How to make the text fill all the QLabel's space?

Question

I am working on a PyQt5 project, but would be happy to read C++/Qt answer as well, because C++ solutions may work on Python too.

I have a MainWindow with a horizonta

Answer 1

This is a quick sketch that might point to a solution. Instead of deriving from a label, an event filter can be installed on any label to make its text fill the available space. The filter uses the existing scaledContents property, and extends its applicability to text content.

The label's font size is adjusted using Newton's algorithm to fill the available space. Some adjustments would be needed to make it work with labels that enable word wrap; the font size should then never overshoot what fits.

// https://github.com/KubaO/stackoverflown/tree/master/questions/label-text-size-36575192
#include <QtWidgets>

class LabelStretcher : public QObject {
   Q_OBJECT
public:
   LabelStretcher(QObject * parent = 0) : QObject(parent) {
      apply(qobject_cast<QLabel*>(parent));
   }
   void apply(QLabel * label) { if (label) label->installEventFilter(this); }
protected:
   bool eventFilter(QObject * obj, QEvent * ev) Q_DECL_OVERRIDE {
      if (ev->type() != QEvent::Resize) return false;
      auto label = qobject_cast<QLabel*>(obj);
      if (!label || label->text().isEmpty() || !label->hasScaledContents()) return false;
      qDebug() << "pre: " << label->minimumSizeHint() << label->sizeHint() << label->size();

      static auto dSize = [](const QSize & inner, const QSize & outer) -> int {
         auto dy = inner.height() - outer.height();
         auto dx = inner.width() - outer.width();
         return std::max(dx, dy);
      };
      static auto f = [](qreal fontSize, QLabel * label) -> qreal {
         auto font = label->font();
         font.setPointSizeF(fontSize);
         label->setFont(font);
         auto d = dSize(label->sizeHint(), label->size());
         qDebug() << "f:" << fontSize << "d" << d;
         return d;
      };
      static auto df = [](qreal fontSize, QLabel * label) -> qreal {
         if (fontSize < 1.0) fontSize = 1.0;
         return f(fontSize + 0.5, label) - f(fontSize - 0.5, label);
      };

      // Newton's method
      auto font = label->font();
      auto fontSize = font.pointSizeF();
      int i;
      for (i = 0; i < 5; ++i) {
         auto d = df(fontSize, label);
         qDebug() << "d:" << d;
         if (d < 0.1) break;
         fontSize -= f(fontSize, label)/d;
      }
      font.setPointSizeF(fontSize);
      label->setFont(font);
      qDebug() << "post:" << i << label->minimumSizeHint() << label->sizeHint() << label->size();
      return false;
   }
};

int main(int argc, char ** argv) {
   QApplication app{argc, argv};
   QLabel label{"Hello There!"};
   label.setScaledContents(true);
   label.show();
   LabelStretcher stretch(&label);
   return app.exec();
}

#include "main.moc"