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Dictionary Comprehension Method update Method values Method keys Method fromkeys Method pop Method setdefault Method items Method popitem Method get Method copy Method clear

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Python | Dictionary Comprehension

schedule Aug 12, 2023

Last updated

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Python

Syntax

Basic


        
        
            
                
                
                    {key expression: value expression for variable in iterable}

Conditional


        
        
            
                
                
                    {output expression + conditional on output for variable in iterable + conditional on iterable}

Examples

Basic

To create a dictionary with keys 1 ~ 3, and values as 2*key:


        
        
            
                
                
                    doubles = {i: 2*i for i in range(1, 4)}
print(doubles)
                
            
            {1: 2, 2: 4, 3: 6}

For every element i in the range(1,4), we are adding a key-value pair i: 2*i to the doubles dictionary.

To create a new dictionary d_new by doubling the values of an existing dictionary d_old:


        
        
            
                
                
                    d_old = {'a': 1, 'b': 2, 'c': 3}
d_new = {key:value*2 for (key,value) in d_old.items()}
print(d_new)
                
            
            {'a': 2, 'b': 4, 'c': 6}

Conditional

Input condition

To specify a condition on the input:


        
        
            
                
                
                    d_old = {'a': 1, 'b': 2, 'c': 3}
d_new = {key:value*2 for (key,value) in d_old.items() if key != 'c'}
print(d_new)
                
            
            {'a': 2, 'b': 4}

Here, we only loop through (key,value) in the for loop where key does not equal 'c'. Hence, we do not see any key-value pair in the output with key 'c'.

To specify multiple conditions on the input:


        
        
            
                
                
                    d_old = {'a': 1, 'b': 2, 'c': 3}
d_new = {key:value*2 for (key,value) in d_old.items() if key != 'c' if value % 2 == 0}
                
            
            {'b': 4}

Here we specify two conditions for the input iterable d_old:

1) key must not equal 'c'

2) value must be divisible by 2

Output condition

To specify a condition on the output:


        
        
            
                
                
                    d_old = {'a': 1, 'b': 2, 'c': 3}
d_new = {key:(value*2 if key!='c' else value*3) for (key,value) in d_old.items()}
print(d_new)
                
            
            {'a': 2, 'b': 4, 'c': 9}

Here, we add key:value*2 to d_new if the key does not equal 'c', otherwise we add key:value*3. Notice how only the value of key 'c' has been multiplied by 3 while the value of keys 'a' and 'b' have only been multiplied by 2.

Published by Arthur Yanagisawa

Edited by 0 others

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