Determine if variable is defined in Python [duplic

问题:

Possible Duplicate:
Easy way to check that variable is defined in python?
How do I check if a variable exists in Python?

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 or defined? in Ruby.

if condition:
    a = 42

# is "a" defined here?

if other_condition:
    del a

# is "a" defined here?

回答1:

try:
  thevariable
except NameError:
  print "well, it WASN'T defined after all!"
else:
  print "sure, it was defined."

回答2:

'a' in vars() or 'a' in globals()

if you want to be pedantic, you can check the builtins too
'a' in vars(__builtins__)

回答3:

I think it's better to avoid the situation. It's cleaner and clearer to write:

a = None
if condition:
    a = 42

回答4:

try:
    a # does a exist in the current namespace
except NameError:
    a = 10 # nope

回答5:

One possible situation where this might be needed:

If you are using finally block to close connections but in the try block, the program exits with sys.exit() before the connection is defined. In this case, the finally block will be called and the connection closing statement will fail since no connection was created.

回答6:

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.

回答7:

If one wants to catch attempts to access a not-defined variable inside an object, there is a very easy way of doing that:

class Whatever(object):
  def __getattr__(self, key):
    return None

Here, python first tries to find an attribute within the object or the object tree, and only if that fails the __getattr__(self, key) function is called. This means, if __getattr__ is called we can simply return None.