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What the f*ck Python! 😱

Exploring and understanding Python through surprising snippets.

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Other modes: Interactive Website | Interactive Notebook | CLI

Python, being a beautifully designed high-level and interpreter-based programming language, provides us with many features for the programmer's comfort. But sometimes, the outcomes of a Python snippet may not seem obvious at first sight.

Here's a fun project attempting to explain what exactly is happening under the hood for some counter-intuitive snippets and lesser-known features in Python.

While some of the examples you see below may not be WTFs in the truest sense, but they'll reveal some of the interesting parts of Python that you might be unaware of. I find it a nice way to learn the internals of a programming language, and I believe that you'll find it interesting too!

If you're an experienced Python programmer, you can take it as a challenge to get most of them right in the first attempt. You may have already experienced some of them before, and I might be able to revive sweet old memories of yours! 😅

PS: If you're a returning reader, you can learn about the new modifications here (the examples marked with asterisk are the ones added in the latest major revision).

So, here we go...

Table of Contents

Structure of the Examples

All the examples are structured like below:

▶ Some fancy Title

# Set up the code.
# Preparation for the magic...

Output (Python version(s)):

>>> triggering_statement
Some unexpected output

(Optional): One line describing the unexpected output.

💡 Explanation:

  • Brief explanation of what's happening and why is it happening.
# Set up code
# More examples for further clarification (if necessary)

Output (Python version(s)):

>>> trigger # some example that makes it easy to unveil the magic
# some justified output

Note: All the examples are tested on Python 3.5.2 interactive interpreter, and they should work for all the Python versions unless explicitly specified before the output.


A nice way to get the most out of these examples, in my opinion, is to read them in sequential order, and for every example:

  • Carefully read the initial code for setting up the example. If you're an experienced Python programmer, you'll successfully anticipate what's going to happen next most of the time.
  • Read the output snippets and,
    • Check if the outputs are the same as you'd expect.
    • Make sure if you know the exact reason behind the output being the way it is.
      • If the answer is no (which is perfectly okay), take a deep breath, and read the explanation (and if you still don't understand, shout out! and create an issue here).
      • If yes, give a gentle pat on your back, and you may skip to the next example.

PS: You can also read WTFPython at the command line using the pypi package,

$ pip install wtfpython -U
$ wtfpython

👀 Examples

Section: Strain your brain!

▶ First things first! *

For some reason, the Python 3.8's "Walrus" operator (:=) has become quite popular. Let's check it out,


# Python version 3.8+

>>> a = "wtf_walrus"
>>> a

>>> a := "wtf_walrus"
File "<stdin>", line 1
    a := "wtf_walrus"
SyntaxError: invalid syntax

>>> (a := "wtf_walrus") # This works though
>>> a

2 .

# Python version 3.8+

>>> a = 6, 9
>>> a
(6, 9)

>>> (a := 6, 9)
(6, 9)
>>> a

>>> a, b = 6, 9 # Typical unpacking
>>> a, b
(6, 9)
>>> (a, b = 16, 19) # Oops
  File "<stdin>", line 1
    (a, b = 16, 19)
SyntaxError: invalid syntax

>>> (a, b := 16, 19) # This prints out a weird 3-tuple
(6, 16, 19)

>>> a # a is still unchanged?

>>> b

💡 Explanation

Quick walrus operator refresher

The Walrus operator (:=) was introduced in Python 3.8, it can be useful in situations where you'd want to assign values to variables within an expression.

def some_func():
        # Assume some expensive computation here
        # time.sleep(1000)
        return 5

# So instead of,
if some_func():
        print(some_func()) # Which is bad practice since computation is happening twice

# or