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10 Awesome Pythonic One-Liners Explained

devmount profile image Andreas ・4 min read

Since I wrote my first lines of code in Python, I was fascinated by its simplicity, excellent readability and its popular one-liners in particular. In the following, I want to present and explain some of these one-liners - maybe there are a few you didn't already know and are useful for your next Python project.

1. Swap two variables

# a = 1; b = 2
a, b = b, a
# print(a,b) >> 2 1

Let's start with a classic: swapping the values of variables by simply swapping positions on assignment - the most intuitive way in my opinion. No need to use a temporary variable. It even works with more than two variables.

2. Multiple variable assignment

a, b, *c = [1,2,3,4,5]
# print(a,b,c) >> 1 2 [3, 4, 5]

Swapping variables is actually a special case of Pythons ability to assign multiple variables at once. Here you can use it to assign list elements to the given variables, which is also called unpacking. The * will do packing the remaining values again, which results in a sublist for c. It even works for every other position of * (e.g. the beginning or middle part of the list).

3. Sum over every second element of a list

# a = [1,2,3,4,5,6]
s = sum(a[1::2])
# print(s) >> 12

No need for a special reduce function here, sum just adds the items of every given iterable. The extended slicing syntax [::] is used here to return every second element. You can read it as [start : stop : step], so [1::2] translates to start with the element of index 1 (second element), don't stop until the list ends (no argument given for second parameter) and always take 2 steps.

4. Delete multiple elements

# a = [1,2,3,4,5]
del a[::2]
# print(a) >> [2, 4]

The extended slicing syntax can also be used to delete multiple list elements at once.

5. Read file into array of lines

c = [line.strip() for line in open('file.txt')]
# print(c) >> ['test1', 'test2', 'test3', 'test4']

With Pythons inline for loop you can easily read a file into an array of lines. The strip() is needed to remove the trailing line breaks. If you want to keep them or they don't matter to you, you can use an even shorter one-liner:

c = list(open('file.txt'))
# print(c) >> ['test1\n', 'test2\n', 'test3\n', 'test4\n']

It's really that simple to read a file in Python. Side note: you can also use the readlines() method if you like.

6. Write string to file

with open('file.txt', 'a') as f: f.write('hello world')
# print(list(open('file.txt'))) >> ['test1\n', 'test2\n', 'test3\n', 'test4\n', 'hello world']

With the help of the with statement, you can directly write content to a file. Make sure to use the correct mode to open the file (here 'a' for appending content).

7. List creation

l = [('Hi ' + x) for x in ['Alice', 'Bob', 'Pete']]
# print(l) >> ['Hi Alice', 'Hi Bob', 'Hi Pete']

Lists can be dynamically created from other lists with the inline for loop. You can directly modify the values, like string concatenation in this example.

8. List mapping

l = list(map(int, ['1', '2', '3']))
# print(l) >> [1, 2, 3]

You can also use Pythons map() function to cast every list element to another type.

9. Set creation

squares = { x**2 for x in range(6) if x < 4 }
# print(squares) >> {0, 1, 4, 9}

It's similar with sets. In addition to the inline for loop, you can even directly append a condition!

10. Palindrome check

# phrase = 'deleveled'
isPalindrome = phrase == phrase[::-1]
# print(isPalindrome) >> true

A palindrome is a series of characters that read the same both forwards and backwards. Normally you would need some looping and conditions to check, if a given string is a palindrome. In Python, you just compare the string with its reverse string. Instead of using the slicing operator [::-1], you can also use the reverse() function to reverse the string.

Bonus: The Zen of Python

import this

Well, this one needs no explanation. Just try it yourself by simply entering it in your Python shell 😊🎉

Wrap it up

We've seen some (admittedly simple) examples of Python one-liners, that are powerful and well readable at the same time. Maybe you know another helpful one-liner? Share it with us in the comments!


Published: 29th July 2020

Posted on by:

devmount profile

Andreas

@devmount

creator. developer. consultant. freelancer. husband. dad². guitarero. climber. javascript. php. python. css.

Discussion

markdown guide
 

Great article, I really like the idea of the easily consumable content. I may think of how I can do something like this in my own way 😉


I am surprised that you made it through 10 one-liners with no ternary statement.

>>> 'Hi ' if 'Bob' in ['Alice', 'Bob', 'Pete'] else 'Goodbye'
'Hi'
 

My pleasure 😊

I didn't cover everything on purpose, but I totally forgot about the ternary statement 😅 So thank you for this addition! 👍🏻

 
[deleted]
 

Good point, thank you for this clarification!

PS: For the sake of correctness: it's readlines() without the underscore, isn't it?

 

Sorry, I was actually incorrect. I thought readlines() was a generator but apparently it isn't so I deleted my comment.

 
  1. List mapping

Best to use list comprehensions instead of map in most cases. For your example:

l = [int(x) for x in ['1', '2', '3']]
 

I agree, this increases readability even more! Thank you. Is there any other benefit in using list comprehensions instead of the (specially made for this purpose) map() function?

 

map returns a map object that you then turn into a list. Readability. There are other comprehensions that follow naturally from list comprehensions to generate, for example, dicts, sets and iterators.

 

Very good article, can I translate it into Chinese and publish it on my WeChat official account? I will specify the source and author. Hope to get your authorization.

 

Hey there, I'm glad you like my article! You are welcome to translate it into Chinese. Please set links to the article, my dev.to profile and my Twitter profile at the beginning and let me know, when you published it ✔️

 

Thank you for your authorization. This is the link to the translated article. Thanks again!
mp.weixin.qq.com/s/_K40z0t_z-sSNDC...

Wow that was fast! 👏🏻 Thanks for the translation and credits. Can you create actual links or are you limited to only show urls as text?

 
  1. Read file into array of lines

Needs to use rstrip rather than strip if you want to preserve left-hand whitespace in lines.

 

Thank you for this addition. In this example I just wanted to strip both ends. To only remove the linebreaks, you have to specify the linebreak character:

c = [line.rstrip('\n') for line in open('file.txt')]
 

If you know the last line of the file does have a '\n', then line[:-1] is shorter.

 

I didn't know most of these tricks. Good article and great clarity 😁

 

Awesome, thank you! I'm glad it was helpful 😊

 

My favs:

python -m json.tool

python3 -m http.server

xD

 

Nice, thanks for this addition!
Can you add some explanation for those who don't know, what these one-liners do?

 

Great little article, list/set comprehensions are really awesome !

 

Indeed they are! Thanks a lot 😊

 

Nice Article. 10th one is enough to explain you why people love Python so much, for doing similar thing in other languages would require you to write a numerous lines of code :)

 

I'm glad you like it 😊 and it's true: I love Python, because its concepts are so well thought through 👌🏻

 
 

Thanks a lot for encouraging me 🤗