question from beginner

[dario]

Hi, Im new on phyton programming.
On my GPRS modem with embedded Phyton 1.5.2+ version, I have to receive
a string from serial port and after send this one enclosed in an
e-mail.
All OK if the string is directly generated in the code. But it doesn't
works if I wait for this inside a 'while' loop. This is the simple
code:

global stringZVEI

  while stringZVEI=='':
      MOD.sleep(10)
      a=SER.send(' sono nel while stringZVEI==st vuota')
      stringZVEI = SER.readbyte()
      a=SER.send(' stringZVEI=')
      a=SER.send(stringZVEI)

MOD and SER are embedded class maked by third part.

>From my very little debug possibility it seem that loop is executed 1
time only nevertheless stringZVEI is still empty. The line
      a=SER.send(' stringZVEI=')
work correctly but

a=SER.send(stringZVEI) 

doesn't work

Any suggestion?

Thanks
Dario.