List output from function on ipywidgets - function

Question

I constructed a function linked to MySQL which returns a list of children for a given parent id. I would like to output this list of children using ipywidgets.
I am having trouble linking the function to ipywidgets. So far I have:
> from ipywidgets import widgets
>
> text1 = widgets.Text()
> text2 = widgets.Text()
> button = widgets.Button(description = 'Run')
> display(text1)
> display(button) display(text2)
>
> idnum = text1.value
> text2.value= list_children(idnum)
>
> button.on_click(list_children)
The function is the following:
> def list_children(parentid):
> value = parentid
> parent_80 = session.query(Parent).get(value)
> parent_80_children= parent_80.children
> childrenlist=[]
>
> for i in parent_80_children:
> childrenlist.append(i.UWI)
>
> return childrenlist
I keep getting the following error:
AttributeError: 'NoneType' object has no attribute 'children'
as it breaks in this line:
parent_80_children= parent_80.children
The function is correct if I run the python cell so I know it's working, but it breaks when I try to click the widget box "Run". Somehow there is no link between the function and the widget box.
I would like to have the output upon clicking the "Run" widget button as the following:
1771860100
1771860200
1771860300
minus the bullet points.
Any input is appreciated.

Answer 1

When you use the .on_click() method of a button, you need to specify the function you want to run, which you have done correctly. However, when you click the button, the button instance itself gets passed to the function. That explains why, when you click the button, you see that error. The function thinks it is getting a parentid (probably a string or int), but actually receives an instance of ipywidgets.Button instead!
Assuming you want to take the text in the text1 widget as you did in the upper section of your code, you need to pass the Text widget to the function. So, add this to your on_click() call using a partial to specify the text field as an argument. The text widget will be the first parameter, and the Button instance will follow as the 2nd parameter. If you forget that the Button instance gets passed when writing your function definition, you will get an error along the lines of 'takes exactly 1 argument but 2 given'.
Hopefully you can take the example below and modify to suit your needs.
import ipywidgets as widgets
from functools import partial
text1 = widgets.Text()
def list_children(text_field, button):
print(text_field.value)
button = widgets.Button()
button.on_click(partial(list_children, text1))
display(text1, button)