How do I pass parameters to a function from a guizero object?

Question

I am unable to pass any parameter to a function called by a guizero widget either with the attribute "command" at initialization or by invoking an event.

This works as expected (no parameters passed):

from guizero import App, PushButton

def go():
    print (10)

app = App(title="Test")
button = PushButton(app, text = "Click Here", command = go)

app.display()

but the following prints the number 10 once, before the button is clicked and then when I click, nothing happens

from guizero import App, PushButton

def go(n):
    print (n)

app = App(title="Test")
button = PushButton(app, text = "Click Here", command = go(10))

app.display()

The same result I get with this:

from guizero import App, PushButton

def go(n):
    print (n)

app = App(title="Test")
button = PushButton(app, text = "Click Here")
button.when_clicked = go(10)

app.display()

What am I missing?

Thanks in advance!

Answer 1

from guizero import App, PushButton
def go(n):
    print (n)
app = App(title="Test")
button = PushButton(app, text = "Click Here", command = lambda: go(10))
app.display()

Whenever you write go(10) anywhere, you are invoking the function go . You might think you are passing go with arguments, but you aren't because the parentheses next to go() invoke the function right then and there. If you want to pass the function go to another function, and go should also be passed with some arguments, then you need to wrap the function go and pass the wrapped function as the argument "command". Using a lambda function to wrap go(10) is one such way of doing this.

The reason this works is that the lambda function is NOT invoked right then and there. You are saying that that command() should invoke the declared anonymous lambda function eventually, and when that lambda function is called it will itself call go(10) . You are declaring an anonymous lambda function, NOT invoking it. The lambda function will be invoked later on as command() .

Answer 2

In all GUI frameworks command= (or similar) expects callback - it means function's name without () and without parameters.

---

If you use go(10) then it does

result = go(10)
PushButton( ...., command=result)

so it executes go(10) before you even click button - and it assign result to command= . Because go() returns None so you get PushButton( ...., command=None) and when you click button then you get nothing.

---

You have to create new function which you can run without ()

def go_10():
    go(10)

PushButton( ...., command=go_10)

Or you can use lambda to create function directly in command=

PushButton( ...., command=lambda:go(10))

Answer 3

I found the solution!

Using a lambda function is a clever way to work around the callback limitation but I found a more proper way to pass arguments to a function by using the widget properties (pun not intended). As it turns out I missed a very brief reference to the property args that can be set at instantiation and is used to pass a list to the function called by command

So here is my solution:

from guizero import App, PushButton

def go(n):
    print (n)

app = App(title="Test")
button = PushButton(app, text = "Click Here", args = [10], command = go)

app.display()

As expected, it prints '10' every time you click the button and not before.

Thanks for all the other answers!