In this article we go over the most obscure “Hello, world!” Python program I have ever seen.

A really weird code snippet that contains code that is supposed to print “Hello, world!”.
A weird code snippet that allegedly prints “Hello, world!”.

The code

In this article we are going to be unravelling this monstrous piece of Python code:

@lambda _: _()
class _:
    def __format__(_, __):
        _.__class__._ = property(lambda _: print(__))
        return ""

def __() -> f"{_:Hello, world!}": ...


If you run this program with Python 3.9+, the output is "Hello, world!". But how does that work?

I will walk you through what is going on.

First, I am going to tell you how I found out what the code is doing. This is useful because it helps you see how you can tackle a piece of code you don't understand.

This explanation/walkthrough might be a bit confusing because I will be narrating my thoughts and tests as I do them. That's why I will also include a clearer explanation of what is going on, assuming you already understand all the components.

Walking through the code


The first thing I need to do, to understand what is going on, is to give more reasonable names to things.

So, I will go through the code and replace the underscore _ and double-underscore __ names with more useful names:

@lambda cls: cls()
class C:
    def __format__(self, fmt):
        self.__class__.prop = property(lambda _: print(fmt))
        return ""

def _() -> f"{C:Hello, world!}": ...


Here are my name changes and the reason why I made them:

  • The argument _ in @lambda _: _() was changed to cls because the lambda is being used as a decorator, and the whole class beneath is the argument, so the argument to lambda is a class.
  • The class we have, class _, is a generic class, so I'm giving it a generic name like C.
  • The parameters in the dunder method __format__ are “standard”, and you can check the documentation for their usual names, so that is why I went for def __format__(self, fmt): .... By changing the parameters in the definition of __format__, I also had to adjust the first _ in _.__class__._ and the __ in lambda _: print(__).
  • The _ in lambda _: print(...) was left unchanged because that is an actual throwaway value. In other words, the argument to that lambda is never used, so we do not care about the name of the argument. It is customary to use _ as a name for a variable we don't actually care about.
  • For the same reason, I changed the function name from __ to _. Previously, it was called __ in order not to clash with the class definition, but now we can use the name _ because we never really use that function. Thus, we used the underscore as a sink again.
  • Finally, the second _ in _.__class__._ was changed into prop, because that is short for “property”, which is what was getting assigned to _.
  • At the end of the program, _._ is C.prop because the global name _ is the class and the attribute _ is the property that we fiddle with inside __format__.

Now, there are three blocks of code that we need to parse and understand.

The first one is the decorated class definition:

@lambda cls: cls()
class C:
    def __format__(self, fmt):
        self.__class__.prop = property(lambda _: print(fmt))
        return ""

The second one is the annotated function definition:

def _() -> f"{C:Hello, world!}": ...

And the third one is the attribute access:


Decorated class definition

Python decorators are insanely powerful and, to be honest, one of my favourite language features!

They are often misunderstood and their difficulty is overestimated. Their essence is quite graspable, though! The syntax @decorator is just syntactic sugar that takes whatever is under the decorator, passes it into the decorator as its only argument, and reassigns the result of the decorator to the same name.

So, this code:

def foo():

Is actually equivalent to this:

def foo():

foo = decorator(foo)

Notice how

  1. we define the function as usual;
  2. we call the decorator with the function as its only argument, decorator(foo); and
  3. we assign the result back to the name of the function that was also the argument, foo = ....

Therefore, the class definition, that looked like this:

@lambda cls: cls()
class C:

is, therefore, equivalent to this:

class C:

C = (lambda cls: cls())(C)

This may look too confusing, so let me give a name to the lambda:

class C:

f = lambda cls: cls()
C = f(C)

However, if you look at it, the function just takes the class and builds an instance out of it! That is what the lambda decorator is doing here: it is replacing the class with an instance of it.

So, a saner way of writing this code would do without the lambda, and instead create a new variable for the instance of C:

class C:

C_obj = C()

def _() -> f"{C_obj:Hello, world!}": ...


Annotated function definition

Now that we have defined a class, we define our function _ with

def _() -> f"{C_obj:Hello, world!}": ...

Notice that the function is, for all practical purposes, “empty”. Its body is ... which is the same as if we had written pass inside the function. In other words, calling the function does absolutely nothing.

On the other hand, the function annotation does something interesting! The function annotation contains a formatted string: f"{C_obj:Hello, world!}".

  • On the left of the colon, we have C_obj: the object that is being formatted.
  • On the right of the colon, we have Hello, world!, which is the format specification.

Having Hello, world! to the right of the colon might look weird. You are likely to be more used to seeing string formatting like this:

>>> f"x is {x:>10}"
'x is   0.123456'
>>> f"x is {x:.2f}"
'x is 0.12'

However, the format specifier (the things to the right of :) can be anything. It is the object that is being formatted that needs to interpret that format inside the dunder method __format__, the method responsible for doing formatting.

So, what is going on in our code? The class C implements __format__, and that method is called when we reach the formatted string. The argument self is C_obj, the class C instance. The argument fmt is a string containing whatever is to the right of :, which is "Hello, world!" in our case.

Then, we run an assignment:

class C:
    def __format__(self, fmt):
        self.__class__.prop = ...

What is self.__class__.prop? The dunder attribute __class__ gives you a reference to the class of the object, so self.__class__ is a way to access the class of self. In our case, that class is C. In the original weird case, we need to use __class__ to access the class because we lost the name of the class. However, we have better naming now, which means we could write:

class C:
    def __format__(self, fmt):
        C.prop = ...

Now we just need to check what we are assigning to the attribute prop.

property is a built-in that allows you to define properties. (Who would have known?!)

A property is just like an attribute, but instead of giving you a fixed value, it calls a function to compute the value of that attribute.

For example, here is a class that wraps the built-in type list and that provides a property length that returns the length of the list:

class MyList(list):
    def length(self):
        return len(self)

lst = MyList([1, 2, 3])
# 3

In the example above, we used @property, but the @ is just syntactic sugar. We can call property directly, just like we do in our crazy program. Thus, what we are saying is that C.prop will call the anonymous function lambda _: print(fmt) when accessed. The lambda function, in turn, prints the value of fmt when called.

Finally, we return "" because the dunder method __format__ must return a string.

The final touch

To wrap everything up, the program ends with C_instance.prop, thus accessing the attribute prop of C_instance. Now, prop is a property, so we must run the lambda function from before. When we do, we run the expression print(fmt), but what is fmt?

The value of fmt was defined earlier, when the function annotation ran and called the dunder method __format__. Python can remember the value of fmt, which was "Hello, world!", so that is what is printed.

And that is how the most obfuscated “Hello, world!” program works!

The short explanation

For someone who understands all of the pieces of the puzzle involved, a much shorter explanation can be given.

In fact, this program works because the function definition forces the evaluation of the function annotation, which implicitly invokes the dunder method __format__. This ends up creating a property that prints the original format specification ("Hello, world!") when accessed. Finally, the program concludes by accessing that same property, triggering the call.


This short blog post was written as an expanded version of a Twitter thread I posted which, in turn, depended on a tweet posted by Ned Batchelder.

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