Dictionary Methods and Built-in functions

Python provides many built-in functions and methods for working with dictionaries. Table 10.2 of the textbook collects the commonly used ones; each is explained below with a worked example.

len() — number of items

Returns the length, i.e. the number of key: value pairs of the dictionary passed as argument:

>>> marks = {'Anita': 95, 'Bala': 89, 'Chetan': 92, 'Divya': 85}
>>> len(marks)
4

dict() — build a dictionary from key-value pairs

Given a sequence of key-value pairs (for example a list of two-element tuples), dict() assembles them into a dictionary:

>>> pairs = [('Anita', 95), ('Bala', 89), ('Chetan', 92), ('Divya', 85)]
>>> pairs
[('Anita', 95), ('Bala', 89), ('Chetan', 92), ('Divya', 85)]
>>> marks = dict(pairs)
>>> marks
{'Anita': 95, 'Bala': 89, 'Chetan': 92, 'Divya': 85}

keys() — the keys of the dictionary

>>> marks.keys()
dict_keys(['Anita', 'Bala', 'Chetan', 'Divya'])

values() — the values of the dictionary

>>> marks.values()
dict_values([95, 89, 92, 85])

items() — the (key, value) pairs

Returns the items as a list of (key – value) tuple pairs — the same view the Method-2 traversal loop of Section 10.10 iterates over:

>>> marks.items()
dict_items([('Anita', 95), ('Bala', 89), ('Chetan', 92), ('Divya', 85)])

get() — lookup without KeyError

Returns the value corresponding to the key passed as argument. Crucially, if the key is not present, get() returns None instead of raising the KeyError that square-bracket access would:

>>> marks.get('Divya')
85
>>> marks.get('Farhan')      # key absent — returns None, no error
>>>

update() — merge another dictionary in

update() appends the key-value pairs of the dictionary passed as argument to the key-value pairs of the given dictionary. The argument dictionary itself is left unchanged:

>>> marks = {'Anita': 95, 'Bala': 89, 'Chetan': 92, 'Divya': 85}
>>> more  = {'Farhan': 79, 'Gita': 89}
>>> marks.update(more)
>>> marks
{'Anita': 95, 'Bala': 89, 'Chetan': 92, 'Divya': 85, 'Farhan': 79, 'Gita': 89}
>>> more
{'Farhan': 79, 'Gita': 89}

del — delete an item, or the whole dictionary

del dictionary[key] deletes the single item with the given key. Writing del dictionary_name (no key) removes the entire dictionary from memory — after that, even mentioning its name is a NameError: