---

---

---

how to make a person with Exceptions

---

I had a problem when I made a person with conditions, I skipped test_when_person_is_older_than_120, test_when_year_of_birth_is_the_future and test_when_year_of_birth_is_not_an_integer because they are always in a RED state since they cause Exceptions.

Python has a way that allows programs to make a choice when they encounter an Exception and continue running without stopping. It is the try statement.

I want to use the try statement to handle making sure the program raises an Exception

• if the age is older than 120

• if the year_of_birth is not an integer

---

preview

I have these tests by the end of the chapter

person/tests/test_person.py

    def test_when_person_is_older_than_120(self):
        try:
            src.person.Person(
                first_name='first_name',
                last_name='last_name',
                sex='M',
                year_of_birth=self.this_year-121
            )
        except ValueError:
            pass

person/tests/test_person.py

    def test_when_year_of_birth_is_the_future(self):
        try:
            src.person.Person(
                first_name='first_name',
                last_name='last_name',
                sex='F',
                year_of_birth=self.this_year+1,
            )
        except ValueError:
            pass

person/tests/test_person.py

    def test_when_year_of_birth_is_not_an_integer(self):
        try:
            src.person.Person(
                first_name='first_name',
                last_name='last_name',
                sex='M',
                year_of_birth=None,
            )
        except TypeError:
            pass

person/tests/test_person.py

        try:
            src.person.Person(
                first_name='first_name',
                last_name='last_name',
                sex='F',
                year_of_birth=2026.0,
            )
        except TypeError:
            pass

person/tests/test_person.py

        try:
            src.person.Person(
                first_name='first_name',
                last_name='last_name',
                sex='M',
                year_of_birth='2026',
            )
        except TypeError:
            pass

person/tests/test_person.py

        try:
            src.person.Person(
                first_name='first_name',
                last_name='last_name',
                sex='F',
                year_of_birth=(2026,),
            )
        except TypeError:
            pass

---

open the project

• I open a terminal

• I change directory to the project

  cd person
  

• I use pytest-watcher to run the tests automatically

  uv run pytest-watcher . --now --patterns "**/*.py"
  

  the terminal shows

  tests/test_person.py .......sss                 [100%]
  
  ============ 7 passed, 3 skipped in T.UVs ============
  

---

add exception handler to test_when_person_is_older_than_120

RED: make it fail

---

• I open test_person.py from the tests folder

• I comment out the unittest.skip decorator from test_when_person_is_older_than_120 in tests/test_person.py

      # @unittest.skip('fails because age > 120')
      def test_when_person_is_older_than_120(self):
          src.person.Person(
              first_name='first_name',
              last_name='last_name',
              sex='M',
              year_of_birth=self.this_year-121
          )
  
      @unittest.skip('fails because age < 0')
      def test_when_year_of_birth_is_the_future(self):
  

  the terminal is my friend, and shows AssertionError

  E       AssertionError
  

• I open __init__.py from the person folder in the src folder

• I change assert age <= 120 to an if statement that raises an Exception if the age is greater than 120 in src/person/__init__.py

  def calculate_age(year_of_birth):
      assert isinstance(year_of_birth, int)
      age = (
          datetime.date.today().year
        - year_of_birth
      )
      assert age >= 0
      # assert age <= 120
      if age > 120:
          raise Exception
      return age
  

  the terminal is my friend, and shows Exception

  E           Exception
  

• I put the failing statement in an if statement in test_when_person_is_older_than_120 in tests/test_person.py

      # @unittest.skip('fails because age > 120')
      def test_when_person_is_older_than_120(self):
          if(
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=self.this_year-121
              )
          ) == Exception:
              pass
  
      @unittest.skip('fails because age < 0')
      def test_when_year_of_birth_is_the_future(self):
  

  the terminal still shows Exception, because an Exception breaks execution of a program, nothing past the line that caused it will run. In this case, the Exception happens when the instance of the Person object is made so the condition never gets checked.

• I change the if statement to a try statement

      # @unittest.skip('fails because age > 120')
      def test_when_person_is_older_than_120(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=self.this_year-121
              )
          except:
              pass
  

  the test passes because the try statement is like an if statement for Exceptions.

  It tells the program what to do if an Exception is raised. A simple way to think of it is

  • try something

  • except - if something raises an Exception do something else

  pass is a special keyword that allows the try statement to follow Python language rules (the except block must have a body), it does not do anything.

• I remove the commented line from test_when_person_is_older_than_120

          self.assertEqual(underage.can_vote(), False)
          self.assertEqual(underage.can_get_license(), False)
  
      def test_when_person_is_older_than_120(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=self.this_year-121
              )
          except:
              pass
  
      @unittest.skip('fails because age < 0')
      def test_when_year_of_birth_is_the_future(self):
  

• I open another terminal and change directories to person

  cd person
  

• add a git commit message

  git commit -am \
  'add exception handler to test_when_person_is_older_than_120'
  

---

add exception handler to test_when_year_of_birth_is_the_future

RED: make it fail

---

• I go back to the terminal where the tests are running

• I comment out the unittest.skip decorator from test_when_year_of_birth_is_the_future in tests/test_person.py

      # @unittest.skip('fails because age < 0')
      def test_when_year_of_birth_is_the_future(self):
          src.person.Person(
              first_name='first_name',
              last_name='last_name',
              sex='F',
              year_of_birth=self.this_year+1,
          )
  
      @unittest.skip(
          'fails because year_of_birth is not an integer'
      )
      def test_when_year_of_birth_is_not_an_integer(self):
  

  the terminal is my friend, and shows AssertionError

  E       AssertionError
  

---

GREEN: make it pass

---

• I change assert age >= 0 to an if statement that raises an Exception when year_of_birth is in the future, in the calculate_age function in src/person/__init__.py

  def calculate_age(year_of_birth):
      assert isinstance(year_of_birth, int)
      age = (
          datetime.date.today().year
        - year_of_birth
      )
      # assert age >= 0
      if age < 0:
          raise Exception
      # assert age <= 120
      if age > 120:
          raise Exception
      return age
  

  the terminal is my friend, and shows Exception

  E           Exception
  

• I put the failing statement in an if statement in test_when_year_of_birth_is_the_future in tests/test_person.py

      # @unittest.skip('fails because age < 0')
      def test_when_year_of_birth_is_the_future(self):
          if (
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=self.this_year+1,
              )
          ) == Exception:
              pass
  
      @unittest.skip(
          'fails because year_of_birth is not an integer'
      )
      def test_when_year_of_birth_is_not_an_integer(self):
  

  the terminal still shows Exception, because an Exception breaks execution of a program, nothing past the line that caused it will run.

• I change the if statement to a try statement

      # @unittest.skip('fails because age < 0')
      def test_when_year_of_birth_is_the_future(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=self.this_year+1,
              )
          except:
              pass
  

  the test passes because try statement is like an if statement for Exceptions. It tells the program what to do if an Exception is raised

---

REFACTOR: make it better

---

• I remove the commented line from test_when_year_of_birth_is_the_future

      def test_when_person_is_older_than_120(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=self.this_year-121
              )
          except:
              pass
  
      def test_when_year_of_birth_is_the_future(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=self.this_year+1,
              )
          except:
              pass
  
      @unittest.skip(
          'fails because year_of_birth is not an integer'
      )
      def test_when_year_of_birth_is_not_an_integer(self):
  

• I put the two new if statements in the calculate_age function together as one because they return the same thing, in src/person/__init__.py

  def calculate_age(year_of_birth):
      assert isinstance(year_of_birth, int)
      age = (
          datetime.date.today().year
        - year_of_birth
      )
      # assert age >= 0
      # if age < 0:
      #     raise Exception
      # assert age <= 120
      # if age > 120:
      if age < 0 or age > 120:
          raise Exception
      return age
  

  the test is still green.

• I remove the commented lines from the calculate_age function

  def calculate_age(year_of_birth):
      assert isinstance(year_of_birth, int)
      age = (
          datetime.date.today().year
        - year_of_birth
      )
  
      if age < 0 or age > 120:
          raise Exception
      return age
  

• I add a git commit message in the other terminal

  git commit -am \
  'add exception handler to test_when_year_of_birth_is_the_future'
  

---

add exception handler to test_when_year_of_birth_is_not_an_integer

RED: make it fail

---

• I go back to the terminal where the tests are running

• I comment out the unittest.skip decorator from test_when_year_of_birth_is_not_an_integer and remove the comment from year_of_birth=None to test when year_of_birth is None, in tests/test_person.py

      # @unittest.skip(
      #     'fails because year_of_birth is not an integer'
      # )
      def test_when_year_of_birth_is_not_an_integer(self):
          src.person.Person(
              first_name='first_name',
              last_name='last_name',
              sex='M',
              year_of_birth=None,
              # year_of_birth=2026.0,   # fails
              # year_of_birth='2026',   # fails
              # year_of_birth=(2026,),  # fails
          )
  
      def test_dir_person_object(self):
  

  the terminal is my friend, and shows AssertionError

  E       AssertionError
  

---

GREEN: make it pass

---

• I change assert isinstance(year_of_birth, int) to an if statement that raises an Exception when year_of_birth is not an integer in the calculate_age function in src/person/__init__.py

  def calculate_age(year_of_birth):
      # assert isinstance(year_of_birth, int)
      if not isinstance(year_of_birth, int):
          raise Exception
  
      age = (
          datetime.date.today().year
        - year_of_birth
      )
  
      if age < 0 or age > 120:
          raise Exception
      return age
  

  the terminal is my friend, and shows Exception

  E           Exception
  

• I move the failing statement to a try statement for when year_of_birth is None in test_when_year_of_birth_is_not_an_integer in tests/test_person.py

      # @unittest.skip(
      #     'fails because year_of_birth is not an integer'
      # )
      def test_when_year_of_birth_is_not_an_integer(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=None,
              )
          except:
              pass
          # year_of_birth=2026.0,   # fails
          # year_of_birth='2026',   # fails
          # year_of_birth=(2026,),  # fails
  
      def test_dir_person_object(self):
  

  the test passes.

---

REFACTOR: make it better

---

• I make a person with a float as the value for year_of_birth

      except:
          pass
  
      src.person.Person(
          first_name='first_name',
          last_name='last_name',
          sex='F',
          year_of_birth=2026.0,
      )
  
      # year_of_birth='2026',   # fails
      # year_of_birth=(2026,),  # fails
  

  the terminal is my friend, and shows Exception

• I move the failing statement for when the year_of_birth is a float to a try statement

          except:
              pass
  
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=2026.0,
              )
          except:
              pass
  
          # year_of_birth='2026',  # fails
  

  the test passes.

• I make a person with a string as the value for year_of_birth

          except:
              pass
  
          src.person.Person(
              first_name='first_name',
              last_name='last_name',
              sex='M',
              year_of_birth='2026',
          )
  
          # year_of_birth=(2026,),  # fails
  

  the terminal is my friend, and shows Exception

• I move the statement for when the year_of_birth is a string to a try statement

          except:
              pass
  
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth='2026',
              )
          except:
              pass
  
          # year_of_birth=(2026,),  # fails
  

  the test passes.

• I make a person with a tuple as the value for year_of_birth

          except:
              pass
  
          src.person.Person(
              first_name='first_name',
              last_name='last_name',
              sex='F',
              year_of_birth=(2026,),
          )
  
      def test_dir_person_object(self):
  

  the terminal is my friend, and shows Exception

• I move the statement for when the year_of_birth is a tuple to a try statement

          except:
              pass
  
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=(2026,),
              )
          except:
              pass
  
      def test_dir_person_object(self):
  

  the test passes.

• I remove the commented line from the calculate_age function in src/person/__init__.py

  def calculate_age(year_of_birth):
      if not isinstance(year_of_birth, int):
          raise Exception
  
      age = (
          datetime.date.today().year
        - year_of_birth
      )
  
      if age < 0 or age > 120:
          raise Exception
      return age
  

• I add a git commit message in the other terminal

  git commit -am \
  'add exception handler to test_when_year_of_birth_is_not_an_integer'
  

---

raise TypeError when year_of_birth is not an integer

The problem with using except: is that it catches all Exceptions which means it does not tell anyone that reads the code what the actual Exception is.

try:
    something
except:
    something else

is the same as

try:
    something
except Exception:
    something else

because Exception is the mother of all the Exceptions covered so far, they inherit from it.

From the Zen of Python: Explicit is better than implicit. I want to make things clearer.

RED: make it fail

---

• I go back to the terminal where the tests are running

• I change the except clause in test_when_year_of_birth_is_not_an_integer for when the year_of_birth is a tuple to show that it catches Exception

        try:
            src.person.Person(
                first_name='first_name',
                last_name='last_name',
                sex='F',
                year_of_birth=(2026,),
            )
        except Exception:
            pass
  
    def test_dir_person_object(self):
  

  the test is still green.

• I change the except clause to make it only catch TypeError

          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=(2026,),
              )
          except TypeError:
              pass
  
      def test_dir_person_object(self):
  

  the terminal is my friend, and shows Exception

  E           Exception
  

  because Exception is not TypeError even though TypeError is an Exception. I cannot use a child Exception to catch its parent Exception.

---

GREEN: make it pass

---

I change the raise statement in the calculate_age function for when the year_of_birth is not an integer to make it clearer, in src/person/__init__.py

def calculate_age(year_of_birth):
    if not isinstance(year_of_birth, int):
        raise TypeError
        raise Exception

    age = (
        datetime.date.today().year
      - year_of_birth
    )

the test passes because the try statement now only catches/handles TypeError, all other Exceptions will still break execution of this program.

---

REFACTOR: make it better

---

• I change the except clause in test_when_year_of_birth_is_not_an_integer for when year_of_birth is a string to show that it catches Exception

          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth='2026',
              )
          except Exception:
              pass
  

  the test is still green.

• I change the except clause to make it only catch AssertionError

          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth='2026',
              )
          except AssertionError:
              pass
  

  the terminal is my friend, and shows TypeError

  E           TypeError
  

  because TypeError is not AssertionError or a child of AssertionError.

• I change the except clause in test_when_year_of_birth_is_not_an_integer for when year_of_birth is a string to only catch TypeError

          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth='2026',
              )
          except TypeError:
              pass
  

  the test passes.

• I change the except clause in test_when_year_of_birth_is_not_an_integer for when year_of_birth is a float to show that it catches Exception

          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=2026.0,
              )
          except Exception:
              pass
  

  the test is still green.

• I change the except clause to make it only catch NameError

          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=2026.0,
              )
          except NameError:
              pass
  

  the terminal is my friend, and shows TypeError

  E           TypeError
  

  because TypeError is not NameError or a child of NameError.

• I change the except clause in test_when_year_of_birth_is_not_an_integer for when the year_of_birth is a float to make it only catch TypeError

          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=2026.0,
              )
          except TypeError:
              pass
  

  the test passes.

• I change the except clause in test_when_year_of_birth_is_not_an_integer for when the year_of_birth is None to show that it catches Exception

      # @unittest.skip(
      #     'fails because year_of_birth is not an integer'
      # )
      def test_when_year_of_birth_is_not_an_integer(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=None,
              )
          except Exception:
              pass
  

  the test is still green.

• I change the except clause to make it only catch ValueError

      def test_when_year_of_birth_is_not_an_integer(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=None,
              )
          except ValueError:
              pass
  

  the terminal is my friend, and shows TypeError

  E           TypeError
  

  because TypeError is not ValueError or a child of ValueError.

• I change the except clause in test_when_year_of_birth_is_not_an_integer for when the year_of_birth is None to make it catch/handle TypeError

      # @unittest.skip(
      #     'fails because year_of_birth is not an integer'
      # )
      def test_when_year_of_birth_is_not_an_integer(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=None,
              )
          except TypeError:
              pass
  

  the test passes.

• I remove the commented lines from test_when_year_of_birth_is_not_an_integer

      def test_when_year_of_birth_is_the_future(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=self.this_year+1,
              )
          except:
              pass
  
      def test_when_year_of_birth_is_not_an_integer(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=None,
              )
          except TypeError:
              pass
  

• I add a git commit message in the other terminal

  git commit -am \
  'raise TypeError when year_of_birth is not an integer'
  

---

raise ValueError when age is greater than 120 or less than 0

RED: make it fail

---

• I go back to the terminal where the tests are running

• I change the except clause in test_when_year_of_birth_is_the_future to catch ValueError

      def test_when_year_of_birth_is_the_future(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='F',
                  year_of_birth=self.this_year+1,
              )
          except ValueError:
              pass
  
      def test_when_year_of_birth_is_not_an_integer(self):
  

  the terminal is my friend, and shows Exception

  E           Exception
  

  because Exception is not ValueError even though ValueError is an Exception. I cannot use a child Exception to catch its parent Exception.

---

GREEN: make it pass

---

I change the raise statement in the calculate_age function for when the age is greater than 120 or less than 0 to make it clearer, in src/person/__init__.py

def calculate_age(year_of_birth):
    if not isinstance(year_of_birth, int):
        raise TypeError
        raise Exception

    age = (
        datetime.date.today().year
      - year_of_birth
    )

    if age < 0 or age > 120:
        raise ValueError
        raise Exception
    return age

the test passes.

---

REFACTOR: make it better

---

• I remove raise Exception from the calculate_age function

  def calculate_age(year_of_birth):
      if not isinstance(year_of_birth, int):
          raise TypeError
  
      age = (
          datetime.date.today().year
        - year_of_birth
      )
  
      if age < 0 or age > 120:
          raise ValueError
      return age
  

• I change the except clause in test_when_person_is_older_than_120 to show that it catches Exception

      def test_when_person_is_older_than_120(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=self.this_year-121
              )
          except Exception:
              pass
  

  the test is still green.

• I change the except clause to make it only catch TypeError

      def test_when_person_is_older_than_120(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=self.this_year-121
              )
          except TypeError:
              pass
  

  the terminal is my friend, and shows ValueError

  E           ValueError
  

• I change the except clause to make it only catch ValueError

      def test_when_person_is_older_than_120(self):
          try:
              src.person.Person(
                  first_name='first_name',
                  last_name='last_name',
                  sex='M',
                  year_of_birth=self.this_year-121
              )
          except ValueError:
              pass
  

  the test is green again.

• I add a git commit message in the other terminal

  git commit -am \
  'raise ValueError when age > 120 or age < 0'
  

---

close the project

• I close tests/test_person.py and src/person/__init__.py

• I use q on the keyboard to leave the tests. The terminal goes back to the command line.

• I change directory to the parent of person

  cd ..
  

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review

• Exceptions break execution of a program, nothing past the line that causes an Exception will run.

• I can use the try statement to make sure a program can make a decision when an Exception is raised.

• I can use the try statement in a test to show that a program raises an Exception when certain conditions are met.

• I can use the raise statement to make an Exception happen to stop a program from running past a certain point.

My tests still have problems:

• The attribute tests - test_dir_person_object and test_dir_person_instance catch changes to the attributes and methods of the Person object and they are a problem to maintain. There has to be a better way.

• test_joe, test_jane, test_john and test_mary all still have the same three tests. There has to be a better way.

• test_when_year_of_birth_is_not_an_integer has four tests that are basically the same, the only thing that changes are the values for the year_of_birth parameter. There has to be a better way.

• The exception handlers also have a problem

  • If the code raises the expected Exception, the test passes

  • If the code does not raise an Exception, the test passes

  • The only time those tests fail is if the code raises an Exception that the test is not expecting

  there has to be a better way

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code from the chapter

Do you want to see all the CODE I typed in this chapter?

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what is next?

• I know how to make a Python Test Driven Development environment manually.

• I know what a Python module is.

• I know how to run tests automatically.

• I know what an assertion is.

• I know how to make functions.

• I know how to make a person with strings.

• I know how to make functions that take input.

• I know what causes TypeError.

• I know how to place values in strings.

• I know how to make a person say hello with f-strings.

• I know how to separate tests from solutions.

• I know what causes AttributeError.

• I know how to make a person with an object.

• I know that everything in Python is an object.

• I know how to use the unittest library.

• I know how to use the datetime library.

• I know what None is.

• I know how to make a person with conditions.

• I know how Python groups objects into False or True.

• I know how to make a Python Test Driven Development environment automatically.

• I know how to write programs that make decisions.

• I know how to make a Python Test Driven Development environment automatically with variables.

• I know how to make a person with Exceptions.

• how to test that an Exception is raised
• how to handle Exceptions in programs

Would you like to test handling Exceptions in tests?

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