Elliptical Python Programming

https://susam.net/elliptical-python-programming.html

184•sebg•1y ago

Comments

benob•1y ago

TIL that in python, 1--2==3

seplox•1y ago

It's not a python thing. 1-(-2), distribute the negative.

qsort•1y ago

In most C-like languages that would be a syntax error. E.g. in C and C++ as a rule you tokenize "greedily", "1--2" would be tokenized as "1", "unary decrement operator", "2", which is illegal because you're trying to decerment an rvalue.

Python doesn't have "--", which allows the tokenizer to do something else.

nyrikki•1y ago

In C, that is really because Unary minus (negation) has precedence over binary operations.

    +a - b; // equivalent to (+a) - b, NOT +(a - b)
    -c + d; // equivalent to (-c) + d, NOT -(c + d)

https://en.cppreference.com/w/cpp/language/operator_arithmet...

    +-e; // equivalent to +(-e), the unary + is a no-op if “e” is a built-in type
     // because any possible promotion is performed during negation already

The same doesn't apply to, !! Which is applied as iterated binary operations (IIRC)

I am pretty sure the decriment operator came around well after that quirk was established.

seanhunter•1y ago

Peter van der Linden’s book “Expert C Programming” (which is awesome btw) says that one of them (Kernighan, Richie or maybe Ken Thompson I forget) realised early on that the c compiler had the wrong operator precedence for bit twiddling and unary and boolean operators but “at that stage we had a few thousand lines of C code and thought it would be too disruptive to change it”

j2kun•1y ago

Also worth noting that `1 - -2` works and produces 3 in C because the space breaks the operator.

plus•1y ago

For those who are curious, `...` is a placeholder value in Python called Ellipsis. I don't believe it serves any real purpose other than being a placeholder. But it is an object and it implements `__eq__`, and is considered equal to itself. So `...==...` evaluates to `True`. When you prefix a `True` with `-`, it is interpreted as a prefix negation operator and implicitly converts the `True` to a `1`, so `-(...==...)` is equal to `-1`. Then, you add another prefix `-` to turn the `-1` back into `1`.

`--(...==...)--(...==...)` evaluates to `2` because the first block evaluates to 1, as previously mentioned, and then the next `-` is interpreted as an infix subtraction operator. The second `-(...==...)` evaluates to `-1`, so you get `1 - -1` or `2`.

When chaining multiple together, you can leave off the initial `--`, because booleans will be implicitly converted to integers if inserted into an arithmetic expression, e.g. `True - -1` -> `1 - -1` -> `2`.

> There should be one-- and preferably only one --obvious way to do it.

This article is obviously completely tongue-in-cheek, but I feel the need to point out that this sentence is not meant to be a complete inversion of the Perl philosophy of TIMTOWTDI. The word "obvious" is crucial here - there can be more than one way, but ideally only one of the ways is obvious.

pletnes•1y ago

Numpy actively uses … to make slicing multidimensional arrays less verbose. There are also uses in FastAPI along the lines of «go with the default».

abuckenheimer•1y ago

excellent explanation, to add to this since I was curious about the composition, '%c' is an integer presentation type that tells python to format numbers as their corresponding unicode characters[1] so

'%c' * (length_of_string_to_format) % (number, number, ..., length_of_string_to_format_numbers_later)

is the expression being evaluated here after you collapse all of the 1s + math formatting each number in the tuple as a unicode char for each '%c' escape in the string corresponding to its place in the tuple.

[1] https://docs.python.org/3/library/string.html#format-specifi...

elijahbenizzy•1y ago

Ok do this but for JavaScript

voidUpdate•1y ago

https://en.wikipedia.org/wiki/JSFuck

mariocesar•1y ago

If you're curious, the code in ellipsis results in executing:

    print('hello, world')

mturmon•1y ago

Thank you!

I noticed some ** and * in the thing sent to eval(), which (given that the building blocks are small integers) seemed related to prime factorizations.

The initial %c is duplicated 21 times (3*7, if I read correctly), and then string-interpolated (%c%c%c...) against a long tuple of integers. These integers themselves are composed of products of factors combined using * and **.

There is also one tuple "multiplication" embedded within that long tuple of integers -- (a,b)*2 = (a,b,a,b). That is for the 'l' 'l' in "hello".

It's all very clever and amusingly mathy, with a winking allusion to the construction of natural numbers using sets. It made me Godel.

callamdelaney•1y ago

I think we're really starting to over crowd pythons syntax and I'm not a fan.

noddleah•1y ago

you're telling me you never program in python elliptically??

acbart•1y ago

Pretty sure this would have been possible in Python 2.6. The Ellipsis object has been around for a very long time.

MadVikingGod•1y ago

This behavior can be replicated with any class that has two special methods: __neg__ that returns -1 and __sub__ that accepts ints and returns 1-other.

For example if you make this class:

  class _:
       def __neg__(self):
           return -1
       def __sub__(self, other):
           return 1-other

You get similar behavior:

  >>> --_()
  1
  >>> _()--_()
  2

Fun python for everyone.

maxloh•1y ago

You can do this on JavaScript too.

  alert(1)
  // equals to:
  [][(![]+[])[+!+[]]+(!![]+[])[+[]]][([][(![]+[])[+!+[]]+(!![]+[])[+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[][(![]+[])[+!+[]]+(!![]+[])[+[]]])[+!+[]+[+[]]]+([][[]]+[])[+!+[]]+(![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[+!+[]]+([][[]]+[])[+[]]+([][(![]+[])[+!+[]]+(!![]+[])[+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[][(![]+[])[+!+[]]+(!![]+[])[+[]]])[+!+[]+[+[]]]+(!![]+[])[+!+[]]]((![]+[])[+!+[]]+(![]+[])[!+[]+!+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]+(!![]+[])[+[]]+([][(![]+[])[+!+[]]+(!![]+[])[+[]]]+[])[+!+[]+[+!+[]]]+[+!+[]]+([]+[]+[][(![]+[])[+!+[]]+(!![]+[])[+[]]])[+!+[]+[!+[]+!+[]]])()

https://jsfuck.com/

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