Dink Software

To kick off the Python forum i start with a peice of knowledge i only picked up the other day after the spending hours wrestling with different peices of code trying to get it to work. But bare in mind, i do not fully understand why this works myself however i'm currently reading more into it.

Using the same function more than once whilst passing arguments in Tk is not as simple as it seems. Many people --including myself-- start with something like this.

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#This code ommiits many formalities.
Button(frame,command=self.foo(x)).grid()

def foo(y):
    print "I like " + str(y)


The above will not work.

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#This code ommits many formalities.
Button(frame,command=lambda: self.foo(x)).grid()

def foo(y):
    print "I like " + str(y)


Using the built in lambda statement you're able to pass arguments with your functions.

Thread Revisited: Entire months have gone by since i posted this, and now i feel very n00bish. The reason the lambda statement is required, is because when Tk needs to fire a callback, it calls the __call__() method on the object passed to it via the "command" option. By using something like "self.foo(x)", you have to remind yourself that it is essentially representing the returned value from that call. Example:

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# Formalities omitted

def foo(x): return x + 10

my_button = tk.Button(parent_widget, command=foo(10))


Is essentially:

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my_button = tk.Button(parent_widget, command=20)

The "20" is an integer object, which does not have a __call__() method bound to it, so when Tk tries to execute it, it raises "TypeError: 'Object' not callable.".

To remedy this, we utilize lambda. From my understanding, lambda can be simply put as a "function not bound to a name"; importantly this means it returns an object which has the shiny __call__() method! This means we can wrap out "foo(10)" inside a lambda, which will return the object which Tk will call, which will in turn call foo(10). Tada, you've got your callback!

* Okay, it's not a sophisticated answer, but it'll do right?

I don't understand it either, perhaps you could start with a few more basic tips and tutorials for us noobs lol .

Dink wrote:I don't understand it either, perhaps you could start with a few more basic tips and tutorials for us noobs lol .

Yep, i totally agree with you, but i thought i'd cover a wide variety of things, from the basics to UI creation. What sort of basics would you like teaching? I am hoping we could all help each other understand things better.

Python 101, from the ground up... perhaps throw in homework exercises as well etc to complete...

ahh my eyes I don't understand anything lol.

So you want me to start with 'print' statements?

Holiverh wrote:So you want me to start with 'print' statements?

Yes, if you could start with the basics it would be helpful.

I revisited this thread, and updated my original post with a proper (Well, a better...) explanation of why the use of lambdas are required with Tk callbacks.

So you're basically calling the function with the parameter by value?

Dink wrote:So you're basically calling the function with the parameter by value?

Basically: If you set the -command option to "foo()" the interpreter will execute the function and pass the returned value from "foo()" as an argument instead of the function it self.

We wrap it in a "lambda" (unamed function) which returns a callable object which calls "foo()".

Any clearer?