为什么在反转python中的字符串时为什么要使用join（）函数

Question

what is the purpose of ''.join() here

print(''.join(reversed(a)))

can't we reverse a string only with reversed function

print(reversed("python"))

Answer 1

No, you can't do it simply by calling reversed . reversed returns an special reversed iterator object, something that needs to be iterated through in order to produce it's elements.

Not supplying it to .join here will just print out the confusing repr of the reversed object: <reversed object at 0x7f2ba02be320> .

Calling "".join , which takes an iterable as an argument, consumes the reversed object and joins the elements producing the string you see.

Take note that there's other ways to consume it without join that result in a different object, list , for example, will take the reversed object and create a list out of the elements it produces:

a = reversed('abcd')
print(list(a))
['d', 'c', 'b', 'a']

If you need a string in the end, then the best option is .join .

Answer 2

You can manipulate str same as list :

a = 'abcd'
a[::-1] #'dcba'

Answer 3

The purpose of join is to take an iterable and join it by a delimiter to produce a string:

''.join(reversed(a))

a can be an iterable that supports the iteration protocol or an object that overloads __getitem__ . That is, any object that you would normally want to use in a for loop or comprehension or you would like to iterate on to produce its result and make them as a string, that object could possibly be a potential value for join .

reversed returns an object that you need to iterate on in order to fetch its values in join . so ''.join(reversed(a)) means the following: reverse a first and then iterate on the resulting object of reversed and join each extracted object by delimiter specified by join , in your case it's just an empty string.

can't we reverse a string only with reversed function?

Not really! You can reverse a string using this expression instead of reversed function: 'stackoverflow'[::-1] . As usual, there could be several ways to achieve the same effects in programming.