CODEWITHSUNDEEP
pythonooppython-2.7decoratorpython-decorators
Chris vCB
Chris vCB

Reputation: 1053

Decorator on methods checking for instance variable

I have a python class, Something. I would like to create a method, borrowed, which would check for whether the instance variable blue of Something is None.

How would I create @check_none on the instance method so that it would check for an instance variable? Using self in the decorator function did not work ;(

Example:

def check_token(func):
    def inner(*args, **kwargs):
        if self.token == None:
            raise ValueError
        else:
            return func(*args, **kwargs)
    return inner

class Something(object):
   def __init__(self, token=None):
      self.token = token

   @check_token
   def testfunction(self):
      print "Test"

Yields a global name 'self' is not defined error.

Upvotes: 0

Views: 1575

Answers (1)

Martijn Pieters
Martijn Pieters

Reputation: 1123420

Your inner function doesn't have a self argument; add it in and pass it on:

def check_token(func):
    def inner(self, *args, **kwargs):
        if self.token is None:
            raise ValueError
        else:
            return func(self, *args, **kwargs)
    return inner

The inner function replaces the original method when decorated and is passed in the same self argument.

Alternatively, you can use args[0].token, since self is simply the first positional argument.

Note that I replaced your == None test with the recommended is None test.

Demo:

>>> def check_token(func):
...     def inner(self, *args, **kwargs):
...         if self.token is None:
...             raise ValueError
...         else:
...             return func(self, *args, **kwargs)
...     return inner
... 
>>> class Something(object):
...    def __init__(self, token=None):
...       self.token = token
...    @check_token
...    def testfunction(self):
...       print "Test"
... 
>>> Something().testfunction()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in inner
ValueError
>>> Something('token').testfunction()
Test

Upvotes: 4

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