Dictionaries

4. Data Structures: Dictionaries

Dictionaries

The last, but not least complex datatype Python provides is the #\mathtt{\color{#4271ae}{\text{dict}}}#, short for dictionary. A dictionary consists of a collection of key-value pairs. Like lists, dictionaries are mutable. However, unlike lists, elements are accessed via their respective keys, not via indexing. A #\mathtt{\color{#4271ae}{\text{dict}}}# can be defined using curly brackets and colons, or through the use of the #\mathtt{\color{#4271ae}{\text{dict}}\text{()}}# function.

Declaring a dict

#\mathtt{\{}\textit{key}_\textit{0}\mathtt{:}\textit{value}_\textit{0}, \textit{key}_\textit{1}\mathtt{:}\textit{value}_\textit{1},\dots, \textit{key}_\textit{n}\mathtt{:}\textit{value}_\textit{n}\mathtt{\}}#

#\color{#4271ae} {\mathtt{dict}}\mathtt{(}\textit{key}_\textit{0}\mathtt{=}\textit{value}_\textit{0}, \textit{key}_\textit{1}\mathtt{=}\textit{value}_\textit{1},\dots, \textit{key}_\textit{n}\mathtt{=}\textit{value}_\textit{n}\mathtt{)}#

#\color{#4271ae} {\mathtt{dict}}\mathtt{(}\mathtt{\text{[(}}\textit{key}_\textit{0},\textit{value}_\textit{0}\mathtt{\text{)}},\mathtt{\text{(}}\textit{key}_\textit{1},\textit{value}_\textit{1}\mathtt{\text{)}},\dots,\mathtt{\text{(}}\textit{key}_\textit{n},\textit{value}_\textit{n}\mathtt{\text{)]}}\mathtt{)}#

Where key must be an immutable object and value can be any valid Python object, including mutables. A dictionary has no particular order, eventhough its contents are displayed in the order they were defined when printed.

Like the other objects discussed, dictionaries also have their common syntax and specific methods. First, we will take a look at how to access and add elements.

Accessing and Adding

The syntax for accessing dictionary elements is quite similar to that of lists, the same notation, where the identifier is followed by square brackets, is used. However, instead of using the position of an element, its key is referenced between the brackets.

>>> d = {"one": 1, "two": 2, "three": 3}
>>> d["two"]

The same syntax can be used to either assign a new value to the key or when a new unique key is specified, to add a new key-value pair to the dictionary.

>>> d["two"] = 5
>>> d

{'one': 1, 'two': 5, 'three': 3}

>>> d["four"] = 4
>>> d

{'one': 1, 'two': 5, 'three': 3, 'four': 4}

To delete an item from the dictionary, you can use the #\mathtt{\color{#8959A8}{\text{del}}}# keyword in combination with the same syntax.

>>> del d["one"]
>>> d

{'two': 5, 'three': 3, 'four': 4}

Next, we will take a look at the methods provided by the #\mathtt{\color{#4271ae}{\text{dict}}}# type.

Dict methods

In addition to the syntax provided for accesing elements, you can also access elements using the safer #\mathtt{\color{#4271ae}{\text{get}}\text{()}}# method. #\mathtt{\color{#4271ae}{\text{get}}\text{(}\textit{key}\text{)}}# searches the dictionary for the key and returns the associated value if it is found, it returns #\mathtt{\color{#F5871F}{\text{None}}}# otherwise.

>>> d = {"a": 1, "b": 2, "c": 3, "d": 4}
>>> d.get("a")

>>> d.get("e")

None

The #\mathtt{\color{#4271ae}{\text{get}}\text{()}}# method is considered safer than directly indexing a dictionary because directly indexing a dictionary with a key that is not actually present in said dictionary results in an error.

>>> d["e"]

KeyError: 'e'

Another method that can be used to obtain elements is the #\mathtt{\color{#4271ae}{\text{pop}}\text{()}}# method. #\mathtt{\color{#4271ae}{\text{pop}}\text{(}\textit{key}\text{)}}# returns the value corresponding to key if the key is present in the dictionary. If succesful, #\mathtt{\color{#4271ae}{\text{pop}}\text{()}}# also removes the key-value pair.

>>> d.pop("d")

>>> d

{'a': 1, 'b': 2, 'c': 3}

The final method that can be used to obtain a single item is the #\mathtt{\color{#4271ae}{\text{popitem}}\text{()}}# method. #\mathtt{\color{#4271ae}{\text{popitem}}\text{()}}# returns the last key-value pair added to the dictionary as a #\mathtt{\color{#4271ae}{\text{tuple}}}#.

>>> d.popitem()

('c', 3)

Multiple values

Naturally, a dictionary also supports methods that allow you to work with multiple values. One of these is the #\mathtt{\color{#4271ae}{\text{clear}}\text{()}}# method, which deletes all key-value pairs contained by the dictionary.

>>> d.clear()
>>> d

{}

The other is the #\mathtt{\color{#4271ae}{\text{update}}\text{()}}# method, which allows you to add or overwrite multiple key-value pairs at once. The #\mathtt{\color{#4271ae}{\text{update}}\text{()}}# method can be used in three ways; with a #\mathtt{\color{#4271ae}{\text{dict}}}# as argument, with a #\mathtt{\color{#4271ae}{\text{list}}}# of tuples as argument, or with keyword arguments.

>>> d.update({"a": 1, "b": 2})
>>> d

{'a': 1, 'b': 2}

>>> d.update([("c", 3)])
>>> d

{'a': 1, 'b': 2, 'c': 3}

>>> d.update(d=4)
>>> d

{'a': 1, 'b': 2, 'c': 3, 'd': 4}

Obtaining contents

A dictionary also provides methods to access some of its contents more directly. The #\mathtt{\color{#4271ae}{\text{keys}}\text{()}}# method returns a #\mathtt{\color{#4271ae}{\text{list}}}# of all keys used in the dictionary.

>>> d.keys()

dict_keys(['a', 'b', 'c', 'd'])

The #\mathtt{\color{#4271ae}{\text{values}}\text{()}}# method returns a list of all values used in the dictionary. Duplicate values are included.

>>> d.values()

dict_values([1, 2, 3, 4])

The #\mathtt{\color{#4271ae}{\text{items}}\text{()}}# method returns a list of tuples containing all key-value pairs contained by the dictionary.

>>> d.items()

dict_items([('a', 1), ('b', 2), ('c', 3), ('d', 4)])

As you can see, the outputs of these methods are not exactly lists. They can be iterated over, but if you want to use other #\mathtt{\color{#4271ae}{\text{list}}}# methods, you would have to explicitly cast the output to a #\mathtt{\color{#4271ae}{\text{list}}}# first.