Why does using python expressions require creating a keyframe?

[magneto]

Hscript expressions immediately work but anytime I use python for expressions, it doesn't start working until I create a keyframe on that parameter.

Any ideas why it's like this? Or is it because hscript does this automatically? If so, then why isn't python made to do the same?

[Erik_JE]

I have never created any keyframes for python expressions. Must be something strange going on.

[lukeiamyourfather]

I have never created any keyframes for python expressions. Must be something strange going on.

The keyframe thing is only for some parameter types like strings, paths, etc. I don't know why this is required but I'm sure a developer would have a good reason why this is the case but once you know the trick its not an issue.

[magneto]

Sorry yes it was strings, paths like Luke said. I don't use python expressions as often but this is mostly what I encountered. Would still be good to know the reason though

[Erik_JE]

The keyframe thing is only for some parameter types like strings, paths, etc. I don't know why this is required but I'm sure a developer would have a good reason why this is the case but once you know the trick its not an issue.

That is so strange but true. I rarely modify strings in Houdini. Now I also want an explanation to why it is like this

[edward]

So you go to type in cubic(), is that an expression or the actual string value?

[Erik_JE]

So you go to type in cubic(), is that an expression or the actual string value?

I think it would make sense to then type 'cubic()' if you actually wanted it as a string value and not an expression.

[lukeiamyourfather]

I think it would make sense to then type 'cubic()' if you actually wanted it as a string value and not an expression.

But why would you require users to put strings in quotes... in a string field.

Just saying.

[Erik_JE]

But why would you require users to put strings in quotes... in a string field.

Just saying.

True, but it is how you make strings when actually coding python so it kinda makes sense but i still get your point.

[magneto]

Shouldn't it be just like hscript expressions?

cubic() is a string.

`cubic()` is an expression.

If hscript can do it, why not python?

Am I missing something?

Edited April 13, 2012 by magneto

[Erik_JE]

Shouldn't it be just like hscript expressions?

cubic() is a string.

`cubic()` is an expression.

If hscript can do it, why not python?

Am I missing something?

Backticks wouldnt make sense in a python fashion imo cause then how would you combine both the expression and the string.

`cubic()` cubic() would break all python logic but

cubic() + ' cubic()' would make sense

[magneto]

What about ~ then? That would work nicely

~cubic()~

[Erik_JE]

What about ~ then? That would work nicely

~cubic()~

[anim]

it may be simply difficult to guess for houdini whether you are writing text or expression since you are in text mode by default

creating keyframe for Python expression in string parms is fine for me

actually you don't have to do it if you use Alt-E in expression mode to create the expression, it will be done for you as if you directly just written expression without setting any keyframe before

and if you imagine that normal expressions are keyframes anyway, it is not that surprising on the other hand it has its own strengths, since you can limit expressions between keyframes

[magneto]

Thanks anim for the Alt+E trick for Python expressions. I only use it for long expressions but with Python it makes more sense to use it.