Improve error handling when accidentally importing a Collection with Catalog

[CloudNiner]

Today I accidentally used pystac.Catalog.from_file to read a collection.json instead of pystac.Collection.from_file and got this error:

TypeError: 'Catalog' object is not subscriptable

I could see a number of people making this mistake or it being a difficult to handle bug in some automated process that reads catalogs. It would be nice if this error was handled such that a specific pystac exception is thrown when this happens or it automatically switches to the collection reader when a collection is detected.

[lossyrob]

This should be handled better so that the error is more interpretable.

It may be worth it to run a check using identify_stac_object_type in all the specific object reads to make sure the user is doing the right thing.

Also note that pystac.read_file will identify the object for you and give you back the relevant STAC object.

[lossyrob]

Looking into this - I tried to replicate and could not. It's unclear where in the code this is throwing. Can you give reproduction steps?

[CloudNiner]

Shoot, sorry for the confusion. It looks like it doesn't throw when the pystac.Catalog.from_file is called, but when you try to perform some operation on the catalog afterwards:

catalog = pystac.Catalog.from_file('catalog/collection.json')
catalog.describe()
* <Catalog id=hand_021>
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-a1e8ca2d347c> in <module>
----> 1 catalog.describe()

~/.pyenv/versions/3.7.8/envs/pystac-0.5.2/lib/python3.7/site-packages/pystac/catalog.py in describe(self, include_hrefs, _indent)
    617         for child in self.get_children():
    618             child.describe(include_hrefs=include_hrefs, _indent=_indent + 4)
--> 619         for item in self.get_items():
    620             s = '{}* {}'.format(' ' * (_indent + 2), item)
    621             if include_hrefs:

~/.pyenv/versions/3.7.8/envs/pystac-0.5.2/lib/python3.7/site-packages/pystac/stac_object.py in get_stac_objects(self, rel)
    360             link = links[i]
    361             if link.rel == rel:
--> 362                 link.resolve_stac_object(root=self.get_root())
    363                 yield link.target
    364 

~/.pyenv/versions/3.7.8/envs/pystac-0.5.2/lib/python3.7/site-packages/pystac/link.py in resolve_stac_object(self, root)
    158                 target_href = make_absolute_href(target_href, start_href)
    159 
--> 160             obj = STAC_IO.read_stac_object(target_href, root=root)
    161             obj.set_self_href(target_href)
    162         else:

~/.pyenv/versions/3.7.8/envs/pystac-0.5.2/lib/python3.7/site-packages/pystac/stac_io.py in read_stac_object(cls, uri, root)
    129         """
    130         d = cls.read_json(uri)
--> 131         return cls.stac_object_from_dict(d, href=uri, root=root)
    132 
    133     @classmethod

~/.pyenv/versions/3.7.8/envs/pystac-0.5.2/lib/python3.7/site-packages/pystac/serialization/__init__.py in stac_object_from_dict(d, href, root)
     28 
     29         # Merge common properties in case this is an older STAC object.
---> 30         merge_common_properties(d, json_href=href, collection_cache=collection_cache)
     31 
     32     info = identify_stac_object(d)

~/.pyenv/versions/3.7.8/envs/pystac-0.5.2/lib/python3.7/site-packages/pystac/serialization/common_properties.py in merge_common_properties(item_dict, collection_cache, json_href)
     73             collection_props = collection.properties
     74         else:
---> 75             collection_id = collection['id']
     76             if 'properties' in collection:
     77                 collection_props = collection['properties']

TypeError: 'Catalog' object is not subscriptable

Here's the catalog I was using:
catalog.tar.gz