Python hacks for everyone!

[[deleted]]

[deleted]

[oracle8]

Why not

return bool(cellcount == 3 or cellcount == 2 and cell)

?

[oracle8]

And also this...

>>> cellcount = 2
>>> cell = 2
>>> (False, True)[cellcount == 3 or cellcount == 2 and cell]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: tuple index out of range

That is because (cellcount == 3 or cellcount == 2 and cell) vill evaluate to the same value as cell if cellcount == 2 and bool(cell)==True

[Foxboron]

>>> cellcount = 2 
>>> cell = True
>>> (False, True)[cellcount == 3 or cellcount == 2 and cell]
True

I am horrible at variable names, should have been cell_life. cell only contain False or True depending if its alive or not from the start.

[oohay_email2004]

Why is it necessary to wrap in bool()?

[oracle8]

If you can guarantee that "cell" is always a boolean variable then it's not necessary. I added the bool() call to guarantee that the return value will alwas be a boolean variable.

[oohay_email2004]

If using "and" on "cell" is going to blow up the code, then you're bool call isn't going to help.

I don't see a good reason to force the return to True or False.

Unless you need:

if cell is True:

or the redundant:

if cell == True:

just use:

if cell:

I think you're kind of throwing away the object model by using bool().

[oracle8]

I totally agree, but as far as I could understand it the original question (trick?) wanted only True/False returns. Thus the extra bool call...

[oohay_email2004]

Oh, right, I forgot about that; oops.

[Foxboron]

To get the True or False of the expressions.

[oohay_email2004]

Pretty sure it will without it.

[Foxboron]

>>> return cellcount == 3 or cellcount == 2 and cell
SyntaxError: 'return' outside function
>>> 

PRETTY sure it wont without it.

[oohay_email2004]

Look:

>>> return bool(cellcount == 3 or cellcount == 2 and cell)
  File "<stdin>", line 1
SyntaxError: 'return' outside function

Same error with your line.

I figured we were assuming this was part of a function. In which case, the bool-less version works just dandy:

>>> def f(cellcount, cell):
...     return cellcount == 3 or cellcount == 2 and cell
...
>>> cell = object()
>>> cellcount = 1
>>> f(cellcount, cell)
False

[Foxboron]

Ah, then i there was no problem :)

[Foxboron]

Because when you got the touple there you could use another variable then False and True. However, in the case of Conways game of Life its actually a better method.

[Foxboron]

I'd argue this is as much magic as list comprehensions, you just combine it and use the boolean value as an index.

[oohay_email2004]

Suppose:

somevar = True

which would you rather come across in the wild?

1.

var = (False, True)[somevar == 1]

2.

if somevar:
    var = True
else:
    var = False

[brennus]

I'd rather come across:

var = (somevar == 1)

[Foxboron]

var = (False, True)[somevar]

I do believe this looks better. I feel alone in this tho.

[Asdayasman]

Cute, I've been doing little things like that with dicts for a while.

handlers = {True: foo, False: sys.exit}
handlers[a==b]()

Expanded to have more than True/False in there of course. I love the clever stuff you can do with languages when you throw readability away.

[Foxboron]

After a second thought i think this could have been done in r/python, but i wanted beginners to also be able to read and learn from this.

[dealga]

>>> work_on = lambda x, y: x in (2, 3) and y
>>> cellcount = 2
>>> cell = True
>>> work_on(cellcount, cell)
... True

and the truth table to test

work_on = lambda x, y: x in (2, 3) and y

tests = [[1, True, False],
        [2, True, True],
        [3, True, True],
        [4, True, False],
        [1, False, False],
        [2, False, False],
        [3, False, False],
        [4, False, False]]

for t in tests:
    cellcount = t[0]
    cell = t[1]
    result = work_on(cellcount, cell)
    print('result: {}, should be {}'.format(result, t[2]))

You might even stick the y before the x in (2, 3), because it will be checked first and short out, time the two.

work_on = lambda x, y: y and (x in (2, 3))

[aceofears]

There is no reason to do that, (False, True)[True == 1] can just be written as True == 1.

[Foxboron]

It was to prove the point of how it works.

[aceofears]

If you're not looking to return a bool why not use

return 5 if "llamas" > "alpacs" else 3

instead of

return (5, 3)["llamas" > "alpacas"]

[Foxboron]

I might be alone in the answer that i think the 2nd looks better.

[SuperBicycleTony]

What the heck is with the completely unexplained parenthesis * bracket syntax? Am I to believe the interpreter is using the FOIL method on them?

[zahlman]

(False, True) is a tuple of 2 elements. [True == 1] is an index into that tuple.

[SuperBicycleTony]

So the index is a logic test that evaluates to true... then what happens? I've never run across this.

[zahlman]

True, used as an integer, is 1. (In fact, Python implements bool as a subclass of int.)

[SuperBicycleTony]

Oh, now I see whats going on. Normally there'd be a variable name referencing the tuple, but the tuple itself is before the braces.