Creating Custom Python Exception Types

Our custom errors don't need to have any implementation, so we're going to create them at the top of our employee.py file to inherit from Exception with pass as the class body.

~/employee.py

class MissingEmployeeError(Exception):
    pass

class DatabaseError(Exception):
    pass

# Rest of file unchanged and omitted

Now we'll need to use these exception types in the proper areas of the Employee class.

Our DatabaseError occurs whenever we can't open and read the database. This can happen in two situations:

• A FileNotFoundError because the file doesn't exist yet
• A PermissionError because we're not allowed to access the file

Everywhere we use open in the Employee class, we need to catch these exceptions and instead raise a DatabaseError.

~/employee.py

# custom errors omitted

class Employee:
    default_db_file = "employee_file.txt"

    @classmethod
    def get_all(cls, file_name=None):
        results = []

        if not file_name:
            file_name = cls.default_db_file

        try:
            with open(file_name, "r") as f:
                lines = [
                    line.strip("\n").split(",") + [index + 1]
                    for index, line in enumerate(f.readlines())
                ]
        except (FileNotFoundError, PermissionError) as err:
            raise DatabaseError(str(err))

        for line in lines:
            results.append(cls(*line))

        return results

    @classmethod
    def get_at_line(cls, line_number, file_name=None):
        if not file_name:
            file_name = cls.default_db_file

        try:
            with open(file_name, "r") as f:
                line = [
                    line.strip("\n").split(",") + [index + 1]
                    for index, line in enumerate(f.readlines())
                ][line_number - 1]
                return cls(*line)
        except (FileNotFoundError, PermissionError) as err:
            raise DatabaseError(str(err))

    def __init__(self, name, email_address, title, phone_number=None, identifier=None):
        self.name = name
        self.email_address = email_address
        self.title = title
        self.phone_number = phone_number
        self.identifier = identifier

    def email_signature(self, include_phone=False):
        signature = f"{self.name} - {self.title}\n{self.email_address}"
        if include_phone and self.phone_number:
            signature += f" ({self.phone_number})"
        return signature

    def save(self, file_name=None):
        if not file_name:
            file_name = self.default_db_file

        try:
            with open(file_name, "r+") as f:
                lines = f.readlines()
                if self.identifier:
                    lines[self.identifier - 1] = self._database_line()
                else:
                    lines.append(self._database_line())
                f.seek(0)
                f.writelines(lines)
        except (FileNotFoundError, PermissionError) as err:
            raise DatabaseError(str(err))

    def _database_line(self):
        return (
            ",".join(
                [self.name, self.email_address, self.title, self.phone_number or ""]
            )
            + "\n"
        )

Now all the areas where our class interacts with the database file will raise the proper error if accessing the file doesn't go according to plan.

Within get_at_line and save, we need to handle the potential IndexError that will be raised if we try to access a line that doesn't exist. This indicates the employee we were searching for wasn't found, so we want to raise the MissingEmployeeError.

~/employee.py

# custom errors omitted

class Employee:
    default_db_file = "employee_file.txt"

    @classmethod
    def get_all(cls, file_name=None):
        results = []

        if not file_name:
            file_name = cls.default_db_file

        try:
            with open(file_name, "r") as f:
                lines = [
                    line.strip("\n").split(",") + [index + 1]
                    for index, line in enumerate(f.readlines())
                ]
        except (FileNotFoundError, PermissionError) as err:
            raise DatabaseError(str(err))

        for line in lines:
            results.append(cls(*line))

        return results

    @classmethod
    def get_at_line(cls, line_number, file_name=None):
        if not file_name:
            file_name = cls.default_db_file

        try:
            with open(file_name, "r") as f:
                line = [
                    line.strip("\n").split(",") + [index + 1]
                    for index, line in enumerate(f.readlines())
                ][line_number - 1]
                return cls(*line)
        except (FileNotFoundError, PermissionError) as err:
            raise DatabaseError(str(err))
        except IndexError:
            raise MissingEmployeeError(f"no employee at line {line_number}")

    def __init__(self, name, email_address, title, phone_number=None, identifier=None):
        self.name = name
        self.email_address = email_address
        self.title = title
        self.phone_number = phone_number
        self.identifier = identifier

    def email_signature(self, include_phone=False):
        signature = f"{self.name} - {self.title}\n{self.email_address}"
        if include_phone and self.phone_number:
            signature += f" ({self.phone_number})"
        return signature

    def save(self, file_name=None):
        if not file_name:
            file_name = self.default_db_file

        try:
            with open(file_name, "r+") as f:
                lines = f.readlines()
                if self.identifier:
                    lines[self.identifier - 1] = self._database_line()
                else:
                    lines.append(self._database_line())
                f.seek(0)
                f.writelines(lines)
        except (FileNotFoundError, PermissionError) as err:
            raise DatabaseError(str(err))
        except IndexError:
            raise MissingEmployeeError(f"no employee at line {self.identifier}")

    def _database_line(self):
        return (
            ",".join(
                [self.name, self.email_address, self.title, self.phone_number or ""]
            )
            + "\n"
        )

Now if we run the test_custom_exceptions.py script, we shouldn't see any issues:

python3.7 test_custom_exceptions.py