Working With Lists

Posted: July 26, 2007
Author: Scott Newman
Category: Python

Combining Lists Together

I have a function that returned a list of category IDs, and I wanted to add them to a running list of overall category IDs. When I tried to append() the output of that function, I got a list of lists.

If you want to join list items to an existing list, don't try to use append(). If you do, you'll get that list added as a single item of the first list. Example:

mylist = [1,2,3]
mylist
>> [1,2,3]
mylist.append([4,5,6])
mylist
>> [1,2,3,[4,5,6]]
len(mylist)
4

In this example, if you want a list of six items, use extend() instead of append():

mylist = [1,2,3]
mylist
>> [1,2,3]
mylist.extend([4,5,6])
mylist
>> [1,2,3,4,5,6]
len(mylist)
6

Sorting Lists

Here's something to remember about sorting lists: the sort() method does not return a value like many Python methods do. Why is that important? Well, if you are trying to keep your code neat and concise, you could run into this:

>> mylist = [10,50,30]
>> print mylist.sort()
None

You need to call the sort() method by itself:

>> mylist = [10,50,30]
>> mylist.sort()
>> print mylist
[10,30,50]

Filtering Out Unique Elements of a List

Python has a cool method called set() that can filter a list and return only unique elements. I use it all the time to remove duplicate category names from lists. One thing to remember is that set() returns a datatype of 'set', so you must use the list() method to make it a list again:

>> mylist = ['Dog', 'Cat','Hamster','Dog']
>> set(mylist) # Not what we want!
set(['Hamster', 'Dog', 'Cat'])
>> list(set(mylist))
['Hamster', 'Dog', 'Cat']

Further Reading

http://diveintopython.org/native_data_types/lists.html#d0e5887