tkinter credit card scan

[phil]

Problem, the focus is on a field which has contents
and I scan the credit card card, which is keyboard input.
The field is altered and a bunch of chars fly acroos the
field.  messy.

Ok, I can solve this in a very clumsy manner.
1. I see the first character of the scan, '%',
configure the field to show ' ' temporarily,
do a get() and after the scan is thru, restore
the contents. clumsy and messy, but ok.

The WORST PROBLEM, is if the previous contents
of the field had been selected, which it would be
if it is the first field in the payment window or
they had tabbed to the field.  The the ist char
of the scan, the '%' replaces the contents before I
can do a get().

Suggestions?