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Python Design Patterns I
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strategy pattern in UML:
The User
dealing with the Abstraction interface. The actual implementation chosen will 
is identical.
Strategy example
T
order they are, and the requirements of the system.
with a 
sort()
method. This object may be a 
QuickSorter
or a 
MergeSorter
object; 
Of course, in Python, we typically just call the 
sorted
function or 
list.sort
method 
look at a better example, don't we?
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Chapter 8
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the screen, centered on it, or scaled to fit. There are other, more complicated, 
basic ones.
screen, with the image manipulated to fit according to the given strategy.
from pygame import image
from pygame.transform import scale
from pygame import Surface
class TiledStrategy:
def make_background(self, img_file, desktop_size):
in_img = image.load(img_file)
out_img = Surface(desktop_size)
for x in range((out_img.get_width(
) // in_img.get_width()) + 1):
for y in range((out_img.get_height(
) // in_img.get_height()) + 1):
out_img.blit(in_img, (in_img.get_width() * x,
in_img.get_height() * y))
return out_img
class CenteredStrategy:
def make_background(self, img_file, desktop_size):
in_img = image.load(img_file)
out_img = Surface(desktop_size)
out_img.fill((0,0,0))
left = (out_img.get_width() - in_img.get_width()) / 2
top = (out_img.get_height() - in_img.get_height()) / 2
out_img.blit(in_img, (left, top))
return out_img
class ScaledStrategy:
def make_background(self, img_file, desktop_size):
in_img = image.load(img_file)
return scale(in_img, desktop_size)
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Python Design Patterns I
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Here we have three strategies, each using 
pygame
to perform their task. Individual 
strategies have a 
make_background
method that accepts the same set of parameters. 
version of the desktop image. 
TiledStrategy
loops over the number of input 
images that would fit in the width and height of the image and copies it into each 
location, repeatedly. 
CenteredStrategy
figures out how much space needs to be left 
on the four edges of the image to center it. 
ScaledStrategy
simply forces the image 
to the output size (ignoring aspect ratio).
an awkward 
if
statement to select the expected one. Every time we wanted to add a 
new strategy, we'd have to make the method even more ungainly.
Strategy in Python
see why?
function. We could just as easily call that function 
__call__
and make the object 
O
first-class functions, the strategy pattern is unnecessary". In truth, Python's first-class 
State pattern
S
systems where it is obvious that an object can be in a specific state, and that certain 
activities may drive it to a different state.
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Chapter 8
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depending on actions invoked upon it.
Here's how it looks in UML:
State example
To
be the parser itself. It will take a string as input and place the tool in an initial parsing 
state. The various parsing states will eat characters, looking for a specific value, and 
which has tags inside of text. Here is an example "simplified XML" file that we'll be 
able to parse:
<book>
<author>Dusty Phillips</author>
<publisher>Packt Publishing</publisher>
<title>Python 3 Object Oriented Programming</title>
<content>
<chapter>
<number>1</number>
<title>Object Oriented Design</title>
</chapter>
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Python Design Patterns I
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<chapter>
<number>2</number>
<title>Objects In Python</title>
</chapter>
</content>
</book>
We know we want a tree of 
Node
objects, but what does a 
Node
look like? Well, 
at the 
Node
class first:
class Node:
def __init__(self, tag_name, parent=None):
self.parent = parent
self.tag_name = tag_name
self.children = []
self.text=""
def __str__(self):
if self.text:
return self.tag_name + ": " + self.text
else:
return self.tag_name
This class sets default attribute values upon initialization. The 
__str__
method is 
supplied to help visualize the tree structure when we're finished.
Now, looking at the example document, we need to consider what states our parser 
can be in. Clearly it's going to start in a state where no nodes have yet been processed. 
We'll need a state for processing opening tags and closing tags. And when we're inside 
a tag with text contents, we'll have to process that as a separate state, too.
but it actually makes more sense to create a new state whose sole purpose is figuring 
ChildNode
we end up with the following states:
FirstTag
ChildNode
OpenTag
CloseTag
Text
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Chapter 8
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The FirstTag state will switch to ChildNode, which is responsible for deciding which 
of the other three states to switch to; when those states are finished, they'll switch 
back to ChildNode. The following state-transition diagram shows the available  
state changes:
it. Let's construct the 
Parser
class first:
class Parser:
def __init__(self, parse_string):
self.parse_string = parse_string
self.root = None
self.current_node = None
self.state = FirstTag()
def process(self, remaining_string):
remaining = self.state.process(remaining_string, self)
if remaining:
self.process(remaining)
def start(self):
self.process(self.parse_string)
access. The 
parse_string
is the text that we are trying to parse. The 
root
node is the 
"top" node in the XML structure. The 
current_node
is the one that we are currently 
adding children to.
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The important feature of this parser is the 
process
method, which accepts the 
self
when it is finished processing. The parser then recursively calls the 
process
method 
on this remaining string to construct the rest of the tree.
Now, let's have a look at the 
FirstTag
state:
class FirstTag:
def process(self, remaining_string, parser):
i_start_tag = remaining_string.find('<')
i_end_tag = remaining_string.find('>')
tag_name = remaining_string[i_start_tag+1:i_end_tag]
root = Node(tag_name)
parser.root = parser.current_node = root
parser.state = ChildNode()
return remaining_string[i_end_tag+1:]
This state finds the index (the 
i_
stands for index) of the opening and closing angle 
brackets on the first tag. You may think this state is unnecessary, since XML requires 
of assuming it is the first character in the document. Note that this code is assuming 
a valid input file. A proper implementation would be religiously testing for invalid 
The
It also assigns it as the 
current_node
, since that's the one we'll be adding children  
to next. 
object to a 
ChildNode
state. It then returns the remainder of the string (after the 
opening tag) to allow it to be processed.
The 
ChildNode
state, which seems quite complicated, turns out to require nothing 
but a simple conditional:
class ChildNode:
def process(self, remaining_string, parser):
stripped = remaining_string.strip()
if stripped.startswith("</"):
parser.state = CloseTag()
elif stripped.startswith("<"):
parser.state = OpenTag()
else:
parser.state = TextNode()
return stripped
Chapter 8
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The 
strip()
call removes whitespace from the string. Then the parser determines 
remainder of the string.
The 
OpenTag
state is similar to the 
FirstTag
state, except that it adds the newly 
created node to the previous 
current_node
object's 
children
and sets it as the new 
current_node
. It places the processor back in the 
ChildNode
state before continuing:
class OpenTag:
def process(self, remaining_string, parser):
i_start_tag = remaining_string.find('<')
i_end_tag = remaining_string.find('>')
tag_name = remaining_string[i_start_tag+1:i_end_tag]
node = Node(tag_name, parser.current_node)
parser.current_node.children.append(node)
parser.current_node = node
parser.state = ChildNode()
return remaining_string[i_end_tag+1:]
CloseTag
basically does the opposite; it sets the parser's 
current_node
back to the 
class CloseTag:
def process(self, remaining_string, parser):
i_start_tag = remaining_string.find('<')
i_end_tag = remaining_string.find('>')
assert remaining_string[i_start_tag+1] == "/"
tag_name = remaining_string[i_start_tag+2:i_end_tag]
assert tag_name == parser.current_node.tag_name
parser.current_node = parser.current_node.parent
parser.state = ChildNode()
return remaining_string[i_end_tag+1:].strip()
The two 
assert
statements help ensure that the parse strings are consistent. The 
if
when it is finished. If the parent of a node is 
None
, it means that we are working on 
the root node. 
Finally, the 
TextNode
state very simply extracts the text before the next close tag and 
sets it as a value on the current node:
class TextNode:
def process(self, remaining_string, parser):
i_start_tag = remaining_string.find('<')
text = remaining_string[:i_start_tag]
Python Design Patterns I
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parser.current_node.text = text
parser.state = ChildNode()
return remaining_string[i_start_tag:]
Now we just have to set up the initial state on the parser object we created. The initial 
state is a 
FirstTag
object, so just add the following to the 
__init__
method:
self.state = FirstTag()
To test the class, let's add a main script that opens an file from the command line, 
parses it, and prints the nodes:
if __name__ == "__main__":
import sys
with open(sys.argv[1]) as file:
contents = file.read()
p = Parser(contents)
p.start()
nodes = [p.root]
while nodes:
node = nodes.pop(0)
print(node)
nodes = node.children + nodes
All it does is open the file, load the contents, and parse them. Then it prints each 
node and its children in order. The 
__str__
method we originally added on the  
the earlier example, it outputs the tree as follows:
book
author: Dusty Phillips
publisher: Packt Publishing
title: Python 3 Object Oriented Programming
content
chapter
number: 1
title: Object Oriented Design
chapter
number: 2
title: Objects In Python
Comparing this to the original simplified XML document tells us the parser  
is working.
Chapter 8
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State versus strategy
Sta 
written our states as first-class functions instead of wrapping them in objects,  
as was suggested for strategy.
the context needs to know which other states that it can switch to.
Singleton pattern
The 
programming language.
The
discussed in Chapter 5. Such objects often need to be referenced by a wide variety of 
and constructors that need them can make code hard to read.
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