Correct way to detect sequence parameter?

Question

I want to write a function that accepts a parameter which can be either a sequence or a single value. The type of value is str, int, etc., but I don't want it to be restricted to a hardcoded list. In other words, I want to know if the parameter X is a sequence or something I have to convert to a sequence to avoid special-casing later. I could do

type(X) in (list, tuple)

but there may be other sequence types I'm not aware of, and no common base class.

-N.

Edit: See my "answer" below for why most of these answers don't help me. Maybe you have something better to suggest.

Answer 1

I think what I would do is check whether the object has certain methods that indicate it is a sequence. I'm not sure if there is an official definition of what makes a sequence. The best I can think of is, it must support slicing. So you could say:

is_sequence = '__getslice__' in dir(X)

You might also check for the particular functionality you're going to be using.

As pi pointed out in the comment, one issue is that a string is a sequence, but you probably don't want to treat it as one. You could add an explicit test that the type is not str.

Answer 2

You could pass your parameter in the built-in len() function and check whether this causes an error. As others said, the string type requires special handling.

According to the documentation the len function can accept a sequence (string, list, tuple) or a dictionary.

You could check that an object is a string with the following code:

x.__class__ == "".__class__

Answer 3

Revised answer:

I don't know if your idea of "sequence" matches what the Python manuals call a "Sequence Type", but in case it does, you should look for the __Contains__ method. That is the method Python uses to implement the check "if something in object:"

if hasattr(X, '__contains__'):
    print "X is a sequence"

My original answer:

I would check if the object that you received implements an iterator interface:

if hasattr(X, '__iter__'):
    print "X is a sequence"

For me, that's the closest match to your definition of sequence since that would allow you to do something like:

for each in X:
    print each

Answer 4

In cases like this, I prefer to just always take the sequence type or always take the scalar. Strings won't be the only types that would behave poorly in this setup; rather, any type that has an aggregate use and allows iteration over its parts might misbehave.

Answer 5

Sequences are described here: http://www.python.org/doc/2.5.2/lib/typesseq.html

So sequences are not the same as iterable objects. I think sequence must implement __getitem__, whereas iterable objects must implement __iter__. So for example string are sequences and don't implement __iter__, xrange objects are sequences and don't implement __getslice__.

But from what you seen to want to do, I'm not sure you want sequences, but rather iterable objects. So go for hasattr("__getitem__", X) you want sequences, but go rather hasattr("__iter__", X) if you don't want strings for example.

Answer 6

The simplest method would be to check if you can turn it into an iterator. ie

try:
    it = iter(X)
    # Iterable
except TypeError:
    # Not iterable

If you need to ensure that it's a restartable or random access sequence (ie not a generator etc), this approach won't be sufficient however.

As others have noted, strings are also iterable, so if you need so exclude them (particularly important if recursing through items, as list(iter('a')) gives ['a'] again, then you may need to specifically exclude them with:

 if not isinstance(X, basestring)

Answer 7

You're asking the wrong question. You don't try to detect types in Python; you detect behavior.

1. Write another function that handles a single value. (let's call it _use_single_val).
2. Write one function that handles a sequence parameter. (let's call it _use_sequence).
3. Write a third parent function that calls the two above. (call it use_seq_or_val). Surround each call with an exception handler to catch an invalid parameter (i.e. not single value or sequence).
4. Write unit tests to pass correct & incorrect parameters to the parent function to make sure it catches the exceptions properly.

    def _use_single_val(v):
        print v + 1  # this will fail if v is not a value type

    def _use_sequence(s):
        print s[0]   # this will fail if s is not indexable

    def use_seq_or_val(item):    
        try:
            _use_single_val(item)
        except TypeError:
            pass

        try:
            _use_sequence(item)
        except TypeError:
            pass

        raise TypeError, "item not a single value or sequence"

EDIT: Revised to handle the "sequence or single value" asked about in the question.

Answer 8

As of 2.6, use abstract base classes.

>>> import collections
>>> isinstance([], collections.Sequence)
True
>>> isinstance(0, collections.Sequence)
False

Furthermore ABC's can be customized to account for exceptions, such as not considering strings to be sequences. Here an example:

class Atomic(object):
    __metaclass__ = abc.ABCMeta
    @classmethod
    def __subclasshook__(cls, other):
        return not issubclass(other, collections.Sequence) or NotImplemented
Atomic.register(basestring)

The Atomic class can be used with isinstance and issubclass as usual. This is still much better than a hard-coded list, because you only need to register the exceptions to the rule, and external users of the code than register their own.

In older versions, there are type checkers in the operator module.

>>> import operator
>>> operator.isSequenceType([])
True
>>> operator.isSequenceType(0)
False

Answer 9

I'm new here so I don't know what's the correct way to do it. I want to answer my answers:

The problem with all of the above mentioned ways is that str is considered a sequence (it's iterable, has __getitem__, etc.) yet it's usually treated as a single item.

For example, a function may accept an argument that can either be a filename or a list of filenames. What's the most Pythonic way for the function to detect the first from the latter?

Should I post this as a new question? Edit the original one?

Answer 10

IMHO, the python way is to pass the list as *list. As in:

myfunc(item)
myfunc(*items)

Answer 11

If strings are the problem, detect a sequence and filter out the special case of strings:

def is_iterable(x):
  if type(x) == str:
    return False
  try:
    iter(x)
    return True
  except TypeError:
    return False

Answer 12

The problem with all of the above mentioned ways is that str is considered a sequence (it's iterable, has getitem, etc.) yet it's usually treated as a single item.

For example, a function may accept an argument that can either be a filename or a list of filenames. What's the most Pythonic way for the function to detect the first from the latter?

Based on the revised question, it sounds like what you want is something more like:

def to_sequence(arg):
    ''' 
    determine whether an arg should be treated as a "unit" or a "sequence"
    if it's a unit, return a 1-tuple with the arg
    '''
    def _multiple(x):  
        return hasattr(x,"__iter__")
    if _multiple(arg):  
        return arg
    else:
        return (arg,)

>>> to_sequence("a string")
('a string',)
>>> to_sequence( (1,2,3) )
(1, 2, 3)
>>> to_sequence( xrange(5) )
xrange(5)

This isn't guaranteed to handle all types, but it handles the cases you mention quite well, and should do the right thing for most of the built-in types.

When using it, make sure whatever receives the output of this can handle iterables.