When is the Button\'s \'command\' Parameter Function Executed?

Button's "command" Parameter Execution at Declaration: Unraveling the Mystery

In Python tkinter, the "command" parameter for the Button widget is intended to specify a function to be executed when the button is pressed. However, a common misconception among beginners is the observation that the function associated with "command" is executed even at the time of button declaration.

To understand this behavior, we need to delve into how Python handles function parameters. When you pass a function as a parameter, there are two possibilities:

• Passing the function object: This is achieved by simply referencing the function name, such as command=Hello.
• Passing the function's return value: This is done by enclosing the function call in parentheses, such as command=Hello().

In the example code you provided, Hello() is passed as a parameter to the Button widget, which means that the function is executed immediately, and its return value is passed to "command." Since Hello() does not return anything, it returns None, which is then assigned to the button's "command" parameter, effectively doing nothing.

To resolve this issue and execute the function only when the button is clicked, you should pass the function object, like this:

<code class="python">hi = Button(frame, text="Hello", command=Hello)</code>

This way, when the button is pressed, the Hello() function will be called and will execute its intended code.

Additionally, if you need to pass arguments to the function, you can use a lambda expression to wrap the function call, as demonstrated below:

<code class="python">hi = Button(frame, text="Hello", command=lambda: Goodnight("Moon"))</code>

In this case, the lambda expression ensures that the Goodnight() function is not executed at the time of button declaration, but rather waits until the button is clicked and the command is executed.

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