Application window geometry specifier

[Rich Shepard]

I want my application's window (main frame) to open centered on the
monitor's screen. This code:
 	# open application centered on screen; set window width and height
         self.appwidth = 600
         self.appheight = 500
         # get screen width and height
         self.scrwidth = self.winfo_screenwidth()
         self.scrheight = self.winfo_screenheight()
         # formula to find screen center
         self.xLeft = (self.scrwidth/2) - (self.appwidth/2)
         self.yTop = (self.scrheight/2) - (self.appheight/2)
         # set geometry
         self.geometry(str(self.appwidth) + "x" + str(self.appheight) +
                       "+" + str(self.xLeft) + "+" + str(self.yTop))

generates this error when run:
  File "/usr/lib64/python3.9/tkinter/__init__.py", line 2036, in wm_geometry
     return self.tk.call('wm', 'geometry', self._w, newGeometry)
_tkinter.TclError: bad geometry specifier "600x500+340.0+262.0"

As the geometry string represents window width times window height plus x-offset
plus y-offset I'm not seeing my error.

Please show me what's wrong with specifier.

TIA,

Rich