What the f*ck Python! 😱

Exploring and understanding Python through surprising snippets.

Translations: Chinese 中文 | Add translation

Other modes: Interactive | 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! :sweat_smile:

PS: If you're a returning reader, you can learn about the new modifications here.

So, here we go...

Table of Contents

• Structure of the Examples
  • ▶ Some fancy Title
• Usage
• 👀 Examples
• Section: Strain your brain!
  • ▶ First things first! *
  • ▶ Strings can be tricky sometimes
  • ▶ Hash brownies
  • ▶ Deep down, we're all the same.
  • ▶ Disorder within order *
  • ▶ Keep trying... *
  • ▶ For what?
  • ▶ Evaluation time discrepancy
  • ▶ How not to use is operator
  • ▶ is not ... is not is (not ...)
  • ▶ A tic-tac-toe where X wins in the first attempt!
  • ▶ The sticky output function
  • ▶ The chicken-egg problem *
  • ▶ Subclass relationships
  • ▶ All-true-ation *
  • ▶ The surprising comma
  • ▶ Strings and the backslashes
  • ▶ not knot!
  • ▶ Half triple-quoted strings
  • ▶ What's wrong with booleans?
  • ▶ Class attributes and instance attributes
  • ▶ Non-reflexive class method *
  • ▶ yielding None
  • ▶ Yielding from... return! *
  • ▶ Nan-reflexivity *
  • ▶ Mutating the immutable!
  • ▶ The disappearing variable from outer scope
  • ▶ The mysterious key type conversion
  • ▶ Let's see if you can guess this?
• Section: Slippery Slopes
  • ▶ Modifying a dictionary while iterating over it
  • ▶ Stubborn del operation
  • ▶ The out of scope variable
  • ▶ Deleting a list item while iterating
  • ▶ Lossy zip of iterators *
  • ▶ Loop variables leaking out!
  • ▶ Beware of default mutable arguments!
  • ▶ Catching the Exceptions
  • ▶ Same operands, different story!
  • ▶ Be careful with chained operations
  • ▶ Name resolution ignoring class scope
  • ▶ Needles in a Haystack *
  • ▶ Splitsies *
  • ▶ Wild imports *
  • ▶ All sorted? *
  • ▶ Midnight time doesn't exist?
• Section: The Hidden treasures!
  • ▶ Okay Python, Can you make me fly?
  • ▶ goto, but why?
  • ▶ Brace yourself!
  • ▶ Let's meet Friendly Language Uncle For Life
  • ▶ Even Python understands that love is complicated
  • ▶ Yes, it exists!
  • ▶ Ellipsis *
  • ▶ Inpinity
  • ▶ Let's mangle
• Section: Appearances are deceptive!
  • ▶ Skipping lines?
  • ▶ Teleportation
  • ▶ Well, something is fishy...
• Section: Miscellaneous
  • ▶ += is faster
  • ▶ Let's make a giant string!
  • ▶ Minor Ones *
• Contributing
• Acknowledgements
• 🎓 License
• Surprise your friends as well!
• More content like this?

Structure of the Examples

All the examples are structured like below:

▶ Some fancy Title

```py

Set up the code.

Preparation for the magic...

```

Output (Python version(s)):

```py

triggering_statement Some unexpected output ``` (Optional): One line describing the unexpected output.

💡 Explanation:

• Brief explanation of what's happening and why is it happening. ```py

Set up code

More examples for further clarification (if necessary)

``` Output (Python version(s)):

```py

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.

Usage

A nice way to get the most out of these examples, in my opinion, is to read them chronologically, 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,

1.

# Python version 3.8+

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

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

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

2 .

# Python version 3.8+

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

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

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

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

>>> a # a is still unchanged?
6

>>> b
16

💡 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
a = some_func()
if a:
    print(a)

# Now you can concisely write
if a := some_func():
        print(a)

Output (> 3.8):

5
5
5

This saved one line of code, and implicitly prevented invoking some_func twice.

• Unparenthesized "assignment expression" (use of walrus operator), is restricted at the top level, hence the SyntaxError in the a := "wtf_walrus" statement of the first snippet. Parenthesizing it worked as expected and assigned a.

• As usual, parenthesizing of an expression containing = operator is not allowed. Hence the syntax error in (a, b = 6, 9).

• The syntax of the Walrus operator is of the form NAME: expr, where NAME is a valid identifier, and expr is a valid expression. Hence, iterable packing and unpacking are not supported which means,

• (a := 6, 9) is equivalent to ((a := 6), 9) and ultimately (a, 9) (where a's value is 6')

  ```py

  (a := 6, 9) == ((a := 6), 9) True x = (a := 696, 9) x (696, 9) x[0] is a # Both reference same memory location True ```

• Similarly, (a, b := 16, 19) is equivalent to (a, (b := 16), 19) which is nothing but a 3-tuple.

---

▶ Strings can be tricky sometimes

1.

>>> a = "some_string"
>>> id(a)
140420665652016
>>> id("some" + "_" + "string") # Notice that both the ids are same.
140420665652016

2.

>>> a = "wtf"
>>> b = "wtf"
>>> a is b
True

>>> a = "wtf!"
>>> b = "wtf!"
>>> a is b
False

3.

>>> a, b = "wtf!", "wtf!"
>>> a is b # All versions except 3.7.x
True

>>> a = "wtf!"; b = "wtf!"
>>> a is b # This will print True or False depending on where you're invoking it (python shell / ipython / as a script)
False

# This time in file some_file.py
a = "wtf!"
b = "wtf!"
print(a is b)

# prints True when the module is invoked!

4.

Output (< Python3.7 )

>>> 'a' * 20 is 'aaaaaaaaaaaaaaaaaaaa'
True
>>> 'a' * 21 is 'aaaaaaaaaaaaaaaaaaaaa'
False

Makes sense, right?

💡 Explanation:

• The behavior in first and second snippets is due to a CPython optimization (called string interning) that tries to use existing immutable objects in some cases rather than creating a new object every time.
• After being "interned," many variables may reference the same string object in memory (saving memory thereby).
• In the snippets above, strings are implicitly interned. The decision of when to implicitly intern a string is implementation-dependent. There are some rules that can be used to guess if a string will be interned or not:
• All length 0 and length 1 strings are interned.
• Strings are interned at compile time ('wtf' will be interned but ''.join(['w', 't', 'f'] will not be interned)
• Strings that are not composed of ASCII letters, digits or underscores, are not interned. This explains why 'wtf!' was not interned due to !. CPython implementation of this rule can be found here
• When a and b are set to "wtf!" in the same line, the Python interpreter creates a new object, then references the second variable at the same time. If you do it on separate lines, it doesn't "know" that there's already wtf! as an object (because "wtf!" is not implicitly interned as per the facts mentioned above). It's a compile-time optimization. This optimization doesn't apply to 3.7.x versions of CPython (check this issue for more discussion).
• A compile unit in an interactive environment like IPython consists of a single statement, whereas it consists of the entire module in case of modules. a, b = "wtf!", "wtf!" is single statement, whereas a = "wtf!"; b = "wtf!" are two statements in a single line. This explains why the identities are different in a = "wtf!"; b = "wtf!", and also explain why they are same when invoked in some_file.py
• The abrupt change in the output of the fourth snippet is due to a peephole optimization technique known as Constant folding. This means the expression 'a'*20 is replaced by 'aaaaaaaaaaaaaaaaaaaa' during compilation to save a few clock cycles during runtime. Constant folding only occurs for strings having a length of less than 20. (Why? Imagine the size of .pyc file generated as a result of the expression 'a'*10**10). [Here's