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         


        

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.

---

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 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().