Author Dusty Phillips Reviewers

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Python Data Structures
In our examples so far, we've already seen many of the built-in Python data 
structures in action. You've probably also covered many of them in introductory 
books or tutorials. In this chapter, we'll be discussing the object-oriented features
of these data structures, when they should be used instead of a regular class, and 
when they should not be used. In particular, we'll be covering:
• 
Tuples and named tuples
• 
Dictionaries
• 
Lists and sets
• 
How and why to extend built-in objects
• 
Three types of queues
Empty objects
Let's start with the most basic Python built-in, one that we've seen many times 
already, the one that we've extended in every class we have created: the 
object
. 
Technically, we can instantiate an 
object
without writing a subclass:
>>> o = object()
>>> o.x = 5
Traceback (most recent call last):
File "", line 1, in
AttributeError: 'object' object has no attribute 'x'
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Python Data Structures
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 156 
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Unfortunately, as you can see, it's not possible to set any attributes on an 
object
that 
was instantiated directly. This isn't because the Python developers wanted to force 
us to write our own classes, or anything so sinister. They did this to save memory; a 
lot of memory. When Python allows an object to have arbitrary attributes, it takes a 
certain amount of system memory to keep track of what attributes each object has, for 
storing both the attribute name and its value. Even if no attributes are stored, memory 
is allocated for 
potential
new attributes. Given the dozens, hundreds, or thousands of 
objects (every class extends object) in a typical Python program; this small amount 
of memory would quickly become a large amount of memory. So, Python disables 
arbitrary properties on 
object
, and several other built-ins, by default.
It is possible to restrict arbitrary properties on our own classes using 
slots
. Slots are beyond the scope of this book, but you now have a 
search term if you are looking for more information. In normal use, 
there isn't much benefit to using slots, but if you're writing an object 
that will be duplicated thousands of times throughout the system, 
they can help save memory, just as they do for 
object
.
It is, however, trivial to create an empty object class of our own; we saw it in our 
earliest example:
class MyObject:
pass
And, as we've already seen, it's possible to set attributes on such classes: