【Python】Function ④ (function None return value | None value application scenario | used for if judgment | define variable without initial content | code example )

1. The return value of the function None

---

1. Empty return value

In Python functions, the return return value is optional, and the function can have no return value;

def 函数名(函数参数):
	"""函数文档字符串（可选）"""
    # 函数体
    # return 返回值 (可选)

If the return value is not defined in the Python function, then the special literal value None is returned, and its type is <class 'NoneType'> ;

None means that it has no practical meaning, returns empty, and does not need to process the return value information;

Returning None in Python is equivalent to

• void in Java/C/C++
• Units in Kotlin

etc void return value;

2. Code example - receive None return value

In the following code, the hello function does not use the return keyword to return the return value;

The hello function does not display a defined return value, in fact, the function returns a None return value;

Use a variable to receive the return value of the function, the return value is None, and the return value type is NoneType;

Code example:

"""
接收 函数 None 返回值示例
"""

# 定义无返回值的函数
def hello():
	print("Hello World")

# 获取函数的空返回值
result = hello()

# 打印返回值
print(result)  # 输出 None

# 打印返回值类型
print(type(result))  # 输出 <class 'NoneType'>

Results of the :

Hello World
None
<class 'NoneType'>

3. Code example - use the return keyword to return None

In this example, there are more return None return values ​​than the previous example, and its execution effect is exactly the same as no return value;

Code example:

"""
接收 函数 None 返回值示例
"""

# 定义无返回值的函数
def hello():
	print("Hello World")
	return None

# 获取函数的空返回值
result = hello()

# 打印返回值
print(result)  # 输出 None

# 打印返回值类型
print(type(result))  # 输出 <class 'NoneType'>

Results of the :

Hello World
None
<class 'NoneType'>

2. Application scenario of None value

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1. Brief introduction of None value application scenarios

Application scenario of function None return value:

• Function return value: indicates that the function has no return value;
• For if judgment: None is equivalent to the Boolean value False;
• Define a variable with no initial content: if you do not need a specific value for the variable when defining the variable, you can temporarily assign None to it;

2. Code example - use None for if judgment

Code example:

"""
使用 None 作为 if 判定条件 代码示例
"""

# 定义函数 返回 None 返回值
def is_adult(age):
	if age > 18:
		return "adult"
	else:
		return None

# 接收返回值
result = is_adult(12)

# 使用返回值进行判定
if not result:
	print("未成年")
else:
	print("成年")

Results of the :

未成年

3. Code example - define a variable without initial content

Code example:

"""
使用 None 定义无初始内容变量 代码示例
"""

# 定义无初始内容变量
name = None
print(name)

# 为 name 设置真实值
name = "Tom"
print(name)

Results of the :

None
Tom