What's “better” the reverse method or the reversed built-in function?

问题

What is typically regarded as more Pythonic/better/faster to use, the reverse method or the reversed built-in function?

Both in action:

_list = list(xrange(4))

print _list

rlist = list(reversed(_list))

print rlist

_list.reverse()

print _list

回答1:

Depends on whether you want to reverse the list in-place (i.e. change the list) or not. No other real difference.

_{Often using reversed leads to nicer code.}

回答2:

foo.reverse() actually reverses the elements in the container. reversed() doesn't actually reverse anything, it merely returns an object that can be used to iterate over the container's elements in reverse order. If that's what you need, it's often faster than actually reversing the elements.

回答3:

There seems to be a great difference. I really thought it's the other way round. Why is rearranging the values in a list faster than creating a new one from an iterator ?

from decorators import bench

_list = range(10 ** 6)

@ bench
def foo():
  list(reversed(_list))

@ bench
def bar():
  _list.reverse()

foo()
bar()

print foo.time
print bar.time

0.167278051376
0.0122621059418

回答4:

Without knowing real stats about performance, _list.reverse() modifies the list itself, whereas reversed(_list) returns an iterator ready to traverse the list in reversed order. That's a big difference itself.

If that's not a problem, object.reverse() seems more readable to me, but maybe you have specific speed requirements. And if reverse() does not belong to 80% of software that's consuming resources, I wouldn't bother (as a general rule of thumb).

回答5:

It is always better to use reversed() if eventually you are going to modify the list in iterator making list immutable and working with immutable data is always better especially when your doing functional programming.

回答6:

_list.reverse() does an in-place reversal and does not return a value
reversed(_list) does not change _list, but returns a reverse iterable object
_list[::-1] does not change _list, but returns reversed slice

example:

_list = [1,2,3]
ret1 = list(reversed(_list))
ret2 = _list[::-1] #reverse order slice
ret3 = _list.reverse() #no value set in ret3
print('ret1,ret2,ret3,_list:',ret1,ret2,ret3,_list)

_list = [1,2,3]
for x in reversed(_list):
    print(x)

output:

ret1,ret2,ret3,_list: [3, 2, 1] [3, 2, 1] None [3, 2, 1]
3
2
1

来源：https://stackoverflow.com/questions/6810036/whats-better-the-reverse-method-or-the-reversed-built-in-function

标签

python

list

methods

reverse

built-in