Python built-in functions (54) -- reversed, pythonreversed

Python built-in functions (54) -- reversed, pythonreversed

English document:

reversed( Seq)

Return a reverse iterator. SeqMust be an object which has __reversed__()Method or supports the sequence protocol ( __len__()Method and __getitem__()Method with integer arguments starting 0).

Note:

1. The function is to reverse a sequence object and reverse its elements from the back to the front to form a new iterator.

>>> A = reversed (range (10) # input range object >>> a # type becomes iterator <range_iterator object at 0x035634E8 >>> list () [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]> a = ['A', 'B', 'C ', 'D'] >>> a ['A', 'B', 'C', 'D'] >>> reversed () # input list object <list_reverseiterator object at 0x031874D0 >>> B = reversed (a) >>> B # type conversion to iterator <list_reverseiterator object at 0x037C4EB0> list (B) ['D', 'C', 'B', 'a']

 

2. If a parameter is not a sequence object, it must define_ Reversed _ method.

# Type Student does not define _ reversed _ method> class Student: def _ init _ (self, name, * args): self. name = name self. scores = [] for value in args: self. scores. append (value) >>> a = Student ('bob', 78,85, 93,96) >>> reversed (a) # The instance cannot reverse Traceback (most recent call last ): file "<pyshell #37>", line 1, in <module> reversed (a) TypeError: argument to reversed () must be a sequence >>> type (. scores) # list type <class 'LIST'> # redefine the type and define _ reversed _ method for it> class Student: def _ init _ (self, name, * args): self. name = name self. scores = [] for value in args: self. scores. append (value) def _ reversed _ (self): self. scores = reversed (self. scores) >>> a = Student ('bob', 78,85, 93,96) >>>. scores # list type [78, 85, 93, 96] >>> type (. scores) <class 'LIST' >>> reversed (a) # The instance can be reversed >>>. scores # After reversal, the type is changed to the iterator <list_reverseiterator object at 0x0342F3B0> type (. scores) <class 'list _ reverseiterator '> list (. scores) [96, 93, 85, 78]