Loop function parameters for sanity check

I have a Python function in which I am doing some sanitisation of the input parameters:

def func(param1, param2, param3):
    param1 = param1 or ''
    param2 = param2 or ''
    param3 = param3 or ''

This caters for the arguments being passed as None rather than empty strings. Is there an easier/more concise way to loop round the function parameters to apply such an expression to all of them. My actual function has nine parameters.

From stackoverflow

• You probably want something more like this:

  def func(param1='', param2='', param3=''):
  

  It's far more concise and idiomatic.

  See http://docs.python.org/tutorial/controlflow.html#default-argument-values

  Mat : I have some default argument values in my real function, but I am still concerned where parameters are passed with a value of 'None'. For example, I could call func(None, None).

  mipadi : That won't do what he wants -- you could still pass 'None' as a parameter.

  Kamil Kisiel : Right, sorry, I misunderstood the question. In that case, I recommend Kiv's answer below.

• def func(x='', y='', z='hooray!'):
      print x, y, z
  
  In [2]: f('test')
  test  hooray!
  
  In [3]: f('test', 'and')
  test and hooray!
  
  In [4]: f('test', 'and', 'done!')
  test and done!
  

• You could do some list manipulation:

  def func(param1, param2, param3):
      param1, param2, param3 = map(lambda x: x or '', (param1, param2, param3))
  

  but I'm not sure that's better than just writing out the nine lines, since once you get to nine parameters, that's a heinously long line.

  You could change the declaration of the function:

  def func(*args):
      param1, param2, param3 = map(lambda x: x or '', args)
  

  but then you lose the documentation that comes from having real parameter names, as well as the possibility of changing the defaults, etc. And you still have a pretty fugly line there to unpack them.

  I say write out the nine lines, or change the function to have fewer parameters: nine is kind of a lot anyway!

• This looks like a good job for a decorator. How about this:

  def sanitized(func):
      def sfunc(*args, **kwds):
          return func(*[arg or '' for arg in args],
                      **dict((k, v or '') for k,v in kwds.iteritems()))
      sfunc.func_name = func.func_name
      sfunc.func_doc = func.func_doc
      return sfunc
  

  You would use this on your function like so:

  @sanitized
  def func(param1, param2, param3):
      print param1, param2, param3
  

  Then the parameters will be replaced by the empty string if they are false:

  >>> func('foo', None, 'spam')
  foo  spam
  

  (Note that this will still mess up the function signature as Ned Batchelder points out in his answer. To fix that you could use Michele Simionato's decorator module-- I think you'd just need to add a @decorator before the definition of sanitized)

  Kiv : Does this work with keyword arguments though?

  Kiv : I think you also need to pass along the kwargs too: **dict((k, arg or '') for k, arg in kwargs.items()))

  dF : Fixed to support kwargs, thanks Kiv

  Kiv : Yes, this is by far the best solution (other than refactoring the original code to not require it.)

  Autoplectic : you should probably change the `arg or ''` to `'' if arg is None else arg` or something similar just in case arg is 0 or {} or something non-None.