Connecting Slots and Signals in PyQt4 Within a Loop

In PyQt4, establishing connections between slots and signals is a fundamental aspect of event handling. However, when attempting to connect multiple signals emitted from buttons within a loop, unexpected behavior may arise.

To illustrate this issue, consider the following code:

def __init__(self):
    for i in range(0, 10):
        self._numberButtons  = [QPushButton(str(i), self)]
        self.connect(self._numberButtons[i], SIGNAL('clicked()'), lambda: self._number(i))

def _number(self, x):
    print(x)

Upon clicking any of the buttons, the output consistently displays '9', regardless of which button was pressed. Understanding the reason behind this behavior involves exploring the intricacies of Python's scoping rules.

In Python, the scoping of a variable is determined by the nearest enclosing function. In this example, both the lambda function and the '_number' function are defined within the '__init__' function. Since the loop assigns the same value ('9') to 'i' for each iteration, the lambda function always captures the last value assigned to 'i' within the '__init__' function, which is '9'.

To resolve this issue, one approach is to pass 'i' as a keyword argument with a default value within the lambda function, ensuring that a separate binding of 'i' is created for each instance.

self._numberButtons[i].clicked.connect(lambda checked, i=i: self._number(i))

Alternatively, functools.partial can be employed to create a specialized function that binds 'i' to the specific value associated with each button.

self._numberButtons[i].clicked.connect(partial(self._number, i))

These modifications guarantee that each callback function captures the correct value of 'i', enabling appropriate responses to button clicks.