Author Dusty Phillips Reviewers

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They have strict ordering of elements
• 
They support the append operation efficiently
They tend to be slow, however, if you are inserting elements anywhere but the end of 
the list (especially so if it's the beginning of the list). As we discussed in the section on 
sets, they are also slow for checking if an element exists in the list, and by extension, 
searching. Storing data in a sorted order or reordering the data can also be inefficient.
Let's look at the three types of containers provided by the Python 
queue
module.
FIFO queues
FIFO stands for 
First In First Out
and represents the most commonly understood 
definition of the word "queue". Imagine a line of people standing in line at a bank or 
cash register. The first person to enter the line gets served first, the second person in 
line gets served second, and if a new person desires service, they join the end of the 
line and wait their turn.
The Python 
Queue
class is just like that. It is typically used as a sort of communication 
medium when one or more objects is producing data and one or more other objects is 
consuming the data in some way, probably at a different rate. Think of a messaging 
application that is receiving messages from the network, but can only display one 
message at a time to the user. The other messages can be buffered in a queue in the 
order they are received. FIFO queues are utilized a lot in such concurrent applications. 
(We'll talk more about concurrency in 
Chapter 12
, 
Testing Object-oriented Programs
.)
The 
Queue
class is a good choice when you don't need to access any data inside
the data structure except the next object to be consumed. Using a list for this would 
be less efficient because under the hood, inserting data at (or removing from) the 
beginning of a list can require shifting every other element in the list.
Queues have a very simple API. A 
Queue
can have "infinite" (until the computer runs 
out of memory) capacity, but it is more commonly bounded to some maximum size. 
The primary methods are 
put()
and 
get()
, which add an element to the back of 
the line, as it were, and retrieve them from the front, in order. Both of these methods 
accept optional arguments to govern what happens if the operation cannot successfully 
complete because the queue is either empty (can't get) or full (can't put). The default 
behavior is to block or idly wait until the 
Queue
object has data or room available 
to complete the operation. You can have it raise exceptions instead by passing the 
block=False
parameter. Or you can have it wait a defined amount of time before 
raising an exception by passing a 
timeout
parameter.
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Python Data Structures