[FIXED] prevent typos in variable keyword args

Issue

In Python3 I can do (thanks to pep 3102):

def some_fun(a, *args, log=None, verbose=0):
    pass

and be sure that if I call this with:

some_fun(1, 2, 3, lob=debug_log)

I get a type error on the unexpected keyword argument lob.

On Python2 I cannot define some_fun() with keyword-only arguments after an arbitrary argument list. I have to do:

def some_fun(a, *args, **kw):
    log  = kw.get("log", None)
    verbose = kw.get("verbose", 0)

this works all fine and dandy when called correctly, but I would like to get a type error just as with Python3 when I provide one or more wrong keyword arguments to some_fun().

Solution

Instead of using .get() to retrieve the values, use .pop() and check if kw is empty after popping all keyword-only arguments.

I use a small helper function for this:

def check_empty_kwargs(kwargs):
   import inspect
   try:
      keys = kwargs.keys()
      assert len(keys) == 0
   except AssertionError:
      # uncomment if you want only one of the unexpected kwargs in the msg
      # keys = keys[:1]
      msg = "{0}() got an unexpected keyword argument{1} {2}".format(
         inspect.stack()[1][3], # caller name
         's' if len(keys) > 1 else '',
         ', '.join(["'{0}'".format(k) for k in keys]))
      raise TypeError(msg)

And you would use it like:

def some_fun(a, *args, **kw):
    log  = kw.pop("log", None)
    verbose = kw.pop("verbose", 0)
    check_empty_kwargs(kw)

calling that with (assuming debug_log is defined)

some_fun(1, 2, 3, lob=debug_log)
....
TypeError: some_fun() got an unexpected keyword argument 'lob'

The traceback will (of course) be different from Python3

Answered By - Anthon