空值 None
空值 None，就是空对象的值，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