add dummy file name for some tiffseq

[dylanmcreynolds]

This addresses a bug with the tiff sequence code.

When the files of a tiffsequence don't start with letters, just numbers, a hash was used for the key for that node.

So:

tiff_files
  - 001.tiff
  - 002.tiff
  - a001.tiff
  - a002.tiff
  - b001.tiff
  - b002.tiff

would result in

['d3a7b120-91f8-41ef-a1ff-f3e312f23525',
 'a',
 'b']

The unit test wasn't expecting a random name for this node, so it didn't test for it.

This PR detects that case and adds an _ in as the key name. On a call with @taxe10 and @danielballan, we debated this default root name for only a couple seconds. I'm happy to hear better ideas.

[padraic-shafer]

This seems ok good. :)

Other thoughts, just to throw them out there: '', ' ', or None.

>>> d1 = {'': 0, 'a':1, 'b': 2}
>>> d1['']
0

>>> d2 = {' ': 0, 'a':1, 'b': 2}
>>> d2[' ']
0

>>> d3 = {None: 0, 'a':1, 'b': 2}
>>> d3[None]
0

# But not...
>>> d3[]
  File "<stdin>", line 1
    d3[]
       ^
SyntaxError: invalid syntax

[danielballan]

Since the keys also end up in path parameters, this unfortunately rules out a couple of those options:

• '' ends up as /api/v1/metadata/tif// (emphasis on //) which HTTP elides into /.
• None has to to coerced to a string of some kind, so its special status as an out-of-band sentinel does not survive.
• Space technically works but may invite confusion.

I think we'll be happy if we stick with "The key must be a non-empty string."

[danielballan]

Feedback from @Wiebke, who thought about related issues in her work on #555, would be very welcome as well.

[dylanmcreynolds]

I almost feel like something super explicit might be better, even if a bit verbose:

['_unnamed_sequence',
 'a',
 'b']

[padraic-shafer]

Explicit and brief would be even better: "_default", "_unnamed", "_blank", "_na", etc.

I favor including a non-alphanumeric to try to minimize name clashes with the user-defined sequences. Something that does not require url-encoding would be good. I suppose a leading "_" will list this key at the beginning for most sorting routines?

EDIT: Or if only letters are allowed for the sequence names, that would leave "0" (zero) available. Is that too cryptic? Or too easy to confuse ['0'] with [0] or ['O']?

[danielballan]

👍 for brevity. Any of these seem good to me:

"_default", "_unnamed", "_blank", "_na", etc.

Using a number would get weird in this context:

c['tif']['0'][0]  # first image

[dylanmcreynolds]

I committed _unnamed. I like it because it's pretty explicit.

[danielballan]

Looks good to me, but we should wait for @Wiebke to weigh in before we merge.

[Wiebke]

I think this is a sensible solution for unnamed sequences and I liked _unnamed as well.