• From C++ QLineEdit::setReadOnly() is not an overridable, virtual function.
• From PyQt, there is a "wrapper" around the whole of QLineEdit, and that has chosen to supply a function, which happens to also be named setReadOnly [but could have been called anything, e.g. pyqtSetReadOnly], which (in Python) is a virtual, overridable function.
• PyQt's QLineEdit.setReadOnly() presumably calls the base C++ QLineEdit::setReadOnly() to effect its work.

The big difference then is:

• In the case of a genuine virtual Qt C++ function, which I have chosen to override in PyQt, if Qt code internally calls the virtual function from itself it will hit my Python override.
• But in the case of this non-virtual QLineEdit::setReadOnly(), any internal Qt C++ calls to it will not be affected by the "override" I have defined from Python.

Does the above sound right?

EDIT I confirmed with the PyQt creator that this is indeed the way it works.

• From C++ QLineEdit::setReadOnly() is not an overridable, virtual function.
• From PyQt, there is a "wrapper" around the whole of QLineEdit, and that has chosen to supply a function, which happens to also be named setReadOnly [but could have been called anything, e.g. pyqtSetReadOnly], which (in Python) is a virtual, overridable function.
• PyQt's QLineEdit.setReadOnly() presumably calls the base C++ QLineEdit::setReadOnly() to effect its work.

The big difference then is:

• In the case of a genuine virtual Qt C++ function, which I have chosen to override in PyQt, if Qt code internally calls the virtual function from itself it will hit my Python override.
• But in the case of this non-virtual QLineEdit::setReadOnly(), any internal Qt C++ calls to it will not be affected by the "override" I have defined from Python.

Does the above sound right?

EDIT I confirmed with the PyQt creator that this is indeed the way it works.

Python is an entirely different beast. You can modify any element of an object the way you want.

1. The selector reference you provided is for CSS, which I know about. My questions is about Qt's QSS, which has "invented" the :read-only pseudo-state.

2. I know all about virtual functions. My question is: is/how come http://doc.qt.io/qt-5/qlineedit.html#readOnly-prop does not show virtual anywhere, yet it is overridable at least from PyQt?

@SGaist
Thanks, I'm changing over to use QWidget::changeEvent.

Any comment on either of the questions in my second post above:

1. Is QSS QLineEdit:read-only simply the same as/a shortcut for QLineEdit[readonly="true"]?

2. Am I supposed to know from Qt C++ documentation that, e.g. QLineEdit::setReadOnly() is overridable, because I don't see virtual? My PyCharm shows it as "overridable" for me from PyQt, but I don't see where that correspond to the C++ documentation?

• 1. It's possible both of these selection works as similar but they are differente methods of selection -> Read More: https://webplatform.github.io/docs/guides/advanced_selectors_guide/
• 2. Yes, you can reimplement this function, but it is good to know why some functions are implemented as Virtual -> Read More: https://stackoverflow.com/questions/2391679/why-do-we-need-virtual-functions-in-c

Any comment on either of the questions in my second post above:

1. Is QSS QLineEdit:read-only simply the same as/a shortcut for QLineEdit[readonly="true"]?

2. Am I supposed to know from Qt C++ documentation that, e.g. QLineEdit::setReadOnly() is overridable, because I don't see virtual? My PyCharm shows it as "overridable" for me from PyQt, but I don't see where that correspond to the C++ documentation?

You can use an event filter and listen for the ReadOnlyChange event on your QLineEdit objects.

Hope it helps.

1. In my stylesheet I could have written either one of:

QLineEdit:read-only { ... }

or

QLineEdit[readonly="true"] { ... }

for my rule. It didn't seem to matter which, they both respond to the result (after update forcing!) of setReadOnly(). Are they indeed the same, or is there a subtle difference?

2. Above I said a possible solution would be

subclass all my QLineEdits so that I can override setReadOnly()

Now, I write in Python/PyQt, not C++, for my sins. From the (C++) documentation I can reach from page http://doc.qt.io/qt-5/qlineedit.html, Public Functions, click on link void setReadOnly(bool), I get taken to http://doc.qt.io/qt-5/qlineedit.html#readOnly-prop, which describes a "Property". I did not see the definition of setReadOnly() saying virtual somewhere. So can I override it in a derived class? Is there something special about these "properties". Anything special Python/PyQt here?

Thanks.