Dictionary in Python is an unordered collection of data values that are used to store data values like a map. Unlike other Data Types that hold only single value as an element, the Dictionary holds key-value pair. In Dictionary, the key must be unique and immutable. This means that a Python Tuple can be a key whereas a Python List can not. A Dictionary can be created by placing a sequence of elements within curly {} braces, separated by ‘comma’.
Example:
Python3
Dict = { 1 : 'Geeks' , 2 : 'For' , 3 : 'Geeks' }
print ( "Dictionary:" )
print ( Dict )
print ( Dict [ 1 ])
|
Output:
Dictionary:
{1: 'Geeks', 2: 'For', 3: 'Geeks'}
Geeks
Traceback (most recent call last):
File "/home/1ca83108cc81344dc7137900693ced08.py", line 11, in
print(Dict[4])
KeyError: 4
Sometimes, when the KeyError is raised, it might become a problem. To overcome this Python introduces another dictionary like container known as Defaultdict which is present inside the collections module.
Note: For more information, refer to Python Dictionary.
DefaultDict
Defaultdict is a container like dictionaries present in the module collections. Defaultdict is a sub-class of the dictionary class that returns a dictionary-like object. The functionality of both dictionaries and defaultdict are almost same except for the fact that defaultdict never raises a KeyError. It provides a default value for the key that does not exists.
Syntax: defaultdict(default_factory)
Parameters:
- default_factory: A function returning the default value for the dictionary defined. If this argument is absent then the dictionary raises a KeyError.
Example:
Python3
from collections import defaultdict
def def_value():
return "Not Present"
d = defaultdict(def_value)
d[ "a" ] = 1
d[ "b" ] = 2
print (d[ "a" ])
print (d[ "b" ])
print (d[ "c" ])
|
Output:
1
2
Not Present
Inner Working of defaultdict
Defaultdict adds one writable instance variable and one method in addition to the standard dictionary operations. The instance variable is the default_factory parameter and the method provided is __missing__.
- Default_factory: It is a function returning the default value for the dictionary defined. If this argument is absent then the dictionary raises a KeyError.
Example:
Python3
from collections import defaultdict
d = defaultdict( lambda : "Not Present" )
d[ "a" ] = 1
d[ "b" ] = 2
print (d[ "a" ])
print (d[ "b" ])
print (d[ "c" ])
|
Output:
1
2
Not Present
- __missing__(): This function is used to provide the default value for the dictionary. This function takes default_factory as an argument and if this argument is None, a KeyError is raised otherwise it provides a default value for the given key. This method is basically called by the __getitem__() method of the dict class when the requested key is not found. __getitem__() raises or return the value returned by the __missing__(). method.
Example:
Python3
from collections import defaultdict
d = defaultdict( lambda : "Not Present" )
d[ "a" ] = 1
d[ "b" ] = 2
print (d.__missing__( 'a' ))
print (d.__missing__( 'd' ))
|
Output:
Not Present
Not Present
Using List as default_factory
When the list class is passed as the default_factory argument, then a defaultdict is created with the values that are list.
Example:
Python3
from collections import defaultdict
d = defaultdict( list )
for i in range ( 5 ):
d[i].append(i)
print ( "Dictionary with values as list:" )
print (d)
|
Output:
Dictionary with values as list:
defaultdict(<class 'list'>, {0: [0], 1: [1], 2: [2], 3: [3], 4: [4]})
Using int as default_factory
When the int class is passed as the default_factory argument, then a defaultdict is created with default value as zero.
Example:
Python3
from collections import defaultdict
d = defaultdict( int )
L = [ 1 , 2 , 3 , 4 , 2 , 4 , 1 , 2 ]
for i in L:
d[i] + = 1
print (d)
|
Output:
defaultdict(<class 'int'>, {1: 2, 2: 3, 3: 1, 4: 2})