How can I tell if variable is defined

[Peter Otten]

Brown, Rodrick  wrote:

> How could I do the following check in Python
> 
> In Perl I could do something like if ((defined($a)) { ... }
> 
> I tried if var is not None:
> However this doesn't seem to work as described.

But in Python this often is the most idiomatic way to check whether a 
variable was set explicitly. Example:

>>> def foo(a=None):
...     if a is None:
...             print "using default"
...     else:
...             print "using a =", a
...
>>> foo()
using default
>>> foo(42)
using a = 42

If you tell us what you are actually trying to do we can probably come up 
with a solution that is better than the literal answer

try:
   a
except NameError:
   a_is_defined = False
else:
   a_is_defined = True

Peter