What is the most pythonic way to use len on a scalar?

Question

I read this question

python: how to identify if a variable is an array or a scalar

but when using the following code I get a false on an np.array as can be demonstrated below.

import collections

isinstance(np.arange(10), collections.Sequence)
# returns false

I find it a bit annoying that I can't do len(1) and simply get 1.

The only work around I can think of is a try except statement such as the following:

a = 1
try:
    print len(a)
except TypeError:
    print 1

Is there a more Pythonic way to do this?

At the *very least* qualify your `except`. Only `TypeError` needs to be caught here. How did you end up with a situation where you need got such a mix of types in the first place however? — Martijn Pieters, Oct 23 '14 at 16:15
Anything wrong with the second answer in your link, which specifically mentions numpy? — Mark Ransom, Oct 23 '14 at 16:24
@ Mark Ranson hm. I think it probably works. I was hoping that someone would say oh sure there is something like np.len(a) which does this kind of thing. It seems there isn't. — evan54, Oct 23 '14 at 21:14
@MartijnPieters well it's due to using `numpy`, `mpmath` or a single number. So the set of types it can be is finite (of course) but still would be nice not to list what happens under each case. — evan54, Oct 23 '14 at 21:15

score 8 · Accepted Answer · 2014-10-23T16:39:15.827

8

collections.Sequence only applies to sequence objects, which are a very specific type of iterable object. Incidentally, a numpy.ndarray (which is returned by numpy.arange) is not a sequence.

You need to test for either collections.Iterable, which represents any iterable object:

>>> isinstance([1, 2, 3], collections.Iterable)
True
>> isinstance(np.arange(10), collections.Iterable)
True
>>> isinstance(1, collections.Iterable)
False
>>>

or collections.Sized, which represents any object that works with len:

>>> isinstance([1, 2, 3], collections.Sized)
True
>>> isinstance(np.arange(10), collections.Sized)
True
>>> isinstance(1, collections.Sized)
False
>>>

You can then use a conditional expression or similar to do what you want:

print len(a) if isinstance(a, collections.Iterable) else 1

print len(a) if isinstance(a, collections.Sized) else 1

For a complete list of the available abstract base classes in the collections module, see Collections Abstract Base Classes in the Python docs.

edited Oct 23 '14 at 16:39

answered Oct 23 '14 at 16:19

If you just want to know whether `len` will work or not, `collections.Sized` would be more general. But it depends what the OP actually wants to do with the objects. – Peter DeGlopper Oct 23 '14 at 16:20
I think I'll go with `collections.Sized` seems more general. Kind of surprised that this kind of function doesn't already exist in `numpy` or smth similar. – evan54 Oct 23 '14 at 21:08
Better use `collections.abc.Sized` now: `DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.10 it will stop working.` – Martin Jun 11 '22 at 05:36

Jon Clements · Answer 2 · 2014-10-23T16:48:04.653

I'll just throw in another potential option:

length = getattr(obj, '__len__', lambda:1)()

So get either the __len__ method from the object, or a function that always returns 1, then call it to get your result.

I wouldn't say it's Pythonic, but avoids an import and exception handling. However, I'd still go with comparing if it's a collections.Sized and a conditional statement and put it in a helper function called len_or_1 or something.

evan54 · Answer 3 · 2014-10-31T02:20:37.777

1

Although this isn't pythonic as it uses numpy here is another neat way to make this work:

import numpy as np
a = 1
aSh = np.shape(a)
if len(aSh) == 0:
    print 1
else:
    print max(aSh)

which gives a behaviour that should work with scalars, lists and matrices.

edited Oct 31 '14 at 02:20

answered Oct 27 '14 at 00:17

evan54

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What is the most pythonic way to use len on a scalar?

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