Python Class Attributes: An Overly Thorough Guide

2018-05-21 5496 words 26 mins read

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This article is originally published at Toptal.

I had a programming interview recently, a phone-screen in which we used a collaborative text editor.

I was asked to implement a certain API, and chose to do so in Python. Abstracting away the problem statement, let’s say I needed a class whose instances stored some

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      data
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and some

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      other_data
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I took a deep breath and started typing. After a few lines, I had something like this:


  
    
      
        
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          class Service(object):

              data = []

          

              def __init__(self, other_data):

                  self.other_data = other_data

              ...

My interviewer stopped me:

Interviewer: “That line:

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      data = []
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. I don’t think that’s valid Python?”

Me: “I’m pretty sure it is. It’s just setting a default value for the instance attribute.”
Interviewer: “When does that code get executed?”
Me: “I’m not really sure. I’ll just fix it up to avoid confusion.”

For reference, and to give you an idea of what I was going for, here’s how I amended the code:




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      class Service(object):<br />
      <br />
      &nbsp; &nbsp; def __init__(self, other_data):<br />
      &nbsp; &nbsp; &nbsp; &nbsp; self.data = []<br />
      &nbsp; &nbsp; &nbsp; &nbsp; self.other_data = other_data<br />
      &nbsp; &nbsp; ...
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As it turns out, we were both wrong. The real answer lay in understanding the distinction between Python class attributes and Python instance attributes.

<p class="is-centered">
  <img src="https://i2.wp.com/uploads.toptal.io/blog/image/302/toptal-blog-image-1392824783170.png?w=688&#038;ssl=1" alt="Python class attributes vs. Python instance attributes" data-recalc-dims="1" />
</p>

_Note: if you have an expert handle on class attributes, you can skip ahead to [use cases][4]._

## Python Class Attributes {#python-class-attributes}

My interviewer was wrong in that the above code _is_ syntactically valid.

I too was wrong in that it isn’t setting a “default value” for the instance attribute. Instead, it’s defining 

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 as a _class_ attribute with value 

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.

In my experience, Python class attributes are a topic that _many_ people know _something_ about, but few understand completely.

### Python Class Variable vs. Instance Variable: What’s the Difference? {#python-class-variable-vs-instance-variable-whats-the-difference}

A Python class attribute is an attribute of the class (circular, I know), rather than an attribute of an _instance_ of a class.

Let’s use a Python class example to illustrate the difference. Here, 

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 is a class attribute, and 

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 is an instance attribute:

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      class MyClass(object):<br />
      &nbsp; &nbsp; class_var = 1<br />
      <br />
      &nbsp; &nbsp; def __init__(self, i_var):<br />
      &nbsp; &nbsp; &nbsp; &nbsp; self.i_var = i_var
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Note that all instances of the class have access to 

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, and that it can also be accessed as a property of the _class itself_:

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      foo = MyClass(2)<br />
      bar = MyClass(3)<br />
      <br />
      foo.class_var, foo.i_var<br />
      ## 1, 2<br />
      bar.class_var, bar.i_var<br />
      ## 1, 3<br />
      MyClass.class_var ## &lt;— This is key<br />
      ## 1
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For Java or C++ programmers, the class attribute is similar—but not identical—to the static member. We’ll see how they differ later.

### Class vs. Instance Namespaces {#class-vs-instance-namespaces}

To understand what’s happening here, let’s talk briefly about **Python namespaces**.

A <a href="http://docs.python.org/2/tutorial/classes.html" target="_blank" rel="noopener noreferrer">namespace</a> is a mapping from names to objects, with the property that there is zero relation between names in different namespaces. They’re usually implemented as Python dictionaries, although this is abstracted away.

Depending on the context, you may need to access a namespace using dot syntax (e.g., 

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          object.name_from_objects_namespace
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) or as a local variable (e.g., 

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). As a concrete example:

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      class MyClass(object):<br />
      &nbsp; &nbsp; ## No need for dot syntax<br />
      &nbsp; &nbsp; class_var = 1<br />
      <br />
      &nbsp; &nbsp; def __init__(self, i_var):<br />
      &nbsp; &nbsp; &nbsp; &nbsp; self.i_var = i_var<br />
      <br />
      ## Need dot syntax as we've left scope of class namespace<br />
      MyClass.class_var<br />
      ## 1
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Amit Agarwal

Python Class Attributes: An Overly Thorough Guide

Authored By Amit Agarwal