Rotating items of list to left

[SaiKrishnaG]

BPO	22124
Nosy	@zware
Files	rotate_left3.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2014-08-02.21:34:29.858>
created_at = <Date 2014-08-02.20:35:33.904>
labels = ['type-bug', 'invalid', 'OS-windows']
title = 'Rotating items of list to left'
updated_at = <Date 2014-08-02.21:34:29.856>
user = 'https://bugs.python.org/SaiKrishnaG'

bugs.python.org fields:

activity = <Date 2014-08-02.21:34:29.856>
actor = 'zach.ware'
assignee = 'none'
closed = True
closed_date = <Date 2014-08-02.21:34:29.858>
closer = 'zach.ware'
components = ['Windows']
creation = <Date 2014-08-02.20:35:33.904>
creator = 'Sai.Krishna.G'
dependencies = []
files = ['36215']
hgrepos = []
issue_num = 22124
keywords = []
message_count = 2.0
messages = ['224586', '224590']
nosy_count = 2.0
nosy_names = ['zach.ware', 'Sai.Krishna.G']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue22124'
versions = ['Python 2.7']

[SaiKrishnaG]

Hi, I am trying to rotate list of 3 to left
for this I am trying to define this function
def rotate_left3(nums)

#argument nums is list
for example
rotate_left3([1, 2, 3])
I am expecting value [2,3,1] in return. I have written the following code
a = nums
a[0]=nums[1]
a[1]=nums[2]
a[2]=nums[0]
return a #this is returning [2,3,2] instead of [2,3,1]
however if I assign
a = [0,0,0] #or any other value other than directly assigning nums
the code works perfectly

[zware]

This is not a bug. The assignment "a = nums" doesn't create a copy of "nums", it just assigns the name "a" to the same object that "nums" refers to. Since lists are mutable, changes made to "a" are visible through the name "nums". By the time you do "a[2] = nums[0]", "nums[0]" has been reassigned.

Have a look at this article: http://nedbatchelder.com/text/names.html

Also, you may want to look at collections.deque and its rotate method.