Command flag in symbolButton keeps erroring for me

[likage]

I am trying out using maya symbolcheckbox and I run into a few issue. In my actual code, I have a 'add', 'remove' and 'clear' button and so if any of these 3 buttons are clicked onto, it will called its own functions. In my code, it was written something as follow:

self.btn_add = cmds.symbolButton( width=60, height=20, annotation='Add object(s)', image='add.xpm', command=lambda * args:self.btnCallback("add", "item_list") )

def uiListCallback(self, *args):
    mode = args[0]
    text_list = args[1]
    if mode == "add":
        # do this
    elif mode == "remove"
        # do that

But when I tried to create a function for each action, I keep getting errors.

This is simple example, but it illustrates the problem I had.

However, if I execute the following code:

def greet():
    print 'hello'

def main():
    cmds.window()
    cmds.columnLayout()
    cmds.symbolCheckBox(image='circle.png', changeCommand=greet())
    cmds.showWindow()
main()

I will get `# Error: TypeError: file <maya console> line 9: Invalid arguments for flag 'changeCommand'.  Expected string or function, got NoneType # `

But if I did it in another way:

def greet():
    print 'hello'

def main():
    cmds.window()
    cmds.columnLayout()
    # () was added into greet under the command flag
    cmds.symbolCheckBox(image='circle.png', command=greet())
    cmds.showWindow()
main()

While I get a popup, but as soon as I click on it, I will get this error - `# Error: greet() takes no arguments (1 given) # `

Where am I doing it wrong?

[Alok Gandhi]

I will get `# Error: TypeError: file <maya console> line 9: Invalid arguments for flag 'changeCommand'.  Expected string or function, got NoneType # `

You need to pass the function object to change commands, currently you are passing the result of calling greet().

I think this should work for you:

import maya.cmds as cmds

def greet(value):
    print 'greet was called with value: {0}'.format(value)

    print 'hello'

def main():
    cmds.window()
    cmds.columnLayout()
    # () was added into greet under the command flag

    cmds.symbolCheckBox(image='circle.png', changeCommand=greet)
    cmds.showWindow()
main()

[likage]

While it works for the example, I tried applying the same concept into my actual code, I am getting the errors..

self.btn_add = cmds.symbolButton( width=60, height=20, annotation='Add object(s)', image='add.xpm', command=self.add_btn_callback("item_list") )

def add_btn_callback(self, "list"):
    # Do the exec when add button is clicked

[Robert White]

Try wrapping it in a lambda?

self.btn_add = cmds.symbolButton( width=60, height=20, annotation='Add object(s)', image='add.xpm', command=lambda *x: self.add_btn_callback("item_list") )

[likage]

Try wrapping it in a lambda?

self.btn_add = cmds.symbolButton( width=60, height=20, annotation='Add object(s)', image='add.xpm', command=lambda *x: self.add_btn_callback("item_list") )

Okay, so it works. And my question is why? 

[Robert White]

Because maya expects you to pass either a string, or a callable to the command flag. 

When you don't wrap the function in a lambda, you're calling the function, and passing the result to the command flag.

[Justin Israel]

Python has a concept called "first class functions". It means a function is an object just like anything else, like strings and lists. They can be passed around as args and saved as references in variables. Callbacks are implemented by have a function passed as an object so that the function can be saved and called later. 

def hello():

    print "hello" 

# call function immediately 

hello() 

# assign function to variable

fn = hello

fn() 

Note that if you don't return a value from a function, the return value is implicitly None. This will explain strange errors about the type when you call the function instead of passing it as a first class function. 

val = hello() 

print type(val) 

For more information here is a tutorial and a general wiki:

https://newcircle.com/bookshelf/python_fundamentals_tutorial/functional_programming

https://en.m.wikipedia.org/wiki/First-class_function

Justin 

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[likage]

Interesting... Thanks for the information, guys!

Never thought that with a simple example that I have used, it can failed almost immediately.

From the flags, though it mentions of `script` I had assumed that it will worked with a normal method too..

[Justin Israel]

On Wed, Mar 15, 2017 at 1:40 PM likage <dissid...@gmail.com> wrote:

From the flags, though it mentions of `script` I had assumed that it will worked with a normal method too..

This refers to the ability to pass script logic in string form. In MEL there are no first class functions so your only option is to pass a string with MEL script to run. In python you can either pass Python script to be evaluated, or a callable object.

 

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[Michael Boon]

Lambdas are particularly confusing, too. Your original attempt

command=lambda *x: self.add_btn_callback("item_list")

doesn't actually call the function add_btn_callback, even though it looks like it does. It creates a new function that will call add_btn_callback later.

I prefer partial instead, because it seems clearer to me what is happening. partial is similar to lambda. It creates a new function that will call your function with given arguments:

from functools import partial
...
command=partial(add_btn_callback, "item_list")

And I should warn you: right now your argument is a constant string "item_list". Things get even weirder if you have arguments whose values might be different when the button is clicked than they are now.

On Wednesday, 15 March 2017 11:49:31 UTC+11, Justin Israel wrote:

On Wed, Mar 15, 2017 at 1:40 PM likage <dissid...@gmail.com> wrote:

From the flags, though it mentions of `script` I had assumed that it will worked with a normal method too..

This refers to the ability to pass script logic in string form. In MEL there are no first class functions so your only option is to pass a string with MEL script to run. In python you can either pass Python script to be evaluated, or a callable object.

 

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