As of 2017, here is a portable solution that works with all versions of Python:

#!/usr/bin/env python
import collections
import six


def iterable(arg):
    return (
        isinstance(arg, collections.Iterable) 
        and not isinstance(arg, six.string_types)
    )


# non-string iterables    
assert iterable(("f", "f"))    # tuple
assert iterable(["f", "f"])    # list
assert iterable(iter("ff"))    # iterator
assert iterable(range(44))     # generator
assert iterable(b"ff")         # bytes (Python 2 calls this a string)

# strings or non-iterables
assert not iterable(u"ff")     # string
assert not iterable(44)        # integer
assert not iterable(iterable)  # function

Since Python 2.6, with the introduction of abstract base classes, isinstance (used on ABCs, not concrete classes) is now considered perfectly acceptable. Specifically:

from abc import ABCMeta, abstractmethod

class NonStringIterable:
    __metaclass__ = ABCMeta

    @abstractmethod
    def __iter__(self):
        while False:
            yield None

    @classmethod
    def __subclasshook__(cls, C):
        if cls is NonStringIterable:
            if any("__iter__" in B.__dict__ for B in C.__mro__):
                return True
        return NotImplemented

This is an exact copy (changing only the class name) of Iterable as defined in _abcoll.py (an implementation detail of collections.py)... the reason this works as you wish, while collections.Iterable doesn't, is that the latter goes the extra mile to ensure strings are considered iterable, by calling Iterable.register(str) explicitly just after this class statement.

Of course it's easy to augment __subclasshook__ by returning False before the any call for other classes you want to specifically exclude from your definition.

In any case, after you have imported this new module as myiter, isinstance('ciao', myiter.NonStringIterable) will be False, and isinstance([1,2,3], myiter.NonStringIterable)will be True, just as you request -- and in Python 2.6 and later this is considered the proper way to embody such checks... define an abstract base class and check isinstance on it.

By combining previous replies, I'm using:

import types
import collections

#[...]

if isinstance(var, types.StringTypes ) \
    or not isinstance(var, collections.Iterable):

#[Do stuff...]

Not 100% fools proof, but if an object is not an iterable you still can let it pass and fall back to duck typing.

Edit: Python3

types.StringTypes == (str, unicode). The Python3 equivalent is:

if isinstance(var, str ) \
    or not isinstance(var, collections.Iterable):

Edit: Python3.3

types.StringTypes == (str, unicode). The Python3 equivalent is:

if isinstance(var, str ) \
    or not isinstance(var, collections.abc.Iterable):

I realise this is an old post but thought it was worth adding my approach for Internet posterity. The function below seems to work for me under most circumstances with both Python 2 and 3:

def is_collection(obj):
    """ Returns true for any iterable which is not a string or byte sequence.
    """
    try:
        if isinstance(obj, unicode):
            return False
    except NameError:
        pass
    if isinstance(obj, bytes):
        return False
    try:
        iter(obj)
    except TypeError:
        return False
    try:
        hasattr(None, obj)
    except TypeError:
        return True
    return False

This checks for a non-string iterable by (mis)using the built-in hasattr which will raise a TypeError when its second argument is not a string or unicode string.

2.x

I would have suggested:

hasattr(x, '__iter__')

or in view of David Charles' comment tweaking this for Python3, what about:

hasattr(x, '__iter__') and not isinstance(x, (str, bytes))

3.x

the builtin basestring abstract type was removed. Use str instead. The str and bytes types don’t have functionality enough in common to warrant a shared base class.

To explicitly expand on Alex Martelli's excellent hack of collections.py and address some of the questions around it: The current working solution in python 3.6+ is

import collections
import _collections_abc as cabc
import abc


class NonStringIterable(metaclass=abc.ABCMeta):

    __slots__ = ()

    @abc.abstractmethod
    def __iter__(self):
        while False:
            yield None

    @classmethod
    def __subclasshook__(cls, c):
        if cls is NonStringIterable:
            if issubclass(c, str):
                return False
            return cabc._check_methods(c, "__iter__")
        return NotImplemented

and demonstrated

>>> typs = ['string', iter(''), list(), dict(), tuple(), set()]
>>> [isinstance(o, NonStringIterable) for o in typs]
[False, True, True, True, True, True]

If you want to add iter('') into the exclusions, for example, modify the line

            if issubclass(c, str):
                return False

to be

            # `str_iterator` is just a shortcut for `type(iter(''))`*
            if issubclass(c, (str, cabc.str_iterator)):
                return False

to get

[False, False, True, True, True, True]

As you point out correctly, a single string is a character sequence.

So the thing you really want to do is to find out what kind of sequence arg is by using isinstance or type(a)==str.

If you want to realize a function that takes a variable amount of parameters, you should do it like this:

def function(*args):
    # args is a tuple
    for arg in args:
        do_something(arg)

function("ff") and function("ff", "ff") will work.

I can't see a scenario where an isiterable() function like yours is needed. It isn't isinstance() that is bad style but situations where you need to use isinstance().

Adding another answer here that doesn't require extra imports and is maybe more "pythonic", relying on duck typing and the fact that str has had a unicode casefold method since Python 3.

def iterable_not_string(x):
    '''
    Check if input has an __iter__ method and then determine if it's a
    string by checking for a casefold method.
    '''
    try:
        assert x.__iter__

        try:
            assert x.casefold
            # could do the following instead for python 2.7 because
            # str and unicode types both had a splitlines method
            # assert x.splitlines
            return False

        except AttributeError:
            return True

    except AttributeError:
        return False

Python 3.X

Notes:

• You need implement "isListable" method.
• In my case dict is not iterable because iter(obj_dict) returns an iterator of just the keys.
• Sequences are iterables, but not all iterables are sequences (immutable, mutable).
• set, dict are iterables but not sequence.
• list is iterable and sequence.
• str is an iterable and immutable sequence.

Sources:

• https://docs.python.org/3/library/stdtypes.html
• https://opensource.com/article/18/3/loop-better-deeper-look-iteration-python

See this example:

from typing import Iterable, Sequence, MutableSequence, Mapping, Text

class Custom():
    pass

def isListable(obj):
    if(isinstance(obj, type)): return isListable(obj.__new__(obj))
    return isinstance(obj, MutableSequence)

try:
    
    # Listable
    #o = [Custom()]
    #o = ["a","b"]
    #o = [{"a":"va"},{"b":"vb"}]
    #o = list # class type
    
    # Not listable
    #o = {"a" : "Value"}
    o = "Only string"
    #o = 1
    #o = False
    #o = 2.4
    #o = None
    #o = Custom()
    #o = {1, 2, 3} #type set
    #o = (n**2 for n in {1, 2, 3})
    #o = bytes("Only string", 'utf-8')
    #o = Custom # class type
    
    if isListable(o):
        print("Is Listable[%s]: %s" % (o.__class__, str(o)))
    else:
        print("Not Listable[%s]: %s" % (o.__class__, str(o)))
    
except Exception as exc:
    raise exc

If you like to test if the variable is a iterable object and not a "string like" object (str, bytes, ...) you can use the fact that the __mod__() function exists in such "string like" objects for formatting proposes. So you can do a check like this:

>>> def is_not_iterable(item):
...    return hasattr(item, '__trunc__') or hasattr(item, '__mod__')
>>> is_not_iterable('')
True
>>> is_not_iterable(b'')
True
>>> is_not_iterable(())
False
>>> is_not_iterable([])
False
>>> is_not_iterable(1)
True
>>> is_not_iterable({})
False
>>> is_not_iterable(set())
False
>>> is_not_iterable(range(19)) #considers also Generators or Iterators
False