Test the existence of a variable?

[Thierry S.]

Hello.

I would to test the existence of a variable before to use it (like
isset($myVar) in PHP).
I try using "if myVar: ", but there is the error meesage (naturally):
"NameError: name 'myVar' is not defined"

Please, could you tell me what for function exist to test the variable with
Python?

Regards,
-- 
Thierry S.