Shadow Detection?

[Michael Yanowitz]

Hello:

Many times, people are warning things like
"Don't use 'str' as a variable name as it will shadow the
built in str function."
Is there some way to determine if a string is already
defined in some higher scope?
Maybe something like
<code>

if isdefined ('str'):
print 'str is already defined, please choose another name'

</code>

If not, would it be easy to write this?

Thanks in advance: