class test:
a="hi"
def msg(self):
print the variable for which the object is referring to
t= test()
print t.b
From the above code is there any way to tell that the object is referring to a variable b which does not exist
Use the __getattr__ method, see documentation:
In [1]: class test(object):
...: a = 'hi'
...: def __getattr__(self, val):
...: print 'you are accessing ', val
...:
...:
In [2]: t = test()
In [3]: t.b
you are accessing b
In [4]: t.c
you are accessing c
In [5]: t.a
Out[5]: 'hi'
EDIT:
class test(object):
a = 'hi'
def msg(self, var, default='undefined'):
setattr(self, var, default)
return default
def __getattr__(self, val):
print 'attribute %s not found, setting..' % val
return self.msg(val)
>>> t = test()
>>> print t.a
'hi'
>>> print t.b
'attribute b not found, setting..'
'undefined'
>>> t.b = 'this is black magic'
>>> # notice no message is printed here about attribute not found
>>> print t.b
'this is black magic'
EDIT2:
>>> d = {'a': '1'}
>>> d.setdefault('b', 'b')
'b'
>>> d
{'a': '1', 'b': 'b'}
>>> d.setdefault('a', 'b')
'1'
>>> d
{'a': '1', 'b': 'b'}
Yes. You will get a NameError on the print b line, and an AttributeError on the print t.b line.
You can catch these exceptions like this:
try:
print t.b
except AttributeError as e: # or `except AttributeError, e` on Python < 2.6
# Do something...
This should do it.
class tester(object):
a = 'hi'
def __getattr__(self, val):
print 'accessing attribute %s failed!' % val
>>> t = tester()
>>> t.a
'hi'
>>> t.b
accessing attribute b failed!
>>>
EDIT: Removed some redundant code
If that's all the code, then of course there is no b. But, I assume that you want us to assume there could be more code. If there is, or perhaps if this code is imported as a module by other code, then there is no way to tell until it is run.
All members of an object are stored in that objects __dir__. So you can just do this:
>>> "msg" in dir(t)
True
>>> "b" in dir(t)
False
This works for the standard case, and may behave differently if descriptors are involved, or __getattr__ or __getattribute__ are overloaded.
__dir__ is a way for an object to report the attributes it wants you to know about.
AttribuiteErrorif you refer a variable which does not exist.