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Python | Prefix key match in dictionary

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  • Last Updated : 30 Jan, 2023
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Sometimes, while working with dictionaries, we can have a problem in which we need to find the dictionary items that have some constraints on keys. One such constraint can be a prefix match on keys. Let’s discuss certain ways in which this task can be performed. 

Method #1: Using dictionary comprehension + startswith() 

The combination of above two methods can be used to perform this particular task. In this, dictionary comprehension does the basic task of dictionary construction and startswith() performs the utility task of checking keys starting with specific prefix. 

Python3




# Python3 code to demonstrate working of
# Prefix key match in dictionary
# Using dictionary comprehension + startswith()
 
# Initialize dictionary
test_dict = {'tough': 1, 'to': 2, 'do': 3, 'todays': 4, 'work': 5}
 
# printing original dictionary
print("The original dictionary : " + str(test_dict))
 
# Initialize prefix
test_pref = 'to'
 
# Using dictionary comprehension + startswith()
# Prefix key match in dictionary
res = {key: val for key, val in test_dict.items()
       if key.startswith(test_pref)}
 
# printing result
print("Filtered dictionary keys are : " + str(res))


Output

The original dictionary : {'tough': 1, 'to': 2, 'do': 3, 'todays': 4, 'work': 5}
Filtered dictionary keys are : {'tough': 1, 'to': 2, 'todays': 4}

Time complexity: O(n), where n is the number of keys in the dictionary.
Auxiliary Space: O(m), where m is the number of keys in the filtered dictionary

Method #2 : Using map() + filter() + items() + startswith() This particular task can also be performed using the combination of above functions. The map function ties the filter logic of startswith() to each dictionary’s items extracted by items() 

Python3




# Python3 code to demonstrate working of
# Prefix key match in dictionary
# Using map() + filter() + items() + startswith()
 
# Initialize dictionary
test_dict = {'tough': 1, 'to': 2, 'do': 3, 'todays': 4, 'work': 5}
 
# printing original dictionary
print("The original dictionary : " + str(test_dict))
 
# Initialize prefix
test_pref = 'to'
 
# Using map() + filter() + items() + startswith()
# Prefix key match in dictionary
res = dict(filter(lambda item: item[0].startswith(test_pref),
                  test_dict.items()))
 
# printing result
print("Filtered dictionary keys are : " + str(res))


Output

The original dictionary : {'tough': 1, 'to': 2, 'do': 3, 'todays': 4, 'work': 5}
Filtered dictionary keys are : {'tough': 1, 'to': 2, 'todays': 4}

Method #3: Using find() method

Python3




# Python3 code to demonstrate working of
# Prefix key match in dictionary
 
# Initialize dictionary
test_dict = {'tough' : 1, 'to' : 2, 'do' : 3, 'todays' : 4, 'work' : 5}
 
# printing original dictionary
print("The original dictionary : " + str(test_dict))
 
# Initialize prefix
test_pref = 'to'
 
# Using dictionary comprehension + find()
# Prefix key match in dictionary
res = {key:val for key, val in test_dict.items()
                if key.find(test_pref)==0}
 
# printing result
print("Filtered dictionary keys are : " + str(res))


Output

The original dictionary : {'tough': 1, 'to': 2, 'do': 3, 'todays': 4, 'work': 5}
Filtered dictionary keys are : {'tough': 1, 'to': 2, 'todays': 4}

use re.match():

Python3




# Python3 code to demonstrate working of
# Prefix key match in dictionary
# Using re.match()
 
# Importing re module
import re
 
# Initialize dictionary
test_dict = {'tough': 1, 'to': 2, 'do': 3, 'todays': 4, 'work': 5}
 
# printing original dictionary
print("The original dictionary : " + str(test_dict))
 
# Initialize prefix
test_pref = 'to'
 
# Using re.match()
# Prefix key match in dictionary
res = {key: val for key, val in test_dict.items()
        if re.match(test_pref, key)}
 
# printing result
print("Filtered dictionary keys are : " + str(res))
#This code is contributed by Edula Vinay Kumar Reddy


Output

The original dictionary : {'tough': 1, 'to': 2, 'do': 3, 'todays': 4, 'work': 5}
Filtered dictionary keys are : {'tough': 1, 'to': 2, 'todays': 4}

Time Complexity: O(n)
Auxiliary Space: O(n)

Method #5 : Using slicing

Python3




# Python3 code to demonstrate working of
# Prefix key match in dictionary
 
# Initialize dictionary
test_dict = {'tough': 1, 'to': 2, 'do': 3, 'todays': 4, 'work': 5}
 
# printing original dictionary
print("The original dictionary : " + str(test_dict))
 
# Initialize prefix
test_pref = 'to'
 
res=dict()
# Prefix key match in dictionary
for i in list(test_dict.keys()):
    if(i[:len(test_pref)]==test_pref):
        res[i]=test_dict[i]
# printing result
print("Filtered dictionary keys are : " + str(res))


Output

The original dictionary : {'tough': 1, 'to': 2, 'do': 3, 'todays': 4, 'work': 5}
Filtered dictionary keys are : {'tough': 1, 'to': 2, 'todays': 4}

Time Complexity: O(n)
Auxiliary Space: O(n)


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