Why Do My PyQt4 Buttons All Print the Same Value When Connected in a Loop?

Connecting Slots and Signals in PyQt4 in a Loop: Breaking the Binding Chain

In this article, we address a common issue encountered while connecting clicked() signals from buttons created within a loop in PyQt4. We aim to understand why all buttons print out the same value, despite the expectation that each button should trigger a unique action based on its label.

Understanding Scoping and Closure

The behavior observed in the example provided is a consequence of Python's scoping and closure mechanisms. When creating buttons inside a loop, we attempt to connect their clicked() signals to a lambda function. However, Python only introduces new bindings in a namespace through assignment or parameter lists of functions.

Therefore, the lambda function does not have its own binding for the loop variable i, and instead looks for it in the namespace of the enclosing function, __init__(). At the time the lambda is executed, i has already been assigned its final value of 9, leading to all buttons printing out the same output.

Resolution

To address this issue, we can pass i as a keyword argument with a default value to the lambda function. This ensures that i within the lambda is independent of i in __init__(). Alternatively, we can use the functools.partial function, which provides a more readable and less "magic" approach to achieve the same result.

Example with Keyword Argument:

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

Example with functools.partial:

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

By implementing one of these approaches, we can successfully establish the desired binding between each button and its respective action.

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