Python List Operations: Concatenation, Multiplication, Slicing & del
Lists and strings have a lot in common. They are both sequences and, like pythons, they get longer as you feed them. Like a string, we can concatenate and multiply a Python list.
Python List Concatenation & Multiplication
Old MacDonald had a farm, E-I-E-I-O. And on this farm there was a python, E-I-E-I-O. At the prompt, create a Python list with an item, ‘farm’:
>>> t = [‘farm’]
>>> t += ‘EIEIO’
['farm', 'E', 'I', 'E', 'I', 'O']
That’s not what we wanted. Our string, EIEIO, was split into individual items and then added to our list. Why? Because strings are iterable. So how do you add strings to a Python list? With brackets.
>>>t += [‘EIEIO’]
['farm', 'E', 'I', 'E', 'I', 'O', ‘EIEIO’]
Try adding a number to a list…
>>>t += 128
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
How do we add integers to lists? Just like we did with strings.
>>>t += 
Lastly, what do you think will happen when you run this?
>>>t += ‘128’
We can also perform multiplication on the entire list or individual items in the list by index.
And by index:
Eeee! How exciting!
You win! Time for dessert.
Slicing a Python List
Care for a bite of hedgehog? A hedgehog slice is a cake made of layers of chocolate and biscuit. It’s delicious. A group of hedgehogs is called an array. Let’s create a list called ‘array’ and populate it with hedgehogs.
array = [‘Plato’, ‘Lucretius’, ‘Dante’, ‘Hegel’, ‘Dostoevsky’, ‘Nietzsche’, ‘Ibsen’, ‘Proust’, ‘Braudel’]
Using an index we can easily see what the first item is:
But what if we want everything but the first item? We slice it, just like a cake.
['Lucretius', 'Dante', 'Hegel', 'Dostoevsky', 'Nietzsche', 'Ibsen', 'Proust', 'Braudel']
And if we only want the middle?
>>> array [1:-1]
['Lucretius', 'Dante', 'Hegel', 'Dostoevsky', 'Nietzsche', 'Ibsen', 'Proust']
Deleting Items from a Python List
Because our array is populated with prickly and disagreeable hedgehogs, their spines are irritating one another. So we need to clear out a few items.
Use del to remove one item, or a slice of items
>>> del array
['Plato', 'Lucretius', 'Hegel', 'Dostoevsky', 'Nietzsche', 'Ibsen', 'Proust', 'Braudel']
>>> del array[2:4]
['Plato', 'Lucretius', 'Nietzsche', 'Ibsen', 'Proust', 'Braudel']
Meanwhile, Back at the Ranch…
This little piggy learned Python list operations,
This little piggy stayed home,
This little piggy ate a hedgehog slice,
And this little piggy had none.