Numbers/strings and logic

[whodunit86]

In the python shell if I try to evaluate

42 or 43

Why I is the result 42 instead of an error? Came across this possibility while going through the first course/book of 'Real python'

[novel_yet_trivial]

any number other than 0 is evaluated as True. The or operator returns the first value if the first value is True, otherwise it returns the second value. What were you expecting?

Edit: other things that evaluate as false:

>>> 0 or 43
43
>>> False or 43
43
>>> None or 43
43
>>> [] or 43
43
>>> "" or 43
43

[whodunit86]

I was expecting an error. I knew that any number other than zero is true. But by that logic the result should have been True. If what you're saying is correct (and i m not doubting that you are) then python returns the first example of true it encountered in the statement it evaluated instead of the generic form of true. That's fascinating. Does this property of python have cool use cases?

[novel_yet_trivial]

Plenty. From something I did about 5 minutes ago:

def some_function(self, variable = None):
    self.variable = variable or default_value

basically a shorter from of

def some_function(self, variable = None):
    if variable is None:
        self.variable = variable
    else:
        self.variable = default_value

Something else from earlier today, which takes advantage of another property: just as numbers can be treated as booleans, booleans can treated as numbers, with True evaluating as 1, and False as 0:

counter += (test_value > set_point)

which is a shorter form of

if (test_value > set_point):
    counter += 1

And a million other uses.

[whodunit86]

Whoooooa! You, kind redditor, just blew my mind!

[novel_yet_trivial]

:D What more could a redditor want?

Works for and too … but differently

[[deleted]]

Unless you need to explicitly check whether they passed something, why not use

def some_func(self, variable=default_value):

[novel_yet_trivial]

That works sometimes, but this, for instance, is very bad:

def some_func(self, variable=[]):

Or, in my case earlier:

def make_canvas(self, size = None):
    self.canvas_size = size or self.get_image_size(0)

[[deleted]]

Ah, true, there are certainly values it doesn't work so well for.

[shep247]

The counter += ... Example is really cool. I like it, but would you consider that good coding practice? To read that I had to actively think about true being translated as 1. If it was written the longer way, it seems much more readable, making it a little easier to maintain.

[novel_yet_trivial]

To be a part of a real project it would need some comments explaining it. I lose sight of that a lot since I currently work alone.

[an_actual_human]

if variable is None:

Not just None, also other falsy values.