Creating and Using Python Classes

Before we write any code, let's see what we need to do to get the using_classes.py file to execute to the next step by looking at the error:

$ python3.7 using_classes.py
Traceback (most recent call last):
  File "using_classes.py", line 1, in <module>
    from employee import Employee
ModuleNotFoundError: No module named 'employee'

This error shows us the first thing we need to do is create the module and the Employee class within it. The error doesn't tell us anything else, though, so we'll take the smallest step possible to move to the next step. Let's create an empty class now:

~/employee.py

class Employee:
    pass

Running using_classes.py again, we'll see a new error:

$ python3.7 using_classes.py
Traceback (most recent call last):
  File "using_classes.py", line 7, in <module>
    phone_number="555-867-5309",
TypeError: Employee() takes no arguments

Our next error is related to not having an __init__ method that takes arguments. To know what we need to implement, let's look at how the Employee instances are being created in using_classes.py:

~/using_classes.py

from employee import Employee

employee_1 = Employee(
    name="Kevin Bacon",
    title="Executive Producer",
    email_address="[email protected]",
    phone_number="555-867-5309",
)
employee_2 = Employee("Bruce Wayne", "[email protected]", "CEO")

# Rest of code omitted

We can see here the positional order for our arguments is given from the employee_2 line, and the name of the attributes are provided by the keyword argument usage when employee_1 is instantiated. The phone_number attribute is optional since it isn't used to create employee_2. Let's take this knowledge and implement the __init__ method now:

~/employee.py

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

Now we shouldn't have any issues creating our instances in using_classes.py:

$ python3.7 using_classes.py
Traceback (most recent call last):
  File "using_classes.py", line 12, in <module>
    employee_1.email_signature(include_phone=True)
AttributeError: 'Employee' object has no attribute 'email_signature'

The last few expressions in the using_classes.py file demonstrate how the email_signature method should work. The two main things to note are:

• By default, the phone_number attribute is not included in the string that is returned; but if include_phone is true, then it is added to the end of the second line.
• If there is no phone_number, then the phone number portion will not be printed, even if include_phone is true.

Let's implement this method now:

~/employee.py

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

    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

By checking if include_phone and self.phone_number are both true, we're able to determine if we should add the phone number to the signature. Let's run using_classes.py one more time to ensure everything works. We should see no output if we've implemented the method correctly.

python3.7 using_classes.py