Python one liner conditional

Question

Can anyone please explain what this code is for?

shift, range = order < self.order and (1, (to, orig-1)) or (-1, (orig+1, to))

I know it will set shift and range depending on the result of order < self.order

What I don't know is why there is an and and or statement there

possible duplicate of [Does Python have a ternary conditional operator?](http://stackoverflow.com/questions/394809/does-python-have-a-ternary-conditional-operator) — NightShadeQueen, Aug 31 '15 at 04:00
@Cyphase: which is why I linked directly to a relevant answer. That question also gives quite a bit more context to why someone would write a statement like that. — NightShadeQueen, Aug 31 '15 at 04:05
@NightShadeQueen, whoops, I didn't see that you posted both of those, and I didn't notice that the first was directly to an answer :). — Cyphase, Aug 31 '15 at 04:14

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

From the documentation of boolean expressions in Python -

The expression x and y first evaluates x ; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.

So and / or expressions in Python don't necessarily return True or False, instead -

For and it return the last evaluated False value, or if all values are True , they return the last evaluated value.
For or it returns the last evaluated True value, or if all values are False, it returns the last evaluated value.

So , in your case if the condition is true , it returns the value of (1, (to, orig-1)) , otherwise it returns the value of (-1, (orig+1, to)) .

Very simple examples to show this -

>>> 1 < 5 and 2 or 4
2
>>> 6 < 5 and 2 or 4
4

Also, though not directly applicable to the condition , but in a general case, for a condition like -

cond and a or b

if cond is a True-like value and a has a False-like value (like 0 or empty string/list/tuple, etc), it would return b . Example -

>>> 1 < 5 and 0 or 2
2

So, it would be better to use the equivalent if..else ternary expression, Example -

a if cond else b

score 1 · Answer 2 · answered Aug 31 '15 at 04:08

There is a logical error in this code - when order < self.order evaluates to True, an exception will be raised.

I believe this is what is intended:

shift, range = (1, (to, orig - 1)) if order < self.order else (-1, (orig + 1, to))

This will assign 1 to shift and (to, orig - 1) to range when order < self.order is True and -1 to shift and (orig + 1, to) to range when order >= self.order

Python one liner conditional

2 Answers2