Pythonic way to check if something exists?
I think you have to be careful with your terminology, whether something exists and something evaluates to False are two different things. Assuming you want the latter, you can simply do:
if not var:
print 'var is False'
For the former, it would be the less elegant:
try:
var
except NameError:
print 'var not defined'
I am going to take a leap and venture, however, that whatever is making you want to check whether a variable is defined can probably be solved in a more elegant manner.
LBYL style, "look before you leap":
var_exists = 'var' in locals() or 'var' in globals()
EAFP style, "easier to ask forgiveness than permission":
try:
var
except NameError:
var_exists = False
else:
var_exists = True
Prefer the second style (EAFP) when coding in Python, because it is generally more reliable.
To check if a var has been defined:
var = 2
try:
varz
except NameError:
print("No varz")
To check if it is None / False
if varz is None
...or
if not varz