Why Does Using Lambda Expressions to Connect Multiple Buttons to the Same Slot in PyQt Lead to Unexpected Behavior?

Using Lambda Expressions to Connect Slots in PyQt

In Qt, lambda expressions can be used to connect signals to slots. However, there are some limitations to this approach that can lead to unexpected behavior.

Problem:

When connecting multiple buttons to the same slot using a lambda expression, only the last connection works correctly. Specifically, the buttons connected manually through individual calls to connect() are successful, while those connected in a loop using a lambda expression all yield the same incorrect result.

Analysis:

The issue arises from the fact that the lambda expression is evaluated when the signal is fired, and the value of its optional argument is overwritten by the signal's argument. In the case of the QPushButton.clicked signal, this argument represents the state of the button.

Solution:

To resolve the problem, the lambda expression should be modified as follows:

button.clicked.connect(lambda state, x=idx: self.button_pushed(x))

By adding the state as the first argument to the lambda expression, it can be ignored while still allowing the intended value of idx to be passed to the slot.

Understanding Lambda Expression Connection:

When a lambda expression is used to connect a signal, it creates an anonymous function that is evaluated whenever the signal is fired. The parameters of the lambda expression are bound to the arguments of the signal, in this case the state of the button. The slot is then called with the result of the lambda expression as its argument.

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