I was wondering if there is any way to get around using global variables in the callback functions that are used in bind in tkinter.
What I refer to is:
canvas = Canvas(root, width=500, height=500)
canvas.bind('<B1-Motion>', func)
where func is now some function that is triggered when the mouse is dragged. What I want is something like:
canvas.bind('<B1-Motion>', func(arg))
In combination with:
def func(event, arg):
commands
I can see from https://docs.python.org/3/library/tkinter.html that one argument, which is the event itself, is given to the callback function, but it seems like waste of potential to not give this method any way to modify its callback in a different way.
Maybe I am mistaken and there is some technical reason why that is impossible in general or maybe there is an alternative to bind.
I was basically expecting something like:
buttoname = Button(...,...,..., command = Lambda: func(arg))
If anyone has any pointers, it would be much appreciated.
regards
Use a lambda that receives the event parameter and passes it along.
canvas.bind('<B1-Motion>', lambda e: func(e, arg))
Related
Is it possible to have arguments and an event listener on a function? I have two entries that I want to clear on <FocusIn>. I thought it would be simple and bind delete like self.minutes.bind("<FocusIn>", self.minutes.delete(0, "end)), but no that would be too easy. So I created a function to wipe any entry I want whenever focused. My function is simply:
def entry_clear(entry, e):
entry.delete(0, "end")
This results in entry_clear() missing 1 required positional argument: 'e' but if I use it with self it works fine like:
def entry_clear(self, e):
self.minutes.delete(0, "end")
But of course now I have to specify the exact entry I want in the function, rather than being used for any entry. Thanks for any help.
You do not really need to pass the widget itself because tkinter passes an Event object implicitly with bind. This Event object has an attribute called widget which will be the widget that originally triggered the event. So you can just delete the items of that widget directly:
def entry_clear(self, e): # `e` is `Event` object
e.widget.delete(0, "end")
Now to answer your original question:
Is it possible to have arguments and an event listener on a function?
Yes it is possible, but it depends on how you use bind, a fairly common way is like:
ent.bind('<1>', lambda event: callback(event, ent))
Instead of this, I would always use the first method.
This question already has answers here:
Why is my Button's command executed immediately when I create the Button, and not when I click it? [duplicate]
(5 answers)
Closed 1 year ago.
I would like to understand how a button is working using lambda.
I have the following Python code:
from tkinter import *
def comando_click(mensagem):
print(mensagem)
menu_inicial = Tk()
menu_inicial.geometry("500x250+200+200")
botao = Button(menu_inicial, text = "Executar", command=comando_click("Nova_Mensagem"))
botao.pack()
menu_inicial.mainloop()
But my button doesn't work when I click on it, it only shows the print once in the console when I run the code, I added some prints here in the question:
Problem Picture one
Well it seems that when I use the Lambda function in the button it works and I really would like to know why.
Lambda working button Picture one
I just added to the button the lambda :
botao = Button(menu_inicial, text = "Executar", command=lambda:comando_click("Nova_Mensagem"))
Lambda working button Picture two
Why with lambda it works?
It shoudn't work without lambda too since lambda is basically a anonymous function?
I am extremely curious to understand why it works, thank you all for the help :)
Edit: I would like to thank you guys, now I finally understand what was going on and how Python was working. Thank you all very much :D
When you use () with a function name(func(args)), then it is immediately calling/invoking the function while python is executing the line, you do not want that. You want to ONLY call the function when the button is clicked. tkinter will internally call the function for you, all you have to do is give the function name.
Why use lambda? Think of it as a function that returns another function, your code can be lengthened to:
func = lambda: comando_click("Nova_Mensagem")
botao = Button(menu_inicial, text = "Executar", command=func)
func is the function name and if you want to call it, you would say func(). And when you say command=comando_click("Nova_Mensagem") then command has the value returned by command click(because you call the function with ()), which is None and if I'm not wrong, if the given value is None, it will not be called by tkinter. Hence your function is executed just once because of () and as a result of calling the function, you are assigning the value of the function call(None) before the event loop starts processing the events.
Some other methods:
Using partial from functools:
from functools import partial
botao = Button(.....,command=partial(comando_click,"Nova_Mensagem"))
Using a helper function:
def helper(args):
def comando_click():
print(args)
return comando_click
botao = Button(...., command=helper("Nova_Mensagem"))
IMO, lambdas are the easiest way to proceed with calling a function with arguments.
In this code:
command=comando_click("Nova_Mensagem")
you have called the comando_click function, once, and assigned the result (None) to the command argument. Nothing will happen when command is called (in fact you should get a TypeError exception because None is not callable).
In this code:
command=lambda:comando_click("Nova_Mensagem")
you have not actually called comando_click yet -- you have created a new function (using lambda) that will in turn call comando_click when it is called. Every time the button is clicked, your new function will get called.
If the lambda is confusing, you can do the exact same thing with a def like this:
def button_command():
comando_click("Nova_Mensagem")
...
command=button_command # no ()! we don't want to actually call it yet!
The lambda expression is just an alternative to using def when you want to create a small single-use function that doesn't need a name (e.g. you want to make a function that calls another function with a specific argument, exactly as you're doing here).
The issue is that with comando_click("Nova_Mensagem") you are executing the function. So command=None.
In the second case lambda:comando_click("Nova_Mensagem") is returning a lambda, that internally calls comando_click("Nova_Mensagem").
Fix: just put command=comando_click.
If you want to personalize the lambda with arguments you could write something like this:
def handler(args):
def custom_handler():
print(args)
return custom_handler
botao = Button(menu_inicial, text = "Executar", command=handler("my custom string"))
I have managed to get a widget to appear by calling a function, then make it disappear by using the destroy method.
Unfortunately, the only way I managed to do that is by making the object global, which I understand is a bad way of doing things. I have tried several times to destroy the object without using global, but none of them worked.
Here's the relevant code:
def hayHijos():
print("ii")
z=var9.get()
if z==1:
var10 = StringVar()
global numHijosMat
numHijosMat=OptionMenu(app,var10,1,2,3,4,5,6,7,8,9,10)
numHijosMat.grid(column=2)
elif z==0:
print("aa")
numHijosMat.destroy()
var9 = IntVar()
hijosEntrePartes = Checkbutton(app, text="Hijos entre las partes", variable=var9, command=hayHijos)
hijosEntrePartes.var=var9
hijosEntrePartes.grid()
Two general possibilities.
Either you create a class context to keep track of elements such as widgets using class variables (self.widget for example). Therefor have a look at the class documentation
You return and pass the widget to / from your function.
This is not very suitable for callbacks but a general approach
def function(var, widget=None):
""" Widget is an optional argument. If not passed, it is initialized with None """
if var.get()==1:
# some code to create your widget
# make sure to use 'widget = ...' to create it
# to have the reference for your return
# call at the end of the function
else:
# some code to destroy your widget
# e.g. widget.destroy()
return widget
Using this code makes it easy for you to use the widget without making it global. Another variable is used in a global behaviour in your code. "var9". You should also pass this one on.
You would need to adapt your callback using maybe a lambda to pass both.
More recommended would be the class approach over here as lambdas often lack in scope of readability of code and reusability of code. (Could call it also "bad habit" but IMHO this is highly influenced by use case)
If you want to reuse the widget and only want to make it appear / disappear as the use of a Checkbutton suggests, I would rather recommend grid_remove method instead of widget.destroy method
How can I get a returned object from a function that is executed as a Tkinter callback?
import Tkinter as Tk
from functools import partial
def square(x):
return x*x
root = Tk.Tk()
var = Tk.IntVar(root, value=0) #the variable the gets passed to the class call
menu = Tk.OptionMenu(root, var, *[0,1,2,3,4,5]) #a drop-down list to choose a value for the variable
menu.pack()
button = Tk.Button(root, text='click', command = partial(square,var.get())) #a button that calls the class
button.pack()
root.mainloop()
Obviously this is a simplified example. In reality the function called by the button will return objects, which I wish to append to a list of objects that will be held in the main Python namespace for further operations.
Anyway, here the user is able to choose an argument for the function using a GUI, and press a button that will execute the function. The return value of the function, however, seems doomed to be lost to the aether, since the callback won't accept returns. Can this be overcome without the use of an ugly global in the definition of square(x)?
The notion of "returning" values from callbacks doesn't make sense in the context of an event driven program. Callbacks are called as the result of an event, so there's nowhere to return a value to.
As a general rule of thumb, your callbacks should always call a function, rather than using functools.partial or lambda. Those two are fine when needed, but if you're using an object-oriented style of coding they are often unnecessary, and lead to code that is more difficult to maintain than it needs to be.
For example:
def compute():
value = var.get()
result = square(value)
list_of_results.append(result)
button = Tk.Button(root, text='click', command = compute)
...
This becomes much easier, and you can avoid global variables, if you create your application as a class:
class App(...):
...
def compute():
...
result = self.square(self.var.get())
self.results.append(result)
Sorry for being 6 years late, but recently I figured out a good way to do this without making your code messy and hard to maintain.
This is pretty much what DaveTheScientist has said, but I just want to expand on it a little.
Usually, in Tkinter, if you want to have a button call a function, you do the following:
exampleButton = Button(root, text = 'Example', command = someFunc)
This will simply call someFunc whenever the button is pressed. If this function, however, takes arguments, you need to use lambdas and do something like this:
exampleButton = Button(root, text = 'Example', command = lambda: someFunc(arg1, arg2))
The above line of code will run someFunc and use the variables arg1 and arg2 as arguments for that function. Now, what you could do in a program where, a lot of the times, you would need the functions run by buttons to return values, is create a new function which is called by every button.
This function takes the function you want your button to run as a first argument, and that function's arguments afterwards.
def buttonpress(function, *args):
value = function(*args)
Then when you create the button, you do:
exampleButton = Button(root, text = 'Example', command = lambda: buttonpress( someFunc, arg1, arg2 ))
This will run the given function (in this case, someFunc) and store the returned value in the value variable. It also has the advantage that you can use as many arguments as you want for the function your button runs.
Just create an actual function that is called by your button, instead of putting it all inline like that.
button=Tk.Button(parent, text='click', command=someFxn)
def someFxn(): your code
Then in your function just call the var.get(), do your calculation, and then do something with the value.
I was surprised when a function that I wrote in python/tkinter and binded to Ctrl-b behaved strangely (specific: it was losing the value of the selected text, so that text.index(SEL_FIRST) was undefined).
I was surprised when, after changing the more improbable things I binded it instead to something else - and it worked!
I searched but didnt find anything: is Control-b binded to something default in tkinter???
alessandro
If you are talking about the text widget, from the official tk text widget documentation:
"The Left and Right keys move the
insertion cursor one character to the
left or right; they also clear any
selection in the text...Control-b and
Control-f behave the same as Left and
Right, respectively."
Thank you Bryan, here's the link - obviously on effbot http://effbot.org/tkinterbook/tkinter-events-and-bindings.htm:
(...)
You could use the bind_class method to
modify the bindings on the class
level, but that would change the
behavior of all text widgets in the
application. An easier solution is to
prevent Tkinter from propagating the
event to other handlers; just return
the string “break” from your event
handler:
def ignore(event):
return "break"
text.bind("<Return>", ignore)
or
text.bind("<Return>", lambda e: "break")
By the way, if you really want to
change the behavior of all text
widgets in your application, here’s
how to use the bind_class method:
top.bind_class("Text", "<Return>", lambda e: None)
But there are a lot of reasons why you
shouldn’t do this. For example, it
messes things up completely the day
you wish to extend your application
with some cool little UI component you
downloaded from the net. Better use
your own Text widget specialization,
and keep Tkinter’s default bindings
intact:
class MyText(Text):
def __init__(self, master, **kw):
apply(Text.__init__, (self, master), kw)
self.bind("<Return>", lambda e: "break")