An easy way to create Python classes with dataclass

Using the Decorator Pattern

No. 19

Whenever you create a new class, you have to go through the “boilerplate” of setting up the __init__ method and copying some values into the instance variables. For example, in this simple Employee class, you generally would write:

class Employee:
    def __init__(self, frname:str, lname:str, idnum:int):
         self.frname = frname
         self.lname = lname
         self.idnum = idnum

where you declare the arguments to the init method and then copy them into variables for that instance.  Well, if this happens pretty much every time you create a class, why not automate it?

This is what the dataclass decorator does for you. If you use this decorator, your code is reduced to:

@dataclass
class Employee:
     frname: str
     lname: str
     idnum: int

And the init method and the copying is filled in for you. You also need to import the library that contains this function:

from dataclasses import dataclass

but only once per module.

So, when you go to create an instance of Employee, you proceed as usual:

emp = Employee('Sarah', 'Smythe', 123)
print(emp.nameString())

and the arguments are in the same order as in the variable list. In fact, IDEs like PyCharm recognize this decorator (which is just a function call under the covers) and pop up the variable list:

Default values in argument lists

In the same way, you can create a class where some of arguments have default values. In Python itself, you could create a class where most of your employees live in, say, Stamford, CT.

class Employee2:
    def __init__(self, frname: str, lname: str, idnum: int,
                 town='Stamford', state = 'CT', zip='06820'):
        self.frname = frname
        self.lname = lname
        self.idnum = idnum
        self.town = town
        self.state = state
        self.zip = zip

For most people, you can create their class instance and leave out those last three values, since they already have default values:

bill = Employee2('Bill', 'Bogus', 456) 
print(bill.nameString(), bill.town, bill.zip)

But, if you have an outlier employee who lives somewhere else, you can fill in the rest of the arguments:

sandy= Employee2('Sandy', 'Zwyzzo', 123, 'Bridgeport') print(sandy.nameString(), sandy.town, sandy.zip)

Note that Sandy’s zipcode will be wrong, though. You can also pick out specific values to change by name, like the zip code in this example:

robert = Employee2('bob', 'simpson', 345, zip= '06840') print(robert.nameString(), robert.zip)

Using dataclass with default values

Of course, the dataclass decorator handles default values in the same wayl:

@dataclass
class Employee2:
    frname: str
    lname: str
    idnum: int
    town: str = "Stamford"
    state: str = 'CT'
    zip: str = '06820'

And all of the above class instances will work just fine.  Doesn’t that make class creation a lot easier?

You can download all code from GitHub under jameswcooper/newsletter.

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