classes: test_classes

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how to make a class in Python

• use the class keyword

• use TitleCase for the name

• use a name that tells what the collection of attributes and methods (functions) does - this is hard to do and is something I am still learning

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test_defining_classes_w_pass

red: make it fail

I make a new file called test_classes.py in the tests directory

import unittest
import classes


class TestClasses(unittest.TestCase):

    def test_defining_classes_w_pass(self):
        self.assertIsInstance(classes.ClassWithPass(), object)

the terminal shows ModuleNotFoundError because I have an import statement for a module called classes

green: make it pass

• I add ModuleNotFoundError to the list of Exceptions encountered

  # Exceptions Encountered
  # AssertionError
  # ModuleNotFoundError
  

• I make Python module called classes.py and the terminal shows AttributeError which I add to the list of Exceptions encountered

  # Exceptions Encountered
  # AssertionError
  # ModuleNotFoundError
  # AttributeError
  

• I then add the name ClassWithPass to the module

  ClassWithPass
  

  and the terminal shows NameError because ClassWithPass is not defined anywhere

• I add the error to the list of Exceptions encountered

  # Exceptions Encountered
  # AssertionError
  # ModuleNotFoundError
  # AttributeError
  # NameError
  

• I point the name to None

  ClassWithPass = None
  

• and then redefine the variable as a class using thePython class keyword

  class ClassWithPass:
  

  the terminal shows IndentationError because I declared a class without adding any indented text

• I add the new error to the list of Exceptions encountered

  # Exceptions Encountered
  # AssertionError
  # ModuleNotFoundError
  # AttributeError
  # NameError
  # IndentationError
  

• Python has the pass keyword to use as a placeholder for moments like this cue Kelly Clarkson

  class ClassWithPass:
  
      pass
  

  and the terminal shows passing tests

refactor: make it better

Here is a quick review of what has happened so far

• pass is a placeholder

• self.assertIsInstance is a unittest.TestCase method that checks if the first input to the method is an instance of the second input

• the test self.assertIsInstance(classes.ClassWithPass(), object) checks if ClassWithPass is an object

• in Python everything is an object , which means if it exists in Python there is a class definition for it somewhere or it inherits from a class

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test_defining_classes_w_parentheses

red: make it fail

I add another test to TestClasses in test_classes.py to show another way to make a class

def test_defining_classes_w_parentheses(self):
    self.assertIsInstance(classes.ClassWithParentheses(), object)

the terminal shows AttributeError

green: make it pass

• I add a class definition like ClassWithPass to classes.py

  class ClassWithParentheses:
  
      pass
  

  the terminal shows passing tests

• When I make the definition include parentheses

  class ClassWithParentheses():
  
      pass
  

  the terminal shows all tests are still passing.

• I can confidently say that in Python

  • I can define classes with parentheses

  • I can define classes without parentheses

  • pass is a placeholder

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test_defining_classes_w_object

In object oriented programming there is a concept called Inheritance). With Inheritance I can define new objects that inherit from existing objects.

Making new objects is easier because I do not have to reinvent or rewrite things that already exist, I can inherit them instead and change the new objects for my specific use case

To use inheritance I specify the “parent” in parentheses when I define the new object (the child) to make the relationship

red: make it fail

I add another test to TestClasses in test_classes.py

def test_defining_classes_w_object(self):
    self.assertIsInstance(classes.ClassWithObject(), object)

and the terminal shows AttributeError

green: make it pass

• I add a class definition to classes.py

  class ClassWithObject():
  
      pass
  

  the terminal shows all tests passed

• then I change the definition to explicitly state the parent object

  class ClassWithObject(object):
  
      pass
  

  and the terminal still shows passing tests

Here is a little summary

• classes can be defined

  • with parentheses stating what object the class inherits from

  • with parentheses without stating what object the class inherits from

  • without parentheses

  • pass is a placeholder

• classes by default inherit from the object class, because in each of the tests, whether the parent is stated or not, each class I defined is an instance of an object

Zen of Python (PEP 20)

I prefer to use the explicit form of class definitions with the parent object in parentheses, from the Zen of Python: Explicit is better than implicit

Would you like to classes: attributes and methods?

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classes: tests and solutions