cleanup of pretty output renderer

[sloanlance]

Based on a conversation started in the comments of #706, clean up the appearance of the new pretty output renderer. Pertinent points from the conversation…

• Fix length of the output dividers (current length is arbitrary; the length of = and - lines do not match)
• Change to Unicode box characters from ASCII
• Add vertical space between each error
• Change exception messages from Python object reprs (with single-quoted strings) to true JSON (with double-quoted strings)

Details

I think more important than adding even more output renderers is essentially cleaning up the one new one that was added. Specifically, here's some example output from the new pretty renderer:

⊙  printf '7' | jsonschema --output pretty <(printf '{"minimum": 9, "multipleOf": 3, "not": {"const": 7}}')

===[ValidationError]===(<stdin>)===

7 is less than the minimum of 9

Failed validating 'minimum' in schema:
    {'minimum': 9, 'multipleOf': 3, 'not': {'const': 7}}

On instance:
    7
-----------------------------
===[ValidationError]===(<stdin>)===

7 is not a multiple of 3

Failed validating 'multipleOf' in schema:
    {'minimum': 9, 'multipleOf': 3, 'not': {'const': 7}}

On instance:
    7
-----------------------------
===[ValidationError]===(<stdin>)===

{'const': 7} is not allowed for 7

Failed validating 'not' in schema:
    {'minimum': 9, 'multipleOf': 3, 'not': {'const': 7}}

On instance:
    7
-----------------------------

where some things that rub me a bit wrong visually are:

• the width of the ascii dividers (right now it's a bit arbitrary and you can see it doesn't match between the = and - lines -- particularly so if it's a SchemaError vs a ValidationError)
• not using unicode box characters
• needing a bit of vertical space between each error
• the fact that, though it's not obvious there, in the exception message, what's there is reprs of Python objects. Even though usually that's what I want shown because jsonschema normally deals with Python objects, not JSON, here that's not the case -- here in fact the reverse is true -- the CLI takes JSON, and does the deserialization itself. So when showing errors back to users of the CLI, it should show JSON, not Python objects.

[sloanlance]

What do you think of this output, @Julian, with divisions padded to a width of 79 characters per line?

╒══[ValidationError]═══(<stdin>)══════════════════════════════════════════════╕
7 is less than the minimum of 9

Failed validating 'minimum' in schema:
    {'minimum': 9, 'multipleOf': 3, 'not': {'const': 7}}

On instance:
    7
└─────────────────────────────────────────────────────────────────────────────┘

╒══[ValidationError]═══(<stdin>)══════════════════════════════════════════════╕
7 is not a multiple of 3

Failed validating 'multipleOf' in schema:
    {'minimum': 9, 'multipleOf': 3, 'not': {'const': 7}}

On instance:
    7
└─────────────────────────────────────────────────────────────────────────────┘

╒══[ValidationError]═══(<stdin>)══════════════════════════════════════════════╕
{'const': 7} is not allowed for 7

Failed validating 'not' in schema:
    {'minimum': 9, 'multipleOf': 3, 'not': {'const': 7}}

On instance:
    7
└─────────────────────────────────────────────────────────────────────────────┘

[Julian]

Very nice! Definitely an improvement! Thumbs up from me.

[sloanlance]

Perhaps off-topic: The error message '{'const': 7} is not allowed for 7' seems odd. Unlike the other two messages in this example, the value being tested (7) comes at the end of the statement and the JSON code seems unnecessary. I think a better error message would be simply 7 is not allowed.

If you agree, should this be a new issue, or would it be OK for me to change it as part of this issue and PR?

[Julian]

I agree on the awkwardness -- let's tackle it in a new issue -- the thing that is a bit complicated is that what's involved there is less about const and more about not -- not needs a message suitable to invert any condition (schema). So in general it's not easy to have that / know what schema you're inverting. Here of course when it's const it's obvious what better looks like, but not won't know that in general. But yeah definitely if we can find a way to not terribly overcomplicate things I'm all for it.

[sloanlance]

I'll open a new issue for the "not" error message.

[Julian]

Superceded by check-jsonschema which is now the official (and more-featureful) CLI. Anyone interested in this is invited to have a look there (and file any issues necessary).