Python List and Dictionary Comprehension, Exercises of Web Programming and Technologies

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Python List

and

Dictionary

Comprehension

List Comprehension

❑ Python's List Comprehension is a quick and compact syntax

to generate a list from a string or any other list.

❑ Creating a new list by performing an operation on each item

in the existing list is a very straightforward way.

❑ Comprehension of the list is considerably faster than using

the for loop to process a list.

Example: Separate Letters from String

chars=[] for ch in ‘GKTCS INNOVATIONS’ : chars.append(ch) print(chars)

Example to uses a list comprehension to build a list of squares of the numbers between 1 and 10.

One of the applications of list comprehension is to flatten a list comprising of multiple lists into a single list

Applications

Nested if conditions with list

comprehension.

We can use nested if conditions with list comprehension.

What is Dictionary Comprehension in

Python?

❑ Dictionary comprehension is an elegant and concise way to

create dictionaries.

❑ Syntax for dictionary comprehension

dictionary = {key: value for vars in iterable}

❑ Like List Comprehension, Python allows dictionary comprehensions. ❑ We can create dictionaries using simple expressions. ❑ A dictionary comprehension takes the form {key: value for (key, value) in iterable}

Dictionary comprehensions

We can use Dictionary comprehensions with if and else statements and with other expressions too.

How to use Dictionary Comprehension..?

old_price = {'milk': 1.02, 'coffee': 2.5, 'bread': 2.5} dollar_to_pound = 0. new_price = {item: value*dollar_to_pound for (item, value) in old_price.items()} print(new_price)

Output :

{'milk': 0.7752, 'coffee': 1.9, 'bread': 1.9}

Multiple if Conditional Dictionary

Comprehension

original_dict = {'jack': 38 , 'michael': 48 , ‘ guido': 57 , 'john': 33 } new_dict = {k: v for (k, v) in original_dict.items() if v % 2 != 0 if v < 40} print(new_dict)

Output :

{'john': 33}

If-else Conditional Dictionary

Comprehension

original_dict = {'jack': 38, 'michael': 48, 'guido': 57, 'john': 33} new_dict_1 = {k: ('old' if v > 40 else 'young') for (k, v) in original_dict.items()} print(new_dict_1)

Output :

{'jack': 'young', 'michael': 'old', 'guido ’: 'old', 'john': 'young'}

Warnings on Using Dictionary

Comprehension

Even though dictionary comprehensions are great for writing

elegant code that is easy to read, they are not always the right

choice.

We must be careful while using them as :

❑ They can sometimes make the code run slower and consume

more memory.

❑ They can also decrease the readability of the code.