空值 None

空值 None，此对象会由没有显式地设置返回值的函数返回。None 是个内置名称，空对象求值，什么也不会发生。

None

help(None)

Help on NoneType object:

class NoneType(object)
 |  Methods defined here:
 |  
 |  __bool__(self, /)
 |      self != 0
 |  
 |  __repr__(self, /)
 |      Return repr(self).
 |  
 |  ----------------------------------------------------------------------
 |  Static methods defined here:
 |  
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.

# 没有返回值的列表方法
a = [].append(1)
a

# 将 a 打印出来，将打印内置名称 None
print(a)

None