5. Python Dictionary:
Python dictionary is an unordered collection of items. Python's
dictionaries
are kind of hash table type.
While other compound data types have only value as an element, a dictionary has a key : value pair. Dictionaries are optimized to retrieve values when the key is known.
A
dictionary key can be almost any Python type, but are usually
numbers or strings. Values, on the other hand, can be any
arbitrary Python object.
Dictionaries
are enclosed by curly braces ({ }) and values can be assigned and accessed
using square braces ([]).
For Example:# empty dictionary
my_dict = {}
# dictionary with integer keys
my_dict = {1: 'apple', 2: 'ball'}
# dictionary with mixed keys
my_dict = {'name': 'John', 1: [2, 4, 3]}
# using dict()
my_dict = dict({1:'apple', 2:'ball'})
# from sequence having each item as a pair
my_dict = dict([(1,'apple'), (2,'ball')])Accessing elements from a dictionary:
While indexing is used with other container types to access values, dictionary uses keys. Key can be used either inside square brackets or with the
get() method.
The difference while using
get() is that it returns None instead of KeyError, if the key is not found.
For Example:
my_dict = {'name':'Jack', 'age': 26}
print(my_dict['name']) # Output: Jack
print(my_dict.get('age')) # Output: 26
# Trying to access keys which doesn't exist throws error
# my_dict.get('address')
my_dict['address'] # Error
Change or Add elements in a dictionary:
Dictionary are mutable. We can add new items or change the value of existing items using assignment operator(=).
If the key is already present, value gets updated, else a new key: value pair is added to the dictionary.
For Example:
my_dict = {'name':'Jack', 'age': 26}
# update value
my_dict['age'] = 27
print(my_dict) #Output: {'age': 27, 'name': 'Jack'}
# add item
my_dict['address'] = 'Downtown'
print(my_dict) # Output: {'address': 'Downtown', 'age': 27, 'name': 'Jack'}
Delete or Remove elements from a dictionary:
We can remove a particular item in a dictionary by using the method
pop(). This method removes as item with the provided key and returns the value.
The method,
popitem() can be used to remove and return an arbitrary item (key, value) form the dictionary. All the items can be removed at once using the clear() method.
We can also use the
del keyword to remove individual items or the entire dictionary itself.
For Example:
# create a dictionary
squares = {1:1, 2:4, 3:9, 4:16, 5:25}
# remove a particular item
print(squares.pop(4)) # Output: 16
print(squares) # Output: {1: 1, 2: 4, 3: 9, 5: 25}
# remove an arbitrary item
print(squares.popitem()) # Output: (1, 1)
print(squares) # Output: {2: 4, 3: 9, 5: 25}
# delete a particular item
del squares[5]
print(squares) # Output: {2: 4, 3: 9}
# remove all items
squares.clear()
print(squares) # Output: {}
# delete the dictionary itself
del squares
print(squares) # Throws Error
Python Dictionary Methods:
| Method | Description |
|---|---|
| clear() | Remove all items form the dictionary. |
| copy() | Return a shallow copy of the dictionary. |
| fromkeys(seq[, v]) | Return a new dictionary with keys from seq and value equal to v(defaults to None). |
| get(key[,d]) | Return the value of key. If key doesnot exit, return d (defaults to None). |
| items() | Return a new view of the dictionary's items (key, value). |
| keys() | Return a new view of the dictionary's keys. |
| pop(key[,d]) | Remove the item with key and return its value or d if key is not found. If d is not provided and key is not found, raises KeyError. |
| popitem() | Remove and return an arbitary item (key, value). Raises KeyError if the dictionary is empty. |
| setdefault(key[,d]) | If key is in the dictionary, return its value. If not, insert key with a value of d and return d (defaults to None). |
| update([other]) | Update the dictionary with the key/value pairs from other, overwriting existing keys. |
| values() | Return a new view of the dictionary's values |
For Example:
marks = {}.fromkeys(['Math','English','Science'], 0)
print(marks) # Output: {'English': 0, 'Math': 0, 'Science': 0}
for item in marks.items():
print(item)
list(sorted(marks.keys())) # Output: ['English', 'Math', 'Science']
squares = {x: x*x for x in range(6)}
print(squares) # Output: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
odd_squares = {x: x*x for x in range(11) if x%2 == 1}
print(odd_squares) # Output: {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}
Built-in Functions with Dictionary:
| Function | Description |
|---|---|
| all() | Return True if all keys of the dictionary are true (or if the dictionary is empty). |
| any() | Return True if any key of the dictionary is true. If the dictionary is empty, return False. |
| len() | Return the length (the number of items) in the dictionary. |
| cmp() | Compares items of two dictionaries. |
| sorted() | Return a new sorted list of keys in the dictionary. |
For Example:
squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}
print(len(squares)) # Output: 5
print(sorted(squares)) # Output: [1, 3, 5, 7, 9]
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