https://www.programiz.com/python-programming/methods/built-in/filter
The filter() method constructs an iterator from elements of an iterable for which a function returns true.
filter()方法语法：
filter(function, iterable)
参数：
function - function that tests if elements of an iterable returns true or false. If None, the function defaults to Identity function - which returns false if any elements are false.
iterable - iterable which is to be filtered, could be sets, lists, tuples, or containers of any iterators.
返回值：
The filter() method returns an iterator that passed the function check for each element in the iterable.
filter()方法等价于：
# when function is defined
(element for element in iterable if function(element))
# when function is None
(element for element in iterable if element)
