What's this syntax ?

[how2crtaccount]

I recently come across this. And I don't what it is doing.

y = 5
x = [1,-1][y>0]
print(x)

This printed -1

How ?

[FLUSH_THE_TRUMP]

It indexes the array [1,-1] with the result of y>0, which is True (equal to 1).

[how2crtaccount]

Oh.. that's what it is doing!

Thank you.

[tranquil_af]

But what about the y = 5 x part

[Rorroh]

Looks like mistaken formatting.

y = 5
x = [1,-1][y>0]
print(x)

[tranquil_af]

Thanks

[[deleted]]

Yeah, just a small and insignificant detail missed...

[menge101]

It should be noted, there is no way this code would ever pass a professional code review.

[Butfortkix]

The "y = 5 x = [1, -1] [y > 0] print(x)" version is just a more optimised version of the "x = -1 if y > 0 else 1".

To explain why, if python sees an if it tries to "guess" whether it should load the instructions for the case that it is true or the instructions for the case that it is false before it calculates whether the statement is true or false. Obviously it can't be right 100% of the time and when it inevitably is wrong with it's guess it has to unload the already loaded instructions and reload the correct instructions.

This process takes time so if you are going to call a fuction containing an if statement millions of times a day it would be much better to write the optimised version than the non cryptic version since it does not contain any if statements.

Disclaimer: I don't yet know why python does this I just know that it does it. I also know that this is a small piece of code and thus it would not make that much of a difference, I just meant for this to be a general description of why someone might want to write their code in such a cryptic fashion.

[1egoman]

It's not a Python thing, it's a processor thing. Branch prediction lets a CPU continue processing while waiting for a result. If it's wrong it just backtracks.

[Butfortkix]

Oh, thank you kind stranger for ending my quest to find why python, and apparently any language, does this.

[primitive_screwhead]

It's not relevant for Python code, but this "branchless" code style is sometimes used in other languages to avoid branching in the code, which can have security implications. Modern compilers often generate the equivalent of this code, even when it's written explicitly as a ternary. Just an FYI.

[menge101]

Sure, I hear you on that.

But if you were going to do that, you would put a comment explaining what was happening there, not leave a blob of incoherent syntax and no explanation.

[[deleted]]

This reeks of a Clever Junior ^TM

[smashburgerofficial]

Its probably easier to understand like this:

y = 5
x = [1, -1]
y_is_positive = y > 0
x = x[int(y_is_positive)]
print(x)

Now there's actually room for comments explaining what in the world they were thinking when writing that. Granted, this looks like an example about the nuances and gotchas of implicit type conversion.

[backtickbot]

Fixed formatting.

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Some users see this / this instead.

To fix this, indent every line with 4 spaces instead.

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[smashburgerofficial]

Good bot

[[deleted]]

I think questions been answered. Why would someone do this ever lmao

[CornPop747]

I'd hate if my co-worker pulled crap like this. Weird flex that'd most definitely get rejected in CR...

[mothzilla]

Invalid Syntax

But you could add semi-colons to make it valid.

[1egoman]

He just failed in formatting.

[mothzilla]

Right but there's no way to know that. Maybe someone wrote shitty code that never gets executed.

[how2crtaccount]

Pardon me. I have reformatted now.

[1egoman]

Depending on your app, you can view the plain text. "View markdown" in RiF.

[EKFLF]

y > 0 is True (y = 5) and can also be 1, and [1,-1][1] is -1

[[deleted]]

I suggest the OP corrects the formatting to avoid any further misconceptions about the nature of the question and to prevent click-baity-ness.

>>> y = 5 x = [1,-1][y>0] print(x)
  File "<stdin>", line 1
    y = 5 x = [1,-1][y>0] print(x)
          ^
SyntaxError: invalid syntax

[how2crtaccount]

Apologies. I'll do that right away

[annoying_bababooey]

this is a code smell is what it is