Python – Working with Lists

One of the most common data types in Python is the list. A list is basically an array in other languages, however with a list, you can mix different data types in the same list. You can test this in the interactive shell:

>>> a_list = ['dog', 'cat', 1, 3, 1000]
>>> print(a_list)
['dog', 'cat', 1, 3, 1000]
>>> type(a_list[3])
<class 'int'>
>>> type(a_list[1])
<class 'str'>

Working with Python Lists

After declaring a list, we will need to either add data, delete data or assign data to another variable. In order do accomplish these tasks, we will need to use the append() method to add data, pop() or remove() to delete data, and subset the list to retrieve elements or assign to another variable.

Lists are subsetted by using the brackets ([]), a positional number and / or using a colon (:). Using a colon will allow a subset range.

Note: list elements start at 0 not 1.

>>> a_list = ['dog', 'cat', 'bat']
>>> b_str = a_list[0] # Take the first element and assign it to b_str
>>> b_str
'dog'
>>> type(b_str)
<class 'str'>
>>> a_list[-1] # Take the last element from the list 
'bat'
>>> a_list[-2] # Take the second to last element from the list
'cat'
>>> type(a_list[0]) # Lists can contain ints, strings, dictionaries or other lists
<class 'str'>
>>> type(a_list[-1])
<class 'int'>

Using a range with list elements sometimes is prone to defects in code. The number before the colon is the starting point, and the number after is the position to end minus 1. For example, a_list[1:4] starts at element 1 and ends at element 3, not 4. If you’ve developed in other languages, this will take some time to acclimate to Python’s way of list subscripting.

>>> a_list
['cat', 1, 3, 1000]
>>> a_list[0:3] # Take the first element through the second element
['cat', 1, 3]
>>> a_list[1:-1] # Take the second element through the second to last element
'cat', 1, 3]

>>> b_list = ['this', 'new' 'list']
>>> a_list.append(b_list)
>>> a_list
['cat', 1, 3, 1000, ['this', 'newlist']]

>>> my_dct = {'language': 'Python'}
>>> a_list.append(my_dct)
>>> a_list
['cat', 1, 3, 1000, ['this', 'newlist'], {'language': 'Python'}]
>>> a_list[-1].keys() # We can call dictionary methods  
dict_keys(['language'])
>>> a_list[-1].values()
dict_values(['Python'])

Here are some examples of using Python lists by merging two lists into one. This simple example appends the values from second_list onto first_list.

firstList = ['dog', 'cat', 'tiger', 'rhnio']
secondList = ['2 x 18g', '250m swap']

for line in secondList:
   firstList.append(line)

Merge Lists Based on A Condition

The following code merges two lists, but will insert the second list after finding a specific string, in this case a hostname. outerList.pop(0) removes the first element from the list, and then inserts the remaining list into firstList.

firstList = ['dog', 'cat', '2', '500', 'daeo', 'DL580', '8', '128']
secondList = [['dog', '2 x 18g', '250m swap'], ['daeo', '4 x 146g', '16g swap']]

try:
    for outerList in secondList:
        systemName = outerList.pop(0)
        for innerList in outerList:
            indexPos = firstList.index(systemName)
            firstList.insert(indexPos + 1, innerList)
except ValueError as err:
    print('ValueError: {}'.format(err))