Python - List Comprehension

List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list.

Example:

Based on a list of fruits, you want a new list, containing only the fruits with the letter "a" in the name.

Without list comprehension you will have to write a for statement with a conditional test inside:

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = []

for x in fruits:

  if "a" in x:

    newlist.append(x)

print(newlist)

With list comprehension you can do all that with only one line of code:

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = [x for x in fruits if "a" in x]

print(newlist)

Syntax

newlist = [expression for item in iterable if condition == True]

The return value is a new list, leaving the old list unchanged.

Condition

The condition is like a filter that only accepts the items that evaluate to True.

fruits = ["apple", "banana", "cherry", "kiwi", "mango"]

newlist = [x for x in fruits if x != "apple"]

# will result in a new list of:

["banana", "cherry", "kiwi", "mango"]

The condition if x != "apple"  will return True for all elements other than "apple", making the new list contain all fruits except "apple".

The condition is optional and can be omitted to simply make a copy of the list:

newlist = [x for x in fruits]

Python Dictionary Comprehension

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}

Python Dictionary Comprehension Example

Here we have two lists named keys and value and we are iterating over them with the help of zip() function.

# Python code to demonstrate dictionary comprehension

# Lists to represent keys and values

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)}  

print (myDict)

# output is

{'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5}

#We can use below too

# Lists to represent keys and values

keys = ['a','b','c','d','e']

values = [1,2,3,4,5] 

myDict = dict(zip(keys, values))  

print (myDict)

# output is

{'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5}

Using fromkeys() method

Here we are using the fromkeys() method that returns a dictionary with specific keys and values.

dic=dict.fromkeys(range(5), True)

print(dic)

# output is

{0: True, 1: True, 2: True, 3: True, 4: True}

Using dictionary comprehension make dictionary

Example 1:

# Python code to demonstrate dictionary creation using list comprehension

my_dict = {x: x**2 for x in [1,2,3,4,5]}

print (my_dict)

# output is

{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Example 2:

str_dict = {x.upper(): x*3 for x in 'coding '}

print (str_dict)

# output is

{'O': 'ooo', 'N': 'nnn', 'I': 'iii', 'C': 'ccc', 'D': 'ddd', 'G': 'ggg'}

Using conditional statements in dictionary comprehension

Example 1:

We can use Dictionary comprehensions with if and else statements and with other expressions too. This example below maps the numbers to their cubes that are divisible by 4.

# Python code to demonstrate dictionary comprehension using if.

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

print(new_dict)

# output is

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