Python ternary operator in list comprehension

15 Most Powerful Python One-liners You Can’t Skip

One-liner in Python refers to a short code snippet that achieves some powerful operations. It’s popular and widely used in Python community as it makes the code more concise and easier to understand. In this article, I will be sharing some most commonly used Python one-liners that would definitely speed up your coding without compromising any clarity.

Let’s start from the basis.

Ternary operations

Ternary operation allows you to evaluate a value based on the condition being true or false. Instead of writing a few lines of if/else statements, you can simply do it with one line of code:

x = 1 y = 2 result = 1 if x > 0 and y > x else -1 print(result) # 1 #re-assign x to 6 if it is evaluated as False x = x or 6

Assign values for multiple variables

You can assign values for multiple variables simultaneously as per below. (You may want to check this article to understand what is going on under the hood)

key, value = "user", "password" print(key, value) #('user', 'password')

Swap variables

To swap the values of the variables, simply perform the below without having a temp variable which is usually required by other programming languages like Java or C.

key, value = value, key print(key, value) #('password', 'user')

Swap elements in a list

Imagine you have a list of users as per below, and you would like to swap the first element with last element:

users = ["admin", "anonymous1", "anonymous2"]

Since list is mutable, you can re-assign the values of first and last elements by swapping their sequence as per below:

users[0], users[2] = users[2], users[0] # or users[0], users[-1] = users[-1], users[0] print(users) #['anonymous2', 'anonymous1', 'admin']

Further more, if the requirement is to swap the elements at the odd and even positions in a list, e.g. in the below list:

numbers = list(range(10)) #[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

We would like to swap the elements of the 2nd and 1st, 4th and 3rd, and so on. It can be achieved by performing the below list slicing with assignment operation:

numbers[::2], numbers[1::2] = numbers[1::2], numbers[0::2] print(numbers) #[1, 0, 3, 2, 5, 4, 7, 6, 9, 8]

Replace elements in a list

To further expend the above example, if we want to replace the elements on every odd/even position in a list, for instance to 0, we can do the re-assignment with below:

numbers[1::2] = [0]*len(numbers[1::2]) print(numbers) #[0, 0, 2, 0, 4, 0, 6, 0, 8, 0]

Of course there is an alternative way with list comprehension, we shall touch on it later.

Generate list with list comprehension

By using list comprehension, you can easily generate new a list with certain filtering conditions from the current sequence object. For instance, the below will generate a list of even numbers between 1 to 20:

even_nums = [i for i in range(1, 20) if i%2 == 0] print(even_nums) #[2, 4, 6, 8, 10, 12, 14, 16, 18]

Create sub list from a list

Similarly, you can get a sub list from the existing list with the list comprehension as per below:

[i for i in even_nums if i 
Manipulating elements in the list
With list comprehension, you can also transform your list of elements into another format. For instance, to convert the integers to alphabets:
alphabets = [chr(65+i) for i in even_nums] # ['C', 'E', 'G', 'I', 'K', 'M', 'O', 'Q', 'S']
 
Or convert the upper case into lower case:
 
[i.lower() for i in alphabets] #['c', 'e', 'g', 'i', 'k', 'm', 'o', 'q', 's']
 
And all the above can be done without list comprehension as well:
 
list(map(lambda x : chr(65+x), even_nums)) #['C', 'E', 'G', 'I', 'K', 'M', 'O', 'Q', 'S'] list(map(str.lower, alphabets)) #['c', 'e', 'g', 'i', 'k', 'm', 'o', 'q', 's']
 
Another real world example would be to use list comprehension to list out all the .ipynb files from current folder and its sub folders (excluding the checkpoint files):
 
import os [f for d in os.walk(".") if not ".ipynb_checkpoints" in d[0] for f in d[2] if f.endswith(".ipynb")]
 
Flatten a list of sequences
 
If you have a list of sequence objects as per below, and you would like to flatten them into 1 dimensional:
 
You can use multiple for expressions in list comprehension to flatten it:
 
b = [y for x in a for y in x] print(b) #[1, 2, 3, 4, 5, 6, 7]
 
Alternatively, you can make use of the itertools module to get the same result:
 
import itertools list(itertools.chain.from_iterable(a))
 
Ternary operation with list comprehension
 
In the previous ternary operation example, we have discussed how to replace the elements in the even position of a list. Here is the alternative using list comprehension in conjunction with a ternary expression:
 
numbers = list(range(10)) [y if i % 2 == 0 else 0 for i, y in enumerate(numbers)] #[0, 0, 2, 0, 4, 0, 6, 0, 8, 0]
 
Generate a dictionary with dictionary comprehension
 
To derive a dictionary from a list, you can use dictionary comprehension as per below:
 
Generate a set with set comprehension
 
Similar operation is also available for set data type when you want to derive elements from a list to set:
 
When using the built-in data type set, you shall expect that it only keeps the unique values. For instance, you can use set to remove duplicate values:
 
a = [1,2,2,4,6,7] unique = set(a) print(unique) #
 
More Python comprehension examples can be found here.
 
Read file into generator
 
Reading files can be done in one-liner as per below:
 
text = (line.strip() for line in open('response.html', 'r'))
 
Take note the parentheses are used in above generator expression rather than [], when [] is used, it returns a list.
 
One-liner with Python -c command
 
Sometimes you may want to run code snippets without entering into the Python interactive mode. You can execute the code with Python -c option in command line window. For instance, check the current Python version:
 
python -c "import sys; print(sys.version.split()[0])" #3.7.2
 
Or check the value of the environment variable:
 
python -c "import os;print(os.getenv('PATH').split(';'))"
 
Conclusion
 
In this article, we have reviewed through some commonly used Python one-liners which would greatly improve your code readability and coding productivity. There are definitely more to be covered, and every Pythonista would have his/her own list of favorite one-liners. Sometimes you will also need to consider the code performance before you use it rather simply pursuing the conciseness of the code.
 
As the general rule of thumb, you shall not use/innovate something that confusing ,difficult to read or totally not benefiting either in readability or productivity.
 
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Python ‘if…else’ in a List Comprehension (Examples)
 
You can place an if. else statement into a list comprehension in Python.
 
["EVEN" if n % 2 == 0 else "ODD" for n in numbers]
 
Notice that the if. else statement in the above expression is not traditional if. else statement, though. Instead, it’s a ternary conditional operator, also known as the one-line if-else statement in Python.
 
Example
 
Given a list of numbers, let’s construct a list of strings that are odd/even based on what the corresponding number in the list of numbers is.
 
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] # Use a list comprehension to create a new list that includes # "EVEN" for even numbers and "ODD" for odd numbers even_or_odd = ["EVEN" if n % 2 == 0 else "ODD" for n in numbers] print(even_or_odd) # Output: ["ODD", "EVEN", "ODD", "EVEN", "ODD", "EVEN", "ODD", "EVEN", "ODD", "EVEN"]
 
In this example, the if..else clause in the list comprehension checks to see if the number n is even (that is, if n is divisible by 2 with no remainder). If it is, the string “EVEN” is included in the new list; otherwise, the string “ODD” is included.
 
The Traditional Approach
 
Let’s see the traditional for loop with an if. else statement approach for comparison:
 
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] # Create an empty list to store the results even_or_odd = [] # Use a for loop to iterate over the numbers for n in numbers: # Use an if..else statement to determine if the number is even or odd if n % 2 == 0: even_or_odd.append("EVEN") else: even_or_odd.append("ODD") print(even_or_odd) # Output: ["ODD", "EVEN", "ODD", "EVEN", "ODD", "EVEN", "ODD", "EVEN", "ODD", "EVEN"]
 
Here’s a fun little illustration of converting the traditional approach to a list comprehension with an if…else statement:
 
 
 
One-Line If-Else Statements in Python
 
To add an if. else statement into a list comprehension in Python, you need to use a slightly modified version of an if. else statement, called the conditional operator.
 
The conditional operator in Python allows you to write conditional expressions in a shorter and more concise way. It is also known as the ternary operator because it takes three operands.
 
Here is the general syntax for using the conditional operator in Python:
 
Here, x and y are the values that will be returned based on the evaluation of the condition . If the condition evaluates to True , x will be returned; otherwise, y will be returned.
 
Here is an example of using the conditional operator to return the maximum of two numbers:
 
# Define two numbers x = 5 y = 10 # Use the conditional operator to return the maximum of x and y max = x if x > y else y # Print the maximum print(max) # Output: 10
 
The condition checks to see if x is greater than y . If it is, the value of x is returned; otherwise, it returns y .
 
The conditional operator can be useful whenever you want to write a conditional expression in a single line of code. It can make your code more readable and concise by avoiding multi-line if..else statements. Also, some argue it only makes the code shorter but less readable which is why some don’t use conditional operators at all.
 
To include an if. else statement into a list comprehension, you need to pass it as a conditional expression.
 
Example
 
Let’s take a look at another example of list comprehensions an if. else statements.
 
Here is an example of a for loop with an if. else statement that prints whether a number is odd or even in a list of numbers.
 
First, let’s start with the traditional approach:
 
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] for number in numbers: if number % 2 == 0: print(number, "is even") else: print(number, "is odd")
 
1 is odd 2 is even 3 is odd 4 is even 5 is odd 6 is even 7 is odd 8 is even 9 is odd 10 is even
 
This for loop can be converted into a one-line list comprehension expression using a conditional operator if. else :
 
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] print([f" is " for number in numbers])
 
The output of this expression would be the same as the original for loop:
 
1 is odd 2 is even 3 is odd 4 is even 5 is odd 6 is even 7 is odd 8 is even 9 is odd 10 is even
 
Should You Place If…Else Statements in List Comprehensions?
 
Whether or not you should use list comprehension expressions with conditional operators in your code depends on your personal preferences and the specific requirements of your project.
 
Some developers never use one-liner shorthand like list comprehensions or conditional operators.
 
List comprehensions can be a concise and elegant way to write certain types of for loops, but they can also make your code more difficult to read and understand if you are not familiar with the syntax.
 
If you are working on a small project with no collaborators, using list comprehension expressions (with if. else statements) can make your code more concise and easier to write.
 
However, if you are working on a larger project and you are collaborating with other people on a project, it may be more beneficial to use longer and more descriptive for loops that are easier for other people to read and understand.
 
Ultimately, the decision to use list comprehension in your code should be based on the specific needs of your project, the level of experience and familiarity of the people working on the project, and the trade-off between conciseness and readability.
 
My personal take: A list comprehension with an if. else statement looks messy and I’d probably not use such expression ever in my code.
 
Thanks for reading. Happy coding!
 
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