Loop function parameters for sanity check

Question

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.

Answer 1

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

Answer 2

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!

Answer 3

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!

Answer 4

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)