class attributes and Python instance attributes. I’ll just fix it up to avoid confusion.”įor reference, and to give you an idea of what I was going for, here’s how I amended the code: class Service(object):Īs it turns out, we were both wrong.
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