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From the documentation, I think that argparse should pass the entire nargs-long string to the "type=" callable. Instead, it only passes the first argument (of nargs), making it impossible within argparse's framework to check for a tuple of mixed types, e.g., 2 ints and a float (my case), or a string and three numbers, etc.
See attached reproducer.

Nope. The documentation says:
 N (an integer). N arguments from the command line will be gathered together into a list
The type function is applied to each argument independently. It would be easy enough to make this explicit in one of the nargs multi-argument examples by using type as well.

Note that 
 '-t 1 2 3'.split()
becomes
 ['-t', '1', '2', '3']
Your 'type' function sees those 3 strings individually. Try printing 'string' the first thing in your function to see what we mean.

Yes, I know. My function just sees '1', but I think it should see '1 2 3' so that it can figure out what to do. That's impossible (well, impossible without saving state between calls) when it sees the arguments piecemeal. 
Sent from my iPhone
> On Jul 24, 2014, at 9:42 PM, paul j3 <report@bugs.python.org> wrote:
> 
> 
> paul j3 added the comment:
> 
> Note that 
> 
> '-t 1 2 3'.split()
> 
> becomes
> 
> ['-t', '1', '2', '3']
> 
> Your 'type' function sees those 3 strings individually. Try printing 'string' the first thing in your function to see what we mean.
> 
> ----------
> nosy: +paul.j3
> 
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue22049>
> _______________________________________

What you want is a custom Action rather than a custom Type. 
from the documentation:
 >>> class FooAction(argparse.Action):
 ... def __call__(self, parser, namespace, values, option_string=None):
 ... print('%r %r %r' % (namespace, values, option_string))
 ... setattr(namespace, self.dest, values)
'values' will be the list ['1','2','3'], which you test and manipulate, before finally saving it to the 'namespace'.
 ret = (int(values[0]), int(values[1]), float(values[2]))
 setattr(namespace, self.dest, ret)
Setting 'nargs=3' ensures that this action will always get a 3 item list. If the parser can't give it 3 items, it will raise an error rather than call your Action.
'optparse' passed the remaining argument strings to Option's callback, which could consume as many as it wanted. 'argparse' does not give the Actions that power. There is a fundamental difference in the parsing algorithm.

Just had a chance to try this, and this does exactly what I wanted from
"type=". Thank you!
On Fri, Jul 25, 2014 at 4:17 PM, paul j3 <report@bugs.python.org> wrote:
>
> paul j3 added the comment:
>
> What you want is a custom Action rather than a custom Type.
>
> from the documentation:
>
> >>> class FooAction(argparse.Action):
> ... def __call__(self, parser, namespace, values,
> option_string=None):
> ... print('%r %r %r' % (namespace, values, option_string))
> ... setattr(namespace, self.dest, values)
>
> 'values' will be the list ['1','2','3'], which you test and manipulate,
> before finally saving it to the 'namespace'.
>
> ret = (int(values[0]), int(values[1]), float(values[2]))
> setattr(namespace, self.dest, ret)
>
> Setting 'nargs=3' ensures that this action will always get a 3 item list.
> If the parser can't give it 3 items, it will raise an error rather than
> call your Action.
>
> 'optparse' passed the remaining argument strings to Option's callback,
> which could consume as many as it wanted. 'argparse' does not give the
> Actions that power. There is a fundamental difference in the parsing
> algorithm.
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue22049>
> _______________________________________
>