python - 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.