Bài trước: Python căn bản ():
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x1. Python Classes and Objects
x1.1 Classes and Objects
Python is an object oriented programming language.
Almost everything in Python is an object, with its properties and methods.
A Class is like an object constructor, or a "blueprint" for creating objects.
Create a Class
To create a class, use the keyword class:
Example
Create a class named MyClass, with a property named x:
class MyClass:
x = 5
Create Object
Now we can use the class named MyClass to create objects:
Example
Create an object named p1, and print the value of x:
p1 = MyClass()
print(p1.x)
The __init__() Function
The examples above are classes and objects in their simplest form, and are not really useful in real life applications.
To understand the meaning of classes we have to understand the built-in __init__() function.
All classes have a function called __init__(), which is always executed when the class is being initiated.
Use the __init__() function to assign values to object properties, or other operations that are necessary to do when the object is being created.
Example
Create a class named Person, use the __init__() function to assign values for name and age:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
p1 = Person("John", 36)
print(p1.name)
print(p1.age)
Note: The __init__() function is called automatically every time the class is being used to create a new object.
The __str__() Function
The __str__() function controls what should be returned when the class object is represented as a string.
If the __str__() function is not set, the string representation of the object is returned:
Example
The string representation of an object WITHOUT the __str__() function:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
p1 = Person("John", 36)
print(p1)
Example
The string representation of an object WITH the __str__() function:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def __str__(self):
return f"{self.name}({self.age})"
p1 = Person("John", 36)
print(p1)
Object Methods
Objects can also contain methods. Methods in objects are functions that belong to the object.
Let us create a method in the Person class.
Example
Insert a function that prints a greeting, and execute it on the p1 object:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def myfunc(self):
print("Hello my name is " + self.name)
p1 = Person("John", 36)
p1.myfunc()
The self Parameter
The self parameter is a reference to the current instance of the class, and is used to access variables that belong to the class.
It does not have to be named self, you can call it whatever you like, but it has to be the first parameter of any function in the class.
Example
Use the words mysillyobject and abc instead of self:
class Person:
def __init__(mysillyobject, name, age):
mysillyobject.name = name
mysillyobject.age = age
def myfunc(abc):
print("Hello my name is " + abc.name)
p1 = Person("John", 36)
p1.myfunc()
Modify Object Properties
You can modify properties on objects like this.
Example
Set the age of p1 to 40:
p1.age = 40
Delete Object Properties
You can delete properties on objects by using the del keyword.
Example
Delete the age property from the p1 object:
del p1.age
Delete Objects
You can delete objects by using the del keyword.
Example
Delete the p1 object:
del p1
The pass Statement
class definitions cannot be empty, but if you for some reason have a class definition with no content, put in the pass statement to avoid getting an error.
Example
class Person:
pass
x1.2 Exercise
1. When the class object is represented as a string, there is a function that controls what should be returned, which one?
A. __init__()
B. __str__()
C. __return__()
D. __toString__()
2. What is the correct syntax for deleting an object named person in Python?
A. person.delete()
B. delete person
C. del person
D. del (person)
3. Write a Python program, with following requies:
- Create class named Student, include properties: name, age, GPA
- Write the __init__() function to fill information into the object
- Create 3 objects from Student class: s1, s2, s3
- Write the __str__() to print student's information following
this pattern:
STT Name Age GPA
1 Teo 11 3.0
2 Ti 12 3.5
3 Mui 13 3.3
4. Write a Python program, to manage all students of a class, with following requies:
- Create class named Student, include properties: name, age, GPA
- Write the __init__() function to fill information into the object
- User input the number of student (number > 5)
- Create each object for a student. Use a loop (for or while) to input data from the keyboard for each student.
- Write the __str__() to print student's information following this pattern:
STT Name Age GPA
1 Teo 11 3.0
2 Ti 12 3.5
3 Mui 13 3.3
...
- Create the main() function to test the program
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The answer hints:
1(B), 2(C),
3. Write a Python program, with following requires:
- Create class named Student, include properties: name, age, GPA
- Write the __init__() function to fill information into the object
- Create 3 objects from Student class: s1, s2, s3
- Write the __str__() to print student's information following this pattern:
STT Name Age GPA
1 Teo 11 3.0
2 Ti 12 3.5
3 Mui 13 3.3
[codex1_3.py]
# Create class Student
class Student:
def __init__(self, name, age, GPA):
self.name = name
self.age = age
self.GPA = GPA
def __str__(self):
return f"{self.name}\t{self.age}\t{self.GPA:.1f}"
# Create 3 student objects
s1 = Student("Teo", 11, 3.0)
s2 = Student("Ti", 12, 3.5)
s3 = Student("Mui", 13, 3.3)
# Print the student information
print("STT\tName\tAge\tGPA")
print("1\t" + str(s1))
print("2\t" + str(s2))
print("3\t" + str(s3))
4. Write a Python program, to manage all students of a class, with following requires:
- Create class named Student, include properties: name, age, GPA
- Write the __init__() function to fill information into the object
- User input the number of student (number > 5)
- Create each object for a student. Use a loop (for or while) to input data from the keyboard for each student.
- Write the __str__() to print student's information following this pattern:
STT Name Age GPA
1 Teo 11 3.0
2 Ti 12 3.5
3 Mui 13 3.3
...
- Create the main() function to test the program
[codex1_4.py]
class Student:
def __init__(self, name, age, GPA):
self.name = name
self.age = age
self.GPA = GPA
def __str__(self):
return f"{self.name}\t{self.age}\t{self.GPA:.1f}"
def main():
num_students = int(input("Enter the number of students (more than 5): "))
while num_students <= 5:
print("Number of students must be greater than 5.")
num_students = int(input("Enter the number of students: "))
students = []
for i in range(1, num_students + 1):
name = input(f"Enter name
for student {i}: ")
age = int(input(f"Enter age for
student {i}: "))
GPA = float(input(f"Enter GPA for
student {i}: "))
students.append(Student(name, age, GPA))
print("STT\tName\tAge\tGPA")
for i, student in enumerate(students, 1):
print(f"{i}\t{student}")
if __name__ == "__main__":
Cập nhật: 7/11/2024
Bài sau: Python căn bản (18):
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