how to make a person with Exceptions

I had a problem when I made a person with conditions

I skipped test_when_year_of_birth_is_not_an_integer because it is always in a RED state since it causes an Exception.
I commented out the bad year_of_birth values in test_john for when a person is older than 120 because it causes an Exception.

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_too_old_to_be_alive(self):
        try:
            src.person.Person(
                first_name='first_name',
                last_name='last_name',
                sex='F',
                year_of_birth=datetime.date.today().year-121,
            )
        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='M',
                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='M',
                year_of_birth=(2026,),
            )
        except TypeError:
            pass

    def test_dir_person_class(self):

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

the terminal shows

tests/test_person.py .......                    [100%]

============ 7 passed, 1 skipped in T.UVs ============

test_when_person_is_too_old_to_be_alive

RED: make it fail

I open test_person.py from the tests folder

I add test_when_person_is_too_old_to_be_alive for if the age of the person is greater than 120, in tests/test_person.py

    def test_underage_citizen(self):
        person = src.person.Person(
            first_name='first_name',
            last_name='last_name',
            sex='M',
            year_of_birth=datetime.date.today().year-17,
            is_citizen=True,
            passed_test=True,
        )
        self.assertEqual(person.can_vote(), False)
        self.assertEqual(
            person.can_get_license(), False
        )

    def test_when_person_is_too_old_to_be_alive(self):
        src.person.Person(
            first_name='first_name',
            last_name='last_name',
            sex='F',
            year_of_birth=datetime.date.today().year-121,
        )

    @unittest.skip('will always fail')
    def test_when_year_of_birth_is_not_an_integer(self):

the terminal is my friend, and shows AssertionError

E       AssertionError

I use a calculation (datetime.date.today().year-121) as the year of birth so that it will always be 121 years ago.

GREEN: make it pass

I open src/person/__init__.py

I change the assert statement in the calculate_age function for if the age is less than or equal to 120 to raise an Exception if the age is greater than 120

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


def say_hello(
    first_name, last_name, year_of_birth,
):

the terminal is my friend, and shows Exception

E           Exception

I add a try statement to test_when_person_is_too_old_to_be_alive in tests/test_person.py

    def test_when_person_is_too_old_to_be_alive(self):
        try:
            src.person.Person(
                first_name='first_name',
                last_name='last_name',
                sex='F',
                year_of_birth=datetime.date.today().year-121,
            )
        except:
            pass

    @unittest.skip('will always fail')
    def test_when_year_of_birth_is_not_an_integer(self):

the test passes, confirming that when the value for year_of_birth makes the person older than 120 an Exception is is raised.

The try statement is like an if statement for Exceptions. It tells the program what to do if an Exception is 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).

I add a git commit message

git commit -am \
'add test_when_person_is_too_old_to_be_alive'

add exception handler to test_when_year_of_birth_is_not_an_integer

RED: make it fail

I remove 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

        except:
            pass

    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_class(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
    )
    # assert age <= 120
    if age > 120:
        raise Exception
    return age


def say_hello(
    first_name, last_name, year_of_birth,
):

the terminal is my friend, and shows Exception

E           Exception

I add a try statement for when year_of_birth is None to test_when_year_of_birth_is_not_an_integer in 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:
            pass
        # year_of_birth=2026.0,  # fails
        # year_of_birth='2026',  # fails
        # year_of_birth=(2026,), # fails

    def test_dir_person_class(self):

the test passes, showing that Exception is raised when year_of_birth is not an integer.

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='M',
            year_of_birth=2026.0,
        )

        # year_of_birth='2026',  # fails
        # year_of_birth=(2026,), # fails

        # fails if year_of_birth is not an integer

    def test_dir_person_class(self):

the terminal is my friend, and shows Exception

I add a try statement for when the year_of_birth is a float

        except:
            pass

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

        # year_of_birth='2026',  # fails

the test passes, showing that Exception is raised when year_of_birth is not an integer.

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

        # fails if year_of_birth is not an integer

    def test_dir_person_class(self):

the terminal is my friend, and shows Exception

I add a try statement for when the year_of_birth is a string

        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, showing that Exception is raised when year_of_birth is not an integer.

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='M',
            year_of_birth=(2026,),
        )

    def test_dir_person_class(self):

the terminal is my friend, and shows Exception

I add a try statement for when the year_of_birth is a tuple, and remove the other comments since I no longer need them

        except:
            pass

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

    def test_dir_person_class(self):

the test passes, showing that Exception is raised when year_of_birth is not an integer.

I remove the commented lines 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 > 120:
        raise Exception
    return age


def say_hello(
    first_name, last_name, year_of_birth,
):

I add a git commit message

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 change the except clause in test_when_year_of_birth_is_not_an_integer for when the year_of_birth is a tuple to be more specific

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

    def test_dir_person_class(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 be more specific, in src/person/__init__.py

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

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

the test passes because the try statement now only catches/handles TypeError.

try:
    something
except TypeError:
    something else

REFACTOR: make it better

I change the except clause in test_when_year_of_birth_is_not_an_integer for when the year_of_birth is a string to catch AssertionError in tests/test_person.py

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

        try:

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 the year_of_birth is a string to catch TypeError

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

        try:

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 a float to catch NameError

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

        try:

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 catch TypeError

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

        try:

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 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

        try:

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 catch TypeError

    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

        try:

the test passes.

I add a git commit message

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

raise ValueError when age is greater than 120

RED: make it fail

I change the except clause in test_when_person_is_too_old_to_be_alive to catch ValueError

    def test_when_person_is_too_old_to_be_alive(self):
        try:
            src.person.Person(
                first_name='first_name',
                last_name='last_name',
                sex='F',
                year_of_birth=datetime.date.today().year-121,
            )
        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 to be more specific, in src/person/__init__.py

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

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

    if age > 120:
        # raise Exception
        raise ValueError
    return age


def say_hello(
    first_name, last_name, year_of_birth,
):

the test passes.

I remove the commented lines 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 > 120:
        raise ValueError
    return age


def say_hello(
    first_name, last_name, year_of_birth,
):

I add a git commit message

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

close the project

I close 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 ..
```

review

I can use the try statement to make sure a program can make a decision when it runs into an Exception.
I can use the try statement in a test to confirm 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_class and test_dir_person_instance catch changes to the attributes and methods of the Person class 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.

code from the chapter

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

what is next?

Would you like to test handling Exceptions in tests?

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