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:
# Python program to demonstrate # dictionary Dict = { 1 : 'Geeks' , 2 : 'For' , 3 : 'Geeks' } print ( "Dictionary:" ) print ( Dict ) print ( Dict [ 1 ]) # Uncommenting this print(Dict[4]) # will raise a KeyError as the # 4 is not present in the dictionary |
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 dict
class that returns a dictionary-like object. The functionality of both dictionaries and defualtdict are almost same except for the fact that defualtdict 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:
# Python program to demonstrate # defaultdict from collections import defaultdict # Function to return a default # values for keys that is not # present def def_value(): return "Not Present" # Defining the dict 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:
# Python program to demonstrate
# default_factory argument of
# defaultdict
from
collections
import
defaultdict
# Defining the dict and passing
# lambda as default_factory argument
d
=
defaultdict(
lambda
:
"Not Present"
)
d[
"a"
]
=
1
d[
"b"
]
=
2
print
(d[
"a"
])
print
(d[
"b"
])
print
(d[
"c"
])
chevron_rightfilter_noneOutput:
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
, aKeyError
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:
# Python program to demonstrate
# defaultdict
from
collections
import
defaultdict
# Defining the dict
d
=
defaultdict(
lambda
:
"Not Present"
)
d[
"a"
]
=
1
d[
"b"
]
=
2
# Provides the default value
# for the key
print
(d.__missing__(
'a'
))
print
(d.__missing__(
'd'
))
chevron_rightfilter_noneOutput:
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:
# Python program to demonstrate # defaultdict from collections import defaultdict # Defining a dict 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:
# Python program to demonstrate # defaultdict from collections import defaultdict # Defining the dict d = defaultdict( int ) L = [ 1 , 2 , 3 , 4 , 2 , 4 , 1 , 2 ] # Iterate through the list # for keeping the count for i in L: # The default value is 0 # so there is no need to # enter the key first d[i] + = 1 print (d) |
Output:
defaultdict(<class 'int'>, {1: 2, 2: 3, 3: 1, 4: 2})
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