Issue with number slider or GhPython

Grasshopper
1

When number slider has its minimum set to 0.0 and its value to 0.0 it is considered as None(NULL) inside GhPython instead of 0.0

NumberSlider_issue.gh (8.7 KB)

2

That is how python works. It is not considered None, that you do yourself on line 14 and 17.

In python 0 and 0.0 evaluate to False in a comparison or boolean check. So if not 0: will be True and go into the statement block. Hence your x gets set to None, and then your a gets set to x.

Many languages have 0 evaluate to False. You’ll find for instance C and C++ code littered with checks for values that are not 0, nullptr etc.

3

ok, how can I check if variable has a value?

4

Explicitly check against None:

if a==None:
  x=None

From py2.7 interpreter:

>>> if 0 == None: print("hi")
... 
>>>
1 Like
5

I always wondered how different is not and != are

6

Nitpick, but you didn’t use is not, but not.

You could do:

if a is None:
  x = None

or the invers

if a is not None:
  x = 42
else:
  x = None
7 
if not x:
# is different from
if x is not None:
# this is even more confusing

Riddle me this then.

If I use if not x: and the minimum of the slider is not 0, then at value 0 x is not considered as None.

8

My bad, it is considered as None.

9

You must understand that None is a type. 0 is a value of type integer (0.0 a float (or double).

Using is you compare types, so if something is put into your a it will have a type other than None.

Using just the if not a: you get a boolean check where a can be of many types, but still evaluate to True or False.

To False evaluate at least the values 0, 0.0, "", [], {} - you may see a pattern here. Typically a zero value of a type will evaluate to False, any non-zero value evaluates to True.

Perhaps now you can see why your initial code failed for the 0.0case by giving unexpected execution.

1 Like
10

That’s a very good explanation.
Thanks Nathan.

By the way, this explanation fixed a lot of scripts I could not figure out why they are not working properly. :slight_smile: