AllenNLP's data piece could be more pythonic

[DeNeutoy]

allennlp's data API has become pretty stable and effective. However, it's still a bit unwieldy to manipulate individual Fields and Instances. There are a few things we could do to make them easier to use for a newcomer:

1. Manipulating Fields in Instances. Instance could inherit from MutableMapping. (so could MetadataField, in the same vein.)

# Currently:
specific_field = instance.fields["field_name"]

# Ideal:
specific_field = instance["field_name"]
"field_name" in instance
>>> True

2. Iterating over SequenceFields and ListFields using Field.__iter__:

# Before
fields = instance.fields
tokens = [t.text for t in fields['tokens'].tokens]
assert tokens == ["Mali", "government", "officials", "say", "the", "woman", "'s",
                      "confession", "was", "forced", "."]

# After
token_field = instances["tokens"]
assert token_field == ["Mali", "government", "officials", "say", "the", "woman", "'s",
                      "confession", "was", "forced", "."]

3. Representing Index and SpanFields as their values for equality (Field.__eq__):

# Before
assert instance.fields["span_start"].sequence_index == 102
assert (instance.fields["span"].span_start, instance.fields["span"].span_end) == (102, 109)

# After
assert instance["span_start"] == 102
assert instance["span"] == (102, 109)

[matt-gardner]

What's the use case for manipulating individual instances? Debugging? Tests? Something else?

[DeNeutoy]

both, I guess - it's low priority, just struck me when I was looking at some of the data tests.

[joelgrus]

I am a fan of #1 and #2 (I think I might have even implemented 2 in the documentqa PR or in some other PR that never got merged), #3 is less obviously a good idea to me (but I could possibly be convinced)

[matt-gardner]

Some more motivation for actually doing this: https://twitter.com/crcrpar/status/1053303645683408896.

[joelgrus]

we assigned this to me because we thought Kathryn might work on it. but she's not, so I can do it, it will be very quick.

[joelgrus]

in an ideal world you'd have e.g. TextField inherit from Sequence[Token], but it turns out that the Sequence class already has an index method that clashes with TextField.index 😬 but I think I can just add the methods anyway