Class Methods As Alternative Constructors

NITIN GUPTA

 We learned about constructor and its functionality in tutorial #55, where we had to set all the values as parameters to a constructor. Now we will learn how to use a method as a constructor. It has its own advantages. By using a method as a constructor, we would be able to pass the values to it using a string.

Note that we are talking about a class method, not a static method.

The parameters that we have to pass to our constructor would be the class i.e., cls and the string containing the parameters. Moving on towards the working, we have to use a function "split()," that will divide the string into parts. And the parts as results will be stored in a list. We can now pass the parameters to the constructor using the index numbers of the list or by the concept of *args (discussed in tutorial#43 ).

split():

Let us have a brief overview of the split() function. What split() does is, it takes a separator as a parameter. If we do not provide any, then the default separator is any whitespace it encounters. Else we can provide any separator to it such as full stop, hash, dash, colon, etc. After separating the string into parts, the split() function stores it into a list in a sequence. For example:

text = "Python tutorial for absolute beginners."
 t = text.split()
 print(t)

 Here, we are not providing it any separator as a parameter, so it will automatically divide, taking whitespace as a separator. 

The output will be a list, such as:

['Python', 'tutorial', 'for', 'absolute', 'beginners.']

Example of Class methods - alternative constructor:

class Date:
    def __init__(self, year, month, day):
        self.year = year
        self.month = month
        self.day = day

@classmethod 
    def from_dash(cls,string):
          return cls(*string.split("-"))

date1=Date.from_dash("2008-12-5")
print(date1.year)
#Output: 2008

If we want multiple and independent "constructors", we can use class methods. They are usually called factory methods. It does not invoke the default constructor __init__.In the above example, we split the string based on the "-" operator. We first create a class method as a constructor that takes the string and split it based on the specified operator. For this purpose, we use a split() function, which takes the separator as a parameter. This alternative constructor approach is useful when we have to deal with files containing string data separated by a separator.

Code as described/written in the video


class Employee:
    no_of_leaves = 8

    def __init__(self, aname, asalary, arole):
        self.name = aname
        self.salary = asalary
        self.role = arole

    def printdetails(self):
        return f"The Name is {self.name}. Salary is {self.salary} and role is {self.role}"

    @classmethod
    def change_leaves(cls, newleaves):
        cls.no_of_leaves = newleaves

    @classmethod
    def from_dash(cls, string):
        # params = string.split("-")
        # print(params)
        # return cls(params[0], params[1], params[2])
        return cls(*string.split("-"))


harry = Employee("Harry", 255, "Instructor")
rohan = Employee("Rohan", 455, "Student")
karan = Employee.from_dash("Karan-480-Student")

print(karan.no_of_leaves)
# rohan.change_leaves(34)
#
# print(harry.no_of_leaves)

Comments

Post a Comment

Popular posts from this blog

HOW IMPORT WORKS IN PYTHON?

NITIN GUPTA
  How does the import keyword work? When we write a certain module name along with the  import  keyword, it will start searching for a file with that name having an extension  .py . After finding the file, it will import it into our program, which means that it will permit our program to use the functions of the certain module we imported. We can import a module named  “sys”  to see the path that our import statement takes while searching for a module. import sys print ( sys . path ) Copy sys.path  prints out a list of directories. When we tell Python to import something, then it looks in each of the listed locations in order.  A common mistake that most of the beginners do and also the primary reason for making this tutorial is, why can’t we name our file, the same as the name of a module. The reason is associated with the path. When we give our file a name same as the name of a module, then instead of importing the original module, the system ...
Read more

Exercise 5: Health Management System

NITIN GUPTA
  Exercise#5 The task is to create a " Health Management System ." Suppose you are a fitness trainer and nutritionist. You have to deal with three clients, i.e., (Harry, Rohan, Hammad). For each client, you have to design their exercise and diet plan. Instructions: Create a food log file for each client Create an exercise log file for each client. Ask the user whether they want to log or retrieve client data. Write a function that takes the user input of the client's name. After the client's name is entered, it will display a message as "What you want to log- Diet or Exercise". Use function def getdate ( ) : import datetime return datetime . datetime . now ( ) Copy The purpose of this function is to give time with every record of food or exercise added in the file. Write a function to retrieve exercise or food file records for any client. You are advised to participate in solving this problem. This task helps you to become a good pr...
Read more

*args and **kwargs In Python

NITIN GUPTA
  Before discussing *args and **kwargs, we should have a basic knowledge about types of arguments. In Python programming, there are two types of arguments that can be passed in a function. positional arguments keyword arguments Positional arguments  are the one in which an order has to be followed while passing arguments. We have been dealing with them until now. We know how normal functions work with positional arguments, so now we will directly start exploring keyword arguments. But to understand them, we have to know about asterisk or more commonly known in Perl 6 and ruby as a splat operator.  Asterisk is used in python as a mathematical symbol for multiplication, but in case of arguments, it refers to unpacking. The unpacking could be for a list, tuple, or a dictionary. We will discover more about it by defining *args and **kwargs. *args: args is a short form used for arguments. It is used to unpack an argument. In the case of *args, the argument could be a list or t...
Read more