[Tutor] user created lists

[Peter Otten]

Christian Witts wrote:

> On 2012/04/12 06:42 AM, john moore wrote:
>> Hello Pyhton World,
>>
>> I'm new at this and was wondering how I create a number of user specified
>> lists?
>>
>> Example:
>>
>> "How many list would you like to create?"
>> User inputs 5
>> creates five lists,
>> list1 []
>> list2 []
>> list3 []
>> list4 []
>> list5 []
>>
>> I can create one with append, but I don't know how to loop it to create
>> five different named list..

> You can use vars() to create the variables on the fly. vars() is just a
> dictionary containing the variable name as the key, and the data as the
> value so you can do `vars()['list1'] = []` and it's easy enough to
> create them en masse
> 
> # Set the start to 1, and add 1 to what the user inputted
> # as range/xrange doesn't include the top number
> for i in xrange(1, user_input + 1):
>      vars()['list%s' % i] = []

This will stop working once you move your code into a function:

>>> def f():
...     vars()["list1"] = ["a", "b"]
...     print list1
... 
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in f
NameError: global name 'list1' is not defined

I recommend that you bite the bullet and use a dedicated dictionary or list 
to hold your five lists from the very begining:

>>> num_lists = int(raw_input("How many lists? "))
How many lists? 5
>>> list_of_lists = [[] for i in range(num_lists)]

You then have to access the first list as "list_of_list[0]" instead of 
"list1" and so on:

>>> list_of_lists[0].append(1)
>>> list_of_lists[4].append(2)
>>> list_of_lists
[[1], [], [], [], [2]]