Python dictionary comprehension

Dictionary comprehension is an elegant and concise way to create dictionaries from another dictionary. Or you can say transforming or creating one dictionary into another dictionary based on conditions.

A dictionary comprehension takes the form.

{key: value for (key, value) in iterable}

{key_expression: value_expression for item in iterable}

Here, key_expression and value_expression are expressions that define the key and value of each key-value pair, and iterable is the sequence of elements over which you want to iterate.

Python dictionary comprehension

A simple example code creates the dictionary with number-square key/value pair in the program using dictionary comprehension.

square_dict = {num: num*num for num in range(1, 11)}

print(square_dict)

Output:

Make dict using two lists named keys and value.

keys = ['a','b','c','d','e']
values = [1,2,3,4,5]  
  
# but this line shows dict comprehension here  
myDict = { k:v for (k,v) in zip(keys, values)}  
  
# We can use below too
# myDict = dict(zip(keys, values))  
  
print (myDict)

Output: {‘a’: 1, ‘b’: 2, ‘c’: 3, ‘d’: 4, ‘e’: 5}

You can also make a dictionary from a list using comprehension.

myDict = {x: x**2 for x in [1,2,3,4,5]}
print (myDict)

If Conditional Dictionary Comprehension

This example below maps the numbers to their cubes that are not divisible by 4:

newdict = {x: x**3 for x in range(10) if x**3 % 4 == 0}
print(newdict)

Output: {0: 0, 8: 512, 2: 8, 4: 64, 6: 216}

if-else Conditional Dictionary Comprehension

original_dict = {'jack': 38, 'michael': 48, 'guido': 57, 'john': 33}

res = {k: ('old' if v > 40 else 'young')
    for (k, v) in original_dict.items()}
print(res)

Do comment if you have any doubts or suggestions on this Python comprehension topic.

Note: IDE: PyCharm 2021.3.3 (Community Edition)

Windows 10

Python 3.10.1

All Python Examples are in Python 3, so Maybe its different from python 2 or upgraded versions.