Flyweight Design Pattern
Video Lecture
Overview
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Terminology
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Flyweight UML Diagram
Source Code
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./flyweight/flyweight_concept.py
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57 | # pylint: disable=too-few-public-methods
"The Flyweight Concept"
class IFlyweight():
"Nothing to implement"
class Flyweight(IFlyweight):
"The Concrete Flyweight"
def __init__(self, code: int) -> None:
self.code = code
class FlyweightFactory():
"Creating the FlyweightFactory as a singleton"
_flyweights: dict[int, Flyweight] = {} # Python 3.9
# _flyweights = {} # Python 3.8 or earlier
def __new__(cls):
return cls
@classmethod
def get_flyweight(cls, code: int) -> Flyweight:
"A static method to get a flyweight based on a code"
if not code in cls._flyweights:
cls._flyweights[code] = Flyweight(code)
return cls._flyweights[code]
@classmethod
def get_count(cls) -> int:
"Return the number of flyweights in the cache"
return len(cls._flyweights)
class Context():
"""
An example context that holds references to the flyweights in a
particular order and converts the code to an ascii letter
"""
def __init__(self, codes: str) -> None:
self.codes = list(codes)
def output(self):
"The context specific output that uses flyweights"
ret = ""
for code in self.codes:
ret = ret + FlyweightFactory.get_flyweight(code).code
return ret
# The Client
CONTEXT = Context("abracadabra")
# use flyweights in a context
print(CONTEXT.output())
print(f"abracadabra has {len('abracadabra')} letters")
print(f"FlyweightFactory has {FlyweightFactory.get_count()} flyweights")
|
Output
| python ./flyweight/flyweight_concept.py
abracadabra
abracadabra has 11 letters
FlyweightFactory has 5 flyweights
|
Example Use Case
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Example UML Diagram
Source Code
./flyweight/client.py
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30 | "The Flyweight Use Case Example"
from table import Table
from flyweight_factory import FlyweightFactory
TABLE = Table(3, 3)
TABLE.rows[0].columns[0].data = "abra"
TABLE.rows[0].columns[1].data = "112233"
TABLE.rows[0].columns[2].data = "cadabra"
TABLE.rows[1].columns[0].data = "racadab"
TABLE.rows[1].columns[1].data = "12345"
TABLE.rows[1].columns[2].data = "332211"
TABLE.rows[2].columns[0].data = "cadabra"
TABLE.rows[2].columns[1].data = "445566"
TABLE.rows[2].columns[2].data = "aa 22 bb"
TABLE.rows[0].columns[0].justify = 1
TABLE.rows[1].columns[0].justify = 1
TABLE.rows[2].columns[0].justify = 1
TABLE.rows[0].columns[2].justify = 2
TABLE.rows[1].columns[2].justify = 2
TABLE.rows[2].columns[2].justify = 2
TABLE.rows[0].columns[1].width = 15
TABLE.rows[1].columns[1].width = 15
TABLE.rows[2].columns[1].width = 15
TABLE.draw()
print(f"FlyweightFactory has {FlyweightFactory.get_count()} flyweights")
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./flyweight/flyweight.py
| "The Flyweight that contains an intrinsic value called code"
class Flyweight(): # pylint: disable=too-few-public-methods
"The Flyweight that contains an intrinsic value called code"
def __init__(self, code: int) -> None:
self.code = code
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./flyweight/flyweight_factory.py
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23 | "Creating the FlyweightFactory as a singleton"
from flyweight import Flyweight
class FlyweightFactory():
"Creating the FlyweightFactory as a singleton"
_flyweights: dict[int, Flyweight] = {} # Python 3.9
# _flyweights = {} # Python 3.8 or earlier
def __new__(cls):
return cls
@classmethod
def get_flyweight(cls, code: int) -> Flyweight:
"A static method to get a flyweight based on a code"
if not code in cls._flyweights:
cls._flyweights[code] = Flyweight(code)
return cls._flyweights[code]
@classmethod
def get_count(cls) -> int:
"Return the number of flyweights in the cache"
return len(cls._flyweights)
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./flyweight/column.py
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26 | "A Column that is used in a Row"
from flyweight_factory import FlyweightFactory
class Column(): # pylint: disable=too-few-public-methods
"""
The columns are the contexts.
They will share the Flyweights via the FlyweightsFactory.
`data`, `width` and `justify` are extrinsic values. They are outside
of the flyweights.
"""
def __init__(self, data="", width=11, justify=0) -> None:
self.data = data
self.width = width
self.justify = justify # 0:center, 1:left, 2:right
def get_data(self):
"Get the flyweight value from the factory, and apply the extrinsic values"
ret = ""
for data in self.data:
ret = ret + FlyweightFactory.get_flyweight(data).code
ret = f"{ret.center(self.width)}" if self.justify == 0 else ret
ret = f"{ret.ljust(self.width)}" if self.justify == 1 else ret
ret = f"{ret.rjust(self.width)}" if self.justify == 2 else ret
return ret
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./flyweight/row.py
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17 | "A Row in the Table"
from column import Column
class Row(): # pylint: disable=too-few-public-methods
"A Row in the Table"
def __init__(self, column_count: int) -> None:
self.columns = []
for _ in range(column_count):
self.columns.append(Column())
def get_data(self):
"Format the row before returning it to the table"
ret = ""
for column in self.columns:
ret = f"{ret}{column.get_data()}|"
return ret
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./flyweight/table.py
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26 | "A Formatted Table that includes rows and columns"
from row import Row
class Table(): # pylint: disable=too-few-public-methods
"A Formatted Table"
def __init__(self, row_count: int, column_count: int) -> None:
self.rows = []
for _ in range(row_count):
self.rows.append(Row(column_count))
def draw(self):
"Draws the table formatted in the console"
max_row_length = 0
rows = []
for row in self.rows:
row_data = row.get_data()
rows.append(f"|{row_data}")
row_length = len(row_data) + 1
if max_row_length < row_length:
max_row_length = row_length
print("-" * max_row_length)
for row in rows:
print(row)
print("-" * max_row_length)
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Output
| python ./flyweight/client.py
-----------------------------------------
|abra | 112233 | cadabra|
|racadab | 12345 | 332211|
|cadabra | 445566 | aa 22 bb|
-----------------------------------------
FlyweightFactory has 12 flyweights
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New Coding Concepts
String Justification
In ./flyweight/column.py, there are commands center(), ljust() and rjust() .
These are special commands on strings that allow you to pad strings and align them left, right, center depending on total string length.
eg,
| >>> "abcd".center(10)
' abcd '
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| >>> "abcd".rjust(10)
' abcd'
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| >>> "abcd".ljust(10)
'abcd '
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Summary
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