`_cdp_catalog` can be `None` in `SegYReader2D.cdp_numbers`. about segpy HOT 5 CLOSED

It looks like the only use of catalog_traces() is in reader.py, and it doesn't make any attempt to account for None values coming out of catalog_traces(). The only place where a None check is made is when we calculate dimensionality right after calling catalog_traces(). In every other place where the catalogs are referenced, we assume that they're Mapping references.

From what I can see, we can safely replace these Nones with empty dicts. That is, when reader.py sees None come out of catalog_traces(), it could replace these with {}. It looks like that's really what reader.py expects anyway.

We might even be able to go a step further and have catalog_traces() return {} instead of None. But I'm not sure right now if catalog_traces() can return {} already and, if so, if that's semantically different from None.

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As a first experiment (and to temporarily shave this yak so I can do testing), I'm going to turn Nones into empty dicts in reader.py.

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If I remember correctly, you can get Nones in the case that there is no bijective mapping, for example if there are duplicate keys. This shouldn't happen with well-formed SEG-Y, but it does. The CatalogBuilder.create() method returns None if there are duplicates.

I think an "empty" SEG-Y file containing headers but zero traces could legitimately have empty dictionaries.

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Looking at this again, the intent is that clients should not usually construct SegYReader2D directly, and should prefer to go via create_reader(). There's a comment to this effect in the docstring for SegYReader2D.__init__() where it says, "Usually a SegYReader is most easily constructed using the create_reader() function", although this should be more strongly worded.

The logic in _make_reader() which tries to determine the dimensionality of the SEG-Y assumes that, the the case the caller of create_reader() has left dimensionality at the default value of None, that a cdp_catalog is successfully constructed, and so cdp_catalog is not None.

Given this, I think it's entirely reasonable that SegYReader2D.__init__() raises a TypeError if its cdp_cataclog argument is None (or better, if it is not a mapping. There's possibly also a length check we could make here to ensure the various mappings have consistent lengths).

Essentially, if you can't provide a valid cdp_catalog mapping you're not in a position to use SegYReader2D anyway, so you need to fall back to using plain-old SegYReader.

Similarly, SegYReader3D.__init__() should raise a TypeError if its line_catalog argument is not a mapping. The SegYReader base class should sanity check its various arguments in SegYReader.__init__(). These checks weren't initially included because of the assumption that callers would go via create_reader(), but that's definitely one assumption too far.

It's much better if these objects establish their class invariants in their initialisers that having to accommodate testing for various non-sensical states once they've been constructed.

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Should we go ahead and close this? I think so, but could use a second opinion.

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