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

Connecting PyQt4 Slots and Signals in a Loop: Troubleshooting Unexpected Results

When constructing a calculator in PyQt4, connecting button 'clicked()' signals within a loop may lead to unexpected behavior. The issue arises because the expected number is not printed when a button is clicked. Instead, all buttons print the same number.

Understanding the Problem

Python's scoping rules create a closure in this scenario. The loop variable 'i' is used as a parameter for each button's lambda function. However, Python resolves 'i' in the namespace of the enclosing function (__init__()). Since 'i' is modified in the loop and eventually assigned the value of 9, all buttons refer to the same closure, so they all output '9' regardless of which button is clicked.

Resolving the Issue

To fix this issue, we can pass 'i' as a keyword argument with a default value. This introduces a new binding for 'i' in the lambda function's namespace, making it independent of 'i' in __init__().

Solution Using Keyword Argument:

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

Solution Using functools.partial:

Another approach is to use functools.partial, which creates a partial function with pre-bound arguments. This makes the code more readable and eliminates the use of keyword arguments.

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

By implementing these solutions, each button will print the intended number when clicked.

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