How do you know whether a variable has been set at a particular place in the code at runtime? This is not always obvious because (1) the variable could be conditionally set, and (2) the variable could be conditionally deleted. I'm looking for something like defined() in Perl or isset() in PHP.
if condition:
a = 42
# is a defined here?
if other_condition:
del a
# is a defined here?
try:
a # does a exist in the current namespace
except NameError:
a = 10 # nope
try:
thevariable
except NameError:
print "well, it WASN'T defined after all!"
else:
print "sure, it was defined."
Despite the unexplained downvotes, try/except/else is nevertheless the right way to handle anomalous situations in Python. If you trust the anonymous, silent downvoters more than you trust me, go ahead and do it wrong; otherwise, you might want to consider this suggestion as it deserves!-)
'a' in vars() or 'a' in globals()
if you want to be pedantic, you can check the builtins too
'a' in vars(__builtins__)
I think it's better to avoid the situation. It's cleaner and clearer to write:
a = None
if condition:
a = 42
For this particular case it's better to do a = None instead of del a. This will decrement reference count to object a was (if any) assigned to and won't fail when a is not defined. Note, that del statement doesn't call destructor of an object directly, but unbind it from variable. Destructor of object is called when reference count became zero.
The correct answer to the question, "How do I discover if a variable is defined in python?" is the not-very-useful but entirely-pythonic answer:
Read the source file starting at the point where you want to discover if a variable is defined, going up in the file to the start of the source file. If you don't see the variable defined, then it is not defined.
To answer the real question you're asking, "Why is it not obvious how to deal with the situation where I have used the del keyword?"
Don't use the del keyword. It is not useful.