Python property() Function

Python3 内置函数Python3 Built-in Functions

property()It is a built-in function in Python used to create properties.

property is a powerful mechanism that allows us to execute code when accessing attributes. It is often used to implement advanced features such as computed properties and data validation.

Word Definitions： property 意为"属性"，用于定义类的属性。

Basic Syntax and Parameters

Syntax Format

property(fget=None, fset=None, fdel=None, doc=None)

Parameter Description

  • Parameter fget(Optional):
    • Type: Function
    • Description: The function called when getting the property value.
  • Parameter fset(Optional):
    • Type: Function
    • Description: The function called when setting the property value.
  • Parameter fdel(Optional):
    • Type: Function
    • Description: The function called when deleting the property.
  • Parameter doc(Optional):
    • Type: String
    • Description: The docstring of the property.

Function Description

  • Return Value: Returns a property object.
  • Features: Makes property access look like a normal variable, but actually executes a function.

Examples

Example 1: Basic Usage

Example

class Circle:
    def __init__(self, radius):
        self.radius = radius

    # Using property to create computed properties
    def get_area(self):
        """Calculate the area of a circle."""
        return 3.14159 * self.radius ** 2

    def set_radius(self, value):
        """Set the radius (validation can be added)."""
        if value < 0:
            raise ValueError(Radius cannot be negative)
        self.radius = value

    # Create property
    area = property(get_area, set_radius)

# Usage
c = Circle(5)
print(fRadius: {c.radius})  # Output: Radius: 5
print(fArea: {c.area})     # Output: Area: 78.53975

# Set property
c.radius = 10
print(fNew area: {c.area})   # Output: New area: 314.159

# Verify
c.radius = -5  # Will raise an exception

Expected output:

半径: 5
面积: 78.53975
新面积: 314.159

Code explanation:

  1. With property, you can define computed properties.
  2. The setter can add data validation logic.

Example 2: Using @property Decorator (Recommended)

Example

class Student:
    def __init__(self, name, score):
        self.name = name
        self._score = score

    @property
    def score(self):
        """Get score"""
        return self._score

    @score.setter
    def score(self, value):
        """Set score (with validation)"""
        if not 0 <= value <= 100:
            raise ValueError(Score must be between 0 and 100)
        self._score = value

    @property
    def grade(self):
        """Calculate grade based on score"""
        if self._score >= 90:
            return 'A'
        elif self._score >= 80:
            return 'B'
        elif self._score >= 70:
            return 'C'
        elif self._score >= 60:
            return 'D'
        else:
            return 'F'

# Usage
s = Student("Tom", 85)
print(s.name)    # Output: Tom
print(s.score)    # Output: 85
print(s.grade)    # Output: B

s.score = 92     # Set score
print(s.grade)    # Output: A

s.score = 105    # Will raise an exception

Expected output:

Tom
85
B
A

Code explanation:

  • The @property decorator is a more Pythonic way of writing it.
  • You can define getter, setter, and deleter.

Example 3: Read-Only Property

Example

class Person:
    def __init__(self, name):
        self._name = name

    @property
    def name(self):
        """Read-only property"""
        return self._name

    # Without a setter, the property cannot be modified

p = Person("Tom")
print(p.name)  # Output: Tom

# Trying to modify it will raise an error (no setter)
# p.name = "Jack" # will raise AttributeError

# Deleting it will also raise an error
# del p.name # will raise AttributeError

Expected output:

Tom

By defining only a getter and no setter, you can create a read-only property.

Python3 内置函数Python3 Built-in Functions

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