A template method pattern is a pattern that has a template. In this pattern, a superclass has template methods which have some abstract methods. These methods describe how abstract methods are called which are implemented by its subclasses.
Let’s see an example by creating a program which displays characters or strings for five times.
Class that is used in this program
- AbstractDisplay: A class acting as a template method named display that calls abstract methods; open, print and close
- CharDisplay: A class which implements AbstractDisplay
- StringDisplay: A class which implements AbstractDisplay
AbstractDisplay class has a template method which calls abstract methods. It implements how they are used but does not implements these abstract methods.
# abstract_display.py
from abc import ABC, abstractmethod
class AbstractDisplay(ABC):
@abstractmethod
def open(self):
pass
@abstractmethod
def print(self):
pass
@abstractmethod
def close(self):
pass
def display(self):
self.open()
for _ in range(5):
self.print()
self.close()
Let’s define CharDisplay class, a subclass of AbstractDisplay class, that implements abstract methods; open, print, and close. This class is given an a character as an input and formats and prints it out.
# char_display.py
from abstract_display import AbstractDisplay
class CharDisplay(AbstractDisplay):
def __init__(self, ch):
self._ch = ch
def open(self):
print("<<", end="")
def print(self):
print(self._ch, end="")
def close(self):
print(">>")
Let’s create another subclass, StringDisplay class which formats a given string and prints it out.
# string_display
from abstract_display import AbstractDisplay
import sys
class StringDisplay(AbstractDisplay):
def __init__(self, string):
self._string = string
# Get a byte length
self._width = len(string.encode("utf-8"))
def open(self):
self.print_line()
def print(self):
print(f"|{self._string}|")
def close(self):
self.print_line()
def print_line(self):
print('+', end='')
for _ in range(self._width):
print('-', end='')
print('+')
Let’s call this program in which display methods are called from CharDisplay and StringDisplay.
# main.py
from char_display import CharDisplay
from string_display import StringDisplay
def main():
d1 = CharDisplay('H')
d2 = StringDisplay('Hello, world.')
d1.display()
d2.display()
if __name__ == "__main__":
main()
One benefit of template method: it standardizes the logics in the method. Since all algorithms are defined in a template method in a superclass, you only need to modify the method as opposed to the case when you don’t have a template method in which case you need to modify all subclass methods.
kenta suzuki 