QtGui.QOpenGLFunctions is missing binding to glShaderSource()

AttributeError: 'PySide2.QtGui.QOpenGLFunctions' object has no attribute 'glShaderSource'

from OpenGL import GL
from PySide2 import QtGui


if __name__ == "__main__":
    app = QtGui.QGuiApplication()
    off_screen = QtGui.QOffscreenSurface()
    off_screen.create()
    if off_screen.isValid():
        context = QtGui.QOpenGLContext()
        if context.create():
            context.makeCurrent(off_screen)

            ###
            f = QtGui.QOpenGLFunctions(context)
            shader = f.glCreateShader(GL.GL_VERTEX_SHADER)
            f.glShaderSource(shader, "") # <-AttributeError
            f.glCompileShader(shader)
            ####