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dict

Properties​

>>> dir(dict)
['__class__', '__class_getitem__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__ior__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__or__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__ror__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values']

Definition​

A dictionary is a collection of key : value pairs grouped by curly braces {}. Unlike a list, dictionaries are unordered and values are accessed by their key. Pairs within a dictionary can be modified, added, or removed, and can contain multiple types, including other dictionaries and lists.

Using dictionaries​

Instantiation​

Dictionaries can be instantiated using the literal syntax, with the dictionary constructor, or with keyword arguments:

# literal syntax
my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

# constructor
my_dict = dict([('fruit', 'apple'), ('count', 2), ('is_good', True), ('snack', 'string cheese')])

# keyword arguments
my_dict = dict(fruit='apple', count=2, is_good=True, snack='string cheese')

The most common and straighforward way to do this is with the literal syntax.

Accessing elements​

Elemets are accessed by a value's key using bracket notation.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_dict['count']
2

Variables can also be passed into the bracket notation to obtain a value.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_var = 'snack'

>>> my_dict[my_var]
'string cheese'
note

Dot notation is not supported in Python

Modifying elements​

Modify elements using bracket notation.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_dict['fruit': 'banana']

>>> print(my_dict)
{ 'fruit': 'banana', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

Adding elements​

Add a new element by using bracket notation

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_dict['person'] = 'Wolf'

>>> print(my_dict)
{ 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese', 'person': 'Wolf' }

Removing elements​

Remove an element with the del command.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> del my_dict['count']

>>> print(my_dict)
{ 'fruit': 'apple', 'is_good': True, 'snack': 'string cheese' }

Membership testing​

The membership of a key within a dictionary can be tested with the in keyword. Returns True if the element is in the dictionary and False if the element is not in the dictionary.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> 'is_good' in my_dict
True

>>> 'string cheese' in my_dict
False

Dunder methods​

Dunder MethodOperationExample (normal syntax)Example (dunder call)
__getitem__Access valued["a"] → 1d.__getitem__("a")
__setitem__Set key/valued["b"] = 2d.__setitem__("b", 2)
__delitem__Delete keydel d["a"]d.__delitem__("a")
__contains__Membership"a" in dd.__contains__("a")
__len__Lengthlen(d)d.__len__()
__iter__Iterate keysfor k in dit = d.__iter__(); next(it)
__eq__Equality{"a":1} == {"a":1}{"a":1}.__eq__({"a":1})

Dictionary methods​

clear​

Removes all elements from a dictionary.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_dict.clear()

>>> print(my_dict)
{}

copy​

Returns a copy (or clone) of a dictionary

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_other_dict  = my_dict.copy()

>>> my_other_dict['editor'] = 'vs code'

>>> del my_dict['snack']

>>> print(my_dict)
{'fruit': 'apple', 'count': 2, 'is_good': True}

>>> print(my_other_dict)
{'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese', 'editor': 'vs code'}
note

altering making a copy of a dictionary like this:

my_other = my_dict

will only create another variable that referrs to the same dictionary. In such a scenario altering my_dict would also alter my_other.

fromkeys​

Returns a new dictionary from keys from the given list or tuple, and values from the second parameter. The default value for the second parameter is None.

>>> student_list = ['Aaron', 'Max', 'Ryan', 'Betty']

>>> grades = 75

>>> student_grades = dict.fromkeys(student_scores, grades)

>>> print(student_grades)
{'Aaron': 75, 'Max': 75, 'Ryan': 75, 'Betty': 75}

get​

Returns the value of the specified key.

Is there a difference in python between

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> fav_fruit = my_dict.get('fruit')

>>> print(fav_fruit)
'apple'

Unlike accessing with bracket notation, get does not raise a key error if the key is not found.

>>> my_dict['color']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'color'

>>> my_dict.get('color')

Can also be used with default value if the value is not found.

>>> my_dict.get('color', 'purple')
'purple'

items​

Returns a view object of a list of tuples as the key-value pairs.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_dict.items()
dict_items([('fruit', 'apple'), ('count', 2), ('is_good', True), ('snack', 'string cheese')])

keys​

Returns a view object of a list of the keys of a dictionary.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_dict.keys
<built-in method keys of dict object at 0x1043b8100>

>>> my_dict.keys()
dict_keys(['fruit', 'count', 'is_good', 'snack'])

pop​

Returns and removes the given key-value pair.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> snizzack= my_dict.pop('snack')

>>> print(my_dict)
{'fruit': 'apple', 'count': 2, 'is_good': True}

>>> print(snizzack)
string cheese

popitem​

Removes the last key-value pair in a dictionary and returns it as a tuple.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> snizzack = my_dict.popitem()

>>> print(my_dict)
{'fruit': 'apple', 'count': 2, 'is_good': True}

>>> print(snizzack)
('snack', 'string cheese')

setdefault​

If the key exists in the dictionary, returns the value of the given key (just like get) and does not change the dictionary.

If the key does not exist in the dictionary, returns the default value and inserts the key with the provided default value.

Key exists:​

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> result = my_dict.setdefault('fruit', 'orange')
>>> print(result)
'apple'

>>> print(my_dict)
{'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese'}

Key does not exist:​

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> result = my_dict.setdefault('color', 'red')
>>> print(result)
'red'

>>> print(my_dict)
{'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese', 'color': 'red'}

update​

If a key exists in the dictionary, overwrites the value at that key.

If a key does not exist in the dictionary, creates a new key value pair.

Key exists​

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_dict.update({'fruit': 'banana'})
>>> print(my_dict)
{'fruit': 'banana', 'count': 2, 'is_good': True, 'snack': 'string cheese'}

Key does not exist​

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_dict.update({'color': 'red'})
>>> print(my_dict)
{'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese', 'color': 'red'}

Updating multiple values​

>>> my_dict.update({'fruit': 'grape', 'count': 5})
>>> print(my_dict)
{'fruit': 'grape', 'count': 5, 'is_good': True, 'snack': 'string cheese'}

values​

Returns a view object of a list of the values of a dictionary.

>>> my_dict = { 'fruit': 'apple', 'count': 2, 'is_good': True, 'snack': 'string cheese' }

>>> my_dict.values
<built-in method values of dict object at 0x1043b8100>

>>> my_dict.values()
dict_values(['apple', 2, True, 'string cheese'])