Question

e.g. so that these would both work – is it possible?

(val,VAL2) = func(args) 
val = func(args)

Where val is not a tuple

For example I’d like these to work for my custom object something

for item in something:
    do_item(item) #where again item - is not a tuple

for (item,key) in something:
    do_more(key,item)

I thought that I need to implement next() function in two different ways…

edit: as follows from the answers below, this should not really be done.

Answer 1

If you mean, can the function act differently based on the return types the caller is expecting, the answer is no (bar seriously nasty bytecode inspection). In this case, you should provide two different iterators on your object, and write something like:

for item in something:  # Default iterator: returns non-tuple objects
    do_something(item)

for (item,key) in something.iter_pairs(): # iter_pairs returns different iterator
    do_something_else(item, key)

eg. see the dictionary object, which uses this pattern. for key in mydict iterates over the dictionary keys. for k,v in mydict.iteritems() iterates over (key, value) pairs.

[Edit] Just in case anyone wants to see what I mean by “seriously nasty bytecode inspection”, here’s a quick implementation:

import inspect, opcode

def num_expected_results():
    """Return the number of items the caller is expecting in a tuple.

    Returns None if a single value is expected, rather than a tuple.
    """
    f = inspect.currentframe(2)
    code = map(ord, f.f_code.co_code)
    pos = f.f_lasti
    if code[pos] == opcode.opmap['GET_ITER']: pos += 1 # Skip this and the FOR_ITER
    if code[pos] > opcode.EXTENDED_ARG: pos +=5
    elif code[pos] > opcode.HAVE_ARGUMENT: pos +=3
    else: pos += 1
    if code[pos] == opcode.opmap['UNPACK_SEQUENCE']:
        return code[pos+1] + (code[pos+2] << 8)
    return None

Usable something like:

class MagicDict(dict):
    def __iter__(self):
        if num_expected_results() == 2:
            for k,v in self.iteritems():
                yield k,v
        else:
            for k in self.iterkeys(): 
                yield k

d=MagicDict(foo=1, bar=2)

print "Keys:"
for key in d:
    print "   ", key
print "Values"    
for k,v in d:
    print "   ",k,v

Disclaimer: This is incredibly hacky, insanely bad practice, and will cause other programmers to hunt you down and kill you if they ever see it in real code. Only works on cpython (if that). Never use this in production code (or for that matter, probably any code).