PR1 – Preparation for Week 5 – Lists and tuples

Lists

Lists are arguably Python’s most versatile, useful data types, and you will find them every.

In short, a list is a ordered collection of values, and are defined by a comma-separated sequence of values inside square brackets [], as shown below:

>>> thisIsAList = [1,2,3,"I am also an element", True]

The important characteristics of Python lists are as follows:

• Lists are ordered.
• Lists can contain any arbitrary objects.
• List elements can be accessed by index.
• Lists are mutable.

Lists Are Ordered

A list is not merely a collection of objects. It is an ordered collection of objects. The order in which you specify the elements when you define a list is an innate characteristic of that list and is maintained for that list’s lifetime. (You will see a Python data type that is not ordered in the next tutorial on dictionaries.)

Lists that have the same elements in a different order are not the same:

>>> a = ['foo', 'bar', 'baz', 'qux']
>>> b = ['baz', 'qux', 'bar', 'foo']
>>> a == b
False
>>> a is b
False

>>> [1, 2, 3, 4] == [4, 1, 3, 2]
False

Lists Can Contain Arbitrary Objects

A list can contain any assortment of objects. The elements of a list can all be the same type:>>>

>>> a = [2, 4, 6, 8]
>>> a
[2, 4, 6, 8]

Or the elements can be of varying types:

>>> a = [21.42, 'foobar', 3, 4, 'bark', False, 3.14159]
>>> a
[21.42, 'foobar', 3, 4, 'bark', False, 3.14159]

List objects needn’t be unique. A given object can appear in a list multiple times:

>>> a = ['bark', 'meow', 'woof', 'bark', 'cheep', 'bark']
>>> a
['bark', 'meow', 'woof', 'bark', 'cheep', 'bark']

List Elements Can Be Accessed by Index

Individual elements in a list can be accessed using an index in square brackets. This is exactly analogous to accessing individual characters in a string. List indexing is zero-based as it is with strings.

Consider the following list:

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

The first element will have index 0, and the second has the index 1, and so forth…

Here is Python code to access some elements of a:

>>> a[0]
'foo'
>>> a[2]
'baz'
>>> a[5]
'corge'

Virtually everything about string indexing works similarly for lists. For example, a negative list index counts from the end of the list:

>>> a[-1]
'corge'
>>> a[-2]
'quux'
>>> a[-5]
'bar'

Slicing also works. If a is a list, the expression a[m:n] returns the portion of a from index m to, but not including, index n:

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> a[2:5]
['baz', 'qux', 'quux']

Several Python operators and built-in functions can also be used with lists in ways that are analogous to strings:The in and not in operators:

>>> a
['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> 'qux' in a
True
>>> 'thud' not in a
True

The concatenation (+) and replication (*) operators:

>>> a
['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> a + ['grault', 'garply']
['foo', 'bar', 'baz', 'qux', 'quux', 'corge', 'grault', 'garply']
>>> a * 2
['foo', 'bar', 'baz', 'qux', 'quux', 'corge', 'foo', 'bar', 'baz',
'qux', 'quux', 'corge']

The len(), min(), and max() functions:

>>> a
['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> len(a)
6
>>> min(a)
'bar'
>>> max(a)
'qux'

It’s not an accident that strings and lists behave so similarly. They are both special cases of a more general object type called an iterable, which you will encounter in more detail in the upcoming tutorial on definite iteration.

Lists Are Mutable

Most of the data types you have encountered so far have been atomic types. Integer or float objects, for example, are primitive units that can’t be further broken down. These types are immutable, meaning that they can’t be changed once they have been assigned. It doesn’t make much sense to think of changing the value of an integer. If you want a different integer, you just assign a different one.

By contrast, the string type is a composite type. Strings are reducible to smaller parts—the component characters. It might make sense to think of changing the characters in a string. But you can’t. In Python, strings are also immutable.

The list is the first mutable data type you have encountered. Once a list has been created, elements can be added, deleted, shifted, and moved around at will. Python provides a wide range of ways to modify lists.

Modifying a Single List Value

A single value in a list can be replaced by indexing and simple assignment:

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']
>>> a
['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> a[2] = 10
>>> a[-1] = 20
>>> a
['foo', 'bar', 10, 'qux', 'quux', 20]

A list item can be deleted with the del command:

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> del a[3]
>>> a
['foo', 'bar', 'baz', 'quux', 'corge

Prepending or Appending Items to a List

Additional items can be added to the start or end of a list using the + concatenation operator or the += augmented assignment operator:

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> a += ['grault', 'garply']
>>> a
['foo', 'bar', 'baz', 'qux', 'quux', 'corge', 'grault', 'garply']

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> a = [10, 20] + a
>>> a
[10, 20, 'foo', 'bar', 'baz', 'qux', 'quux', 'corge']

Methods That Modify a List

Finally, Python supplies several built-in methods that can be used to modify lists. Information on these methods is detailed below.

a.append(<obj>)

Appends an object to a list.

a.append(<obj>) appends object <obj> to the end of list a:

>>> a = ['a', 'b']
>>> a.append(123)
>>> a
['a', 'b', 123]

a.remove(<obj>)

Removes an object from a list.

a.remove(<obj>) removes object <obj> from list a. If <obj> isn’t in a, an exception is raised:

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']
>>> a.remove('baz')
>>> a
['foo', 'bar', 'qux', 'quux', 'corge']

>>> a.remove('Bark!')
Traceback (most recent call last):
  File "<pyshell#13>", line 1, in <module>
    a.remove('Bark!')
ValueError: list.remove(x): x not in list

a.pop(index=-1)

Removes an element from a list.

This method differs from .remove() in two ways:

1. You specify the index of the item to remove, rather than the object itself.
2. The method returns a value: the item that was removed.

a.pop() simply removes the last item in the list:>>>

>>> a = ['foo', 'bar', 'baz', 'qux', 'quux', 'corge']

>>> a.pop()
'corge'
>>> a
['foo', 'bar', 'baz', 'qux', 'quux']

>>> a.pop()
'quux'
>>> a
['foo', 'bar', 'baz', 'qux']

Python Tuples

Python provides another type that is an ordered collection of objects, called a tuple.

Pronunciation varies depending on whom you ask. Some pronounce it as though it were spelled “too-ple” (rhyming with “Mott the Hoople”), and others as though it were spelled “tup-ple” (rhyming with “supple”). My inclination is the latter, since it presumably derives from the same origin as “quintuple,” “sextuple,” “octuple,” and so on, and everyone I know pronounces these latter as though they rhymed with “supple.”

Defining and Using Tuples

Tuples are identical to lists in all respects, except for the following properties:

• Tuples are defined by enclosing the elements in parentheses (()) instead of square brackets ([]).
• Tuples are immutable.

Here is a short example showing a tuple definition, indexing, and slicing:

>>> t = ('foo', 'bar', 'baz', 'qux', 'quux', 'corge')
>>> t
('foo', 'bar', 'baz', 'qux', 'quux', 'corge')

>>> t[0]
'foo'
>>> t[-1]
'corge'
>>> t[1::2]
('bar', 'qux', 'corge')

Never fear! Our favorite string and list reversal mechanism works for tuples as well:

>>> t[::-1]
('corge', 'quux', 'qux', 'baz', 'bar', 'foo')

Note: Even though tuples are defined using parentheses, you still index and slice tuples using square brackets, just as for strings and lists.

Everything you’ve learned about lists—they are ordered, they can contain arbitrary objects, they can be indexed and sliced, they can be nested—is true of tuples as well. But they can’t be modified!

>>> t = ('foo', 'bar', 'baz', 'qux', 'quux', 'corge')
>>> t[2] = 'Bark!'
Traceback (most recent call last):
  File "<pyshell#65>", line 1, in <module>
    t[2] = 'Bark!'
TypeError: 'tuple' object does not support item assignment

Why use a tuple instead of a list?

• Program execution is faster when manipulating a tuple than it is for the equivalent list. (This is probably not going to be noticeable when the list or tuple is small.)
• Sometimes you don’t want data to be modified. If the values in the collection are meant to remain constant for the life of the program, using a tuple instead of a list guards against accidental modification.
• There is another Python data type that you will encounter shortly called a dictionary, which requires as one of its components a value that is of an immutable type. A tuple can be used for this purpose, whereas a list can’t be.