[Tutor] list to string and string to list

[Martin Walsh]

Martin Walsh wrote:
> johnf wrote:
>> On Thursday 16 April 2009 05:04:39 pm Alan Gauld wrote:
>>> "johnf" <jfabiani at yolo.com> wrote
>>>
>>>>> I want to save the list to the field and when I retrieve the string
>>>>> convert
>>>>> it back to a list.
>>>>>
>>>>> But this does NOT work.
>>>>> mylist=[1,2,3,4]
>>>>> mystr=str(mylist)
>>>>>
>>>>> newlist= list(mystr)
>>>>>
>>>>> I keep thinking there must be a simple way of get this done.
>>>> Is this a good way?
>>>> newlist = eval(mystr)
>>> eval has all sorts of security implications so I wouldn't recommend
>>> it where you are reading data fropm an external source.
>>>
>>> One thing that might work is this:
>>>>>> L = [1,2,3,4,5]
>>>>>> s1 = ','.join(str(n) for n in L)
>>>>>> s1
>>> '1,2,3,4,5'
>>>
>>>>>> newlist = [int(n) for n in s1.split(',')]
>>>>>> newlist
>>> [1, 2, 3, 4, 5]
>>>
>>> Provided your original data doesn't have commas to start with
>>> it should work, I think... And the data needs to be pretty
>>> homogenous to allow a single conversion function.
>> Kent Johnson suggested 
>>
>> newlist = map(int, mystr[1:-1].split(','))
>>
> 
> Maybe that's a question in disguise, but I would think both suggestions
> are valid.
> 
> Another, less valid, suggestion would be to pickle or shelve the list
> before storing it in the database -- although this poses similar
> security implications as eval. And the resulting string is not
> particularly easy to read, if that's important to you.
> 
> import cPickle as pickle
> mylist = [1,2,3,4]
> 
> mystr = pickle.dumps(mylist)
> # '(lp1\nI1\naI2\naI3\naI4\na.'
> 
> newlist = pickle.loads(mystr)
> # [1, 2, 3, 4]

Sorry to reply to my own post, but the json module in python 2.6+
(formerly 3rd party, simplejson) might work for your purposes also. But
I must confess, I'm not that familiar.

import json

s = json.dumps([1, 2, 3, 4])
# '[1, 2, 3, 4]'
l = json.loads(s)
# [1, 2, 3, 4]

> 
> HTH,
> Marty