Change singleStep in a QDoubleSpinBox depending on the cursor position when using arrow keys

Probably the easiest way to achieve this is to make a subclass of QDoubleSpinBox and override the virtual stepBy() method. That is called every time the value is to be stepped up or down. Here is a simple example that shows how to do it for an integer spin box. For a floating point value, one just has to pin the singleStep size to the minimum decimal step size, and parse the text a bit more carefully in stepBy(), taking the decimal point into account.

class MySpinBox : public QSpinBox
{
public:
    explicit MySpinBox(QWidget *parent = nullptr)
        : QSpinBox(parent) {
        setMaximum(9999);
        setValue(5000);
    };

    void stepBy(int steps) override {
        const int cursorPos = qMax(lineEdit()->cursorPosition(), 1);
        const int textLen = lineEdit()->text().length();
        for (int i = textLen - cursorPos; i > 0; i--)
            steps *= 10;
        QAbstractSpinBox::stepBy(steps);
        lineEdit()->setCursorPosition(cursorPos);
    }
};

Following aavit's answer I did also redefine stepBy within PyQt. Since I am looking for a solution that works for floats and negative values, I had to handle some special cases and I display the + sign for positive values by redefining textFromValue. Special cases are commented in the solution below:

import sys
from PyQt5.QtWidgets import QDoubleSpinBox, QMainWindow, QApplication, QLabel


class MyQDoubleSpinBox(QDoubleSpinBox):
    def textFromValue(self, value):
        # show + sign for positive values
        text = super().textFromValue(value)
        if value >= 0:
            text = "+" + text
        return text

    def stepBy(self, steps):
        cursor_position = self.lineEdit().cursorPosition()
        # number of characters before the decimal separator including +/- sign
        n_chars_before_sep = len(str(abs(int(self.value())))) + 1
        if cursor_position == 0:
            # set the cursor right of the +/- sign
            self.lineEdit().setCursorPosition(1)
            cursor_position = self.lineEdit().cursorPosition()
        single_step = 10 ** (n_chars_before_sep - cursor_position)
        # Handle decimal separator. Step should be 0.1 if cursor is at `1.|23` or
        # `1.2|3`.
        if cursor_position >= n_chars_before_sep + 2:
            single_step = 10 * single_step
        # Change single step and perform the step
        self.setSingleStep(single_step)
        super().stepBy(steps)
        # Undo selection of the whole text.
        self.lineEdit().deselect()
        # Handle cases where the number of characters before the decimal separator
        # changes. Step size should remain the same.
        new_n_chars_before_sep = len(str(abs(int(self.value())))) + 1
        if new_n_chars_before_sep < n_chars_before_sep:
            cursor_position -= 1
        elif new_n_chars_before_sep > n_chars_before_sep:
            cursor_position += 1
        self.lineEdit().setCursorPosition(cursor_position)


class Window(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setGeometry(100, 100, 600, 400)
        self.UiComponents()
        self.show()

    def UiComponents(self):
        self.spin = MyQDoubleSpinBox(self)
        self.spin.setGeometry(100, 100, 150, 40)
        self.spin.setRange(-10000, 10000)
        self.spin.setValue(50)
        self.spin.setKeyboardTracking(False)


if __name__ == "__main__":
    App = QApplication(sys.argv)
    window = Window()
    sys.exit(App.exec())