Containers¶

Containers are a data type that contains other objects.

Lists¶

Python's basic container type is the list

We can define our own list with square brackets:

In [1]:

[1, 3, 7]

Out[1]:

[1, 3, 7]

In [2]:

type([1, 3, 7])

Out[2]:

list

Lists do not have to contain just one type:

In [3]:

various_things = [1, 2, "banana", 3.4, [1, 2]]

We access an element of a list with an int in square brackets:

In [4]:

one = 1
two = 2
three = 3

In [5]:

my_new_list = [one, two, three]

In [6]:

middle_value_in_list = my_new_list[1]

In [7]:

middle_value_in_list

Out[7]:

In [8]:

[1, 2, 3][1]

Out[8]:

In [9]:

various_things[2]

Out[9]:

'banana'

In [10]:

index = 2
various_things[index]

Out[10]:

'banana'

Note that list indices start from zero.

We can quickly make a list containing a range of consecutive integer numbers using the built-in range function

In [11]:

count_to_five = list(range(5))
print(count_to_five)

[0, 1, 2, 3, 4]

We can use a string to join together a list of strings:

In [12]:

name = ["Grace", "Brewster", "Murray", "Hopper"]
print(" ".join(name))

Grace Brewster Murray Hopper

And we can split up a string into a list:

In [13]:

"Ernst Stavro Blofeld".split(" ")

Out[13]:

['Ernst', 'Stavro', 'Blofeld']

We can an item to a list:

In [14]:

name.append("BA")
print(" ".join(name))

Grace Brewster Murray Hopper BA

Or we can add more than one:

In [15]:

name.extend(["MS", "PhD"])
print(" ".join(name))

Grace Brewster Murray Hopper BA MS PhD

Or insert values at different points in the list

In [16]:

name.insert(0, "Admiral")
print(" ".join(name))

Admiral Grace Brewster Murray Hopper BA MS PhD

Sequences¶

Many other things can be treated like lists. Python calls things that can be treated like lists sequences.

A string is one such sequence type

In [17]:

print(count_to_five[1])
print("James"[2])

1
m

In [18]:

print(count_to_five[1:3])
print("Hello World"[4:8])

[1, 2]
o Wo

In [19]:

print(len(various_things))
print(len("Python"))

5
6

In [20]:

len([[1, 2], 4])

Out[20]:

Unpacking¶

Multiple values can be unpacked when assigning from sequences, like dealing out decks of cards.

In [21]:

mylist = ["Goodbye", "Cruel"]
a, b = mylist
print(a)

Goodbye

In [22]:

a = mylist[0]
b = mylist[1]

Checking for containment¶

The list we saw is a container type: its purpose is to hold other objects. We can ask python whether or not a container contains a particular item:

In [23]:

"Dog" in ["Cat", "Dog", "Horse"]

Out[23]:

True

In [24]:

"Bird" in ["Cat", "Dog", "Horse"]

Out[24]:

False

In [25]:

2 in range(5)

Out[25]:

True

In [26]:

99 in range(5)

Out[26]:

False

In [27]:

"a" in "cat"

Out[27]:

True

Mutability¶

An list can be modified:

In [28]:

name = "Grace Brewster Murray Hopper".split(" ")
print(name)

['Grace', 'Brewster', 'Murray', 'Hopper']

In [29]:

name[0:3] = ["Admiral"]
name.append("PhD")

print(" ".join(name))

Admiral Hopper PhD

Tuples¶

A tuple is an immutable sequence:

In [30]:

my_tuple = ("Hello", "World")

In [31]:

my_tuple

Out[31]:

('Hello', 'World')

In [32]:

my_tuple[0] = "Goodbye"

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In [32], line 1
----> 1 my_tuple[0] = "Goodbye"

TypeError: 'tuple' object does not support item assignment

str is immutable too:

In [33]:

fish = "Hake"
fish[0] = "R"

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In [33], line 2
      1 fish = "Hake"
----> 2 fish[0] = "R"

TypeError: 'str' object does not support item assignment

But note that container reassignment is moving a label, not changing an element:

In [34]:

fish = "Rake"  ## OK!

Supplementary material: Try the online memory visualiser for this one.

Memory and containers¶

The way memory works with containers can be important:

In [35]:

x = list(range(3))
print(x)

[0, 1, 2]

In [36]:

y = x
print(y)

[0, 1, 2]

In [37]:

z = x[0:3]
y[1] = "Gotcha!"
print(x)
print(y)
print(z)

[0, 'Gotcha!', 2]
[0, 'Gotcha!', 2]
[0, 1, 2]

In [38]:

z[2] = "Really?"
print(x)
print(y)
print(z)

[0, 'Gotcha!', 2]
[0, 'Gotcha!', 2]
[0, 1, 'Really?']

Supplementary material: This one works well at the memory visualiser.

In [39]:

x = ["What's", "Going", "On?"]
y = x
z = x[0:3]

y[1] = "Gotcha!"
z[2] = "Really?"

In [40]:

Out[40]:

["What's", 'Gotcha!', 'On?']

The explanation: While y is a second label on the same object, z is a separate object with the same data.

Nested objects make it even more complicated:

In [41]:

x = [["a", "b"], "c"]
y = x
z = x[0:2]

x[0][1] = "d"
z[1] = "e"

In [42]:

Out[42]:

[['a', 'd'], 'c']

In [43]:

Out[43]:

[['a', 'd'], 'c']

In [44]:

Out[44]:

[['a', 'd'], 'e']

Try the visualiser again.

Identity versus equality¶

Having the same data is different from being the same actual object in memory:

In [45]:

print([1, 2] == [1, 2])
print([1, 2] is [1, 2])

True
False

The == operator checks, element by element, that two containers have the same data. The is operator checks that they are actually the same object.

In [46]:

my3numbers = list(range(3))
print(my3numbers)

[0, 1, 2]

In [47]:

[0, 1, 2] == my3numbers

Out[47]:

True

In [48]:

[0, 1, 2] is my3numbers

Out[48]:

False

But, and this point is really subtle, for immutables, the Python language might save memory by reusing a single instantiated copy. This will always be safe.

In [49]:

word = "Hello"
print("Hello" == word)
print("Hello" is word)

True
True

<>:3: SyntaxWarning: "is" with a literal. Did you mean "=="?
<>:3: SyntaxWarning: "is" with a literal. Did you mean "=="?
/tmp/ipykernel_6296/1808182476.py:3: SyntaxWarning: "is" with a literal. Did you mean "=="?
  print("Hello" is word)