how to tell a variable is iterable but not a string

Question

I have a function that take an argument which can be either a single item or a double item:

def iterable(arg)     if #arg is an iterable:         print "yes"     else:         print "no" 

so that:

 >>> iterable( ("f","f") ) yes  >>> iterable( ["f","f"] ) yes  >>> iterable("ff") no 

The problem is that string is technically iterable, so I can't just catch the ValueError when trying arg[1]. I don't want to use isinstance(), because that's not good practice (or so I'm told).

Answer 1

Use isinstance (I don't see why it's bad practice)

import types if not isinstance(arg, types.StringTypes): 

Note the use of StringTypes. It ensures that we don't forget about some obscure type of string.

On the upside, this also works for derived string classes.

class MyString(str):     pass  isinstance(MyString("  "), types.StringTypes) # true 

Also, you might want to have a look at this previous question.

Cheers.

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NB: behavior changed in Python 3 as StringTypes and basestring are no longer defined. Depending on your needs, you can replace them in isinstance by str, or a subset tuple of (str, bytes, unicode), e.g. for Cython users. As @Theron Luhn mentionned, you can also use six.

Answer 2

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