How to append to a list stored in a custom python parameter?

General TouchDesigner Discussion
1

I have a custom python parameter that holds a list. How can I script a modification of this list?
node.par.Pythonpar.eval().append(123) does not work

2

You tried
node.par.Pythonpar = node.par.Pythonpar.eval().append(123)

depending on what yo have in the par already

?

3

Yes, i did. It results in None as the par value
I had [1,2] in the par already

see: pythonpar.toe (3.7 KB)

4

n.par.Py = n.par.Py.eval() + [123] works.

So why not what you had… not sure. .append() changes an existing list by adding a member. the .eval() creates a temporary list object, then .append appends to that temporary object. maybe it’s lost when the assignment is made.

5

thanks greg, that works

6

The reason the other didn’t work is because the list that you get out of the par (node.par.Pythonpar.eval()) is not the list object that is stored in the par, but a copy of it. So appending to it changes the copy but not the parameter. Greg’s method works because it reassigns the newly added-to copy back to the parameter.

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