Cannot fix PyCharm warnings in simple PyQt program

Question

Cannot fix PyCharm warnings in simple PyQt program

The following program (excerpt from my real code)

from PyQt4 import QtGui
import sys
from PyQt4.QtGui import QMessageBox


def main():
    app = QtGui.QApplication([])

    w = QtGui.QPushButton("Test")

    def on_pressed():
        print("Pressed")
        QMessageBox.warning(w, 'Info', 'Button Pressed')

    w.pressed.connect(on_pressed)
    w.show()

    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

triggers the following three warnings in PyCharm (e.g. at startup Code/Inspect Code...) related to a callQMessageBox.warning

Calling a method on a class using an instance of another class; Passing PyQt4.QtGui.QPushButton.QPushButton instead of PyQt4.QtGui.QMessageBox.QMessageBox. Is it intentional?
Invalid call arguments; QString_1 is empty
Type check; The expected type is "QMessageBox", instead "QPushButton"

and one warning related to calling PyQt connect