I'm new to Python and trying to write a program with tkinter. Why is the Hello-function below executed? As I understand it, the callback would only be executed when the button is pressed? I am very confused...
>>> def Hello():
print("Hi there!")
>>> hi=Button(frame,text="Hello",command=Hello())
Hi there!
>>>
It is called while the parameters for Button are being assigned:
command=Hello()
If you want to pass the function (not it's returned value) you should instead:
command=Hello
in general function_name is a function object, function_name() is whatever the function returns. See if this helps further:
>>> def func():
... return 'hello'
...
>>> type(func)
<type 'function'>
>>> type(func())
<type 'str'>
If you want to pass arguments, you can use a lambda expression to construct a parameterless callable.
>>> hi=Button(frame, text="Hello", command=lambda: Goodnight("Moon"))
Simply put, because Goodnight("Moon") is in a lambda, it won't execute right away, instead waiting until the button is clicked.
def func(par1, par2): in the latter, you simply use the function name without parenthesis, as I outlined in my answer. Since you are admittedly new to python, may I suggest this reading? It's easy to follow and very enjoyable.subprocess module, for example). If this is not an option, what you probably want is using closures. A nice article about their implementation in python is this one.You can also use a lambda expression as the command argument:
import tkinter as tk
def hello():
print("Hi there!")
main = tk.Tk()
hi = tk.Button(main,text="Hello",command=lambda: hello()).pack()
main.mainloop()
