implemented items, to_object and zip functions

[jstewmon]

implements jmespath/jmespath.jep#20

I think to_object may need additional input checking, but I'm not sure what the behavior should be for some error cases. For example, to_object([[1, 2, 3]]) will raise ValueError: dictionary update sequence element #0 has length 3; 2 is required. And to_object([[[1], 1]]) will raise TypeError: unhashable type: 'list'.

Does the spec prescribe a behavior (like a specific error type) for these cases?

[jamesls]

Awesome, thanks for the pull request.

I would expect both of those cases to be errors, potentially type errors. I'm not sure if it's worth updating the type checker to support some sort of record/tuple type (that would just be a list with a fixed structure) or just adding this check in to_object. Something like to_object(array[tuple[str, any]]). I'd be ok with either for now.

As an aside, I noticed the various to_* functions in JMESPath will take any type and have specific behavior defined for each input type. Given this is the first to_* function that doesn't follow this pattern, it might make sense to name this function something different.

[jstewmon]

Regarding the naming convention of to_* functions, would it be reasonable to have to_object return null for inputs that cannot be represented as an object? That would seem to be congruent with the to_number spec, especially if the type checker supports a tuple type because the spec could clearly indicate that any other type results in null. number and object are similar in that there is a limited set of types from which a cast is possible.

That said, as a user I would find a type error is more informative than null. Does to_number result in null instead of a type error because the expectation of all to_* functions is to return either a casted representation of the input or null?

[codecov-io]

Codecov Report

Merging #105 into develop will decrease coverage by 0.49%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           develop     jmespath/jmespath.site#105     +/-   ##
==========================================
- Coverage    97.27%   96.77%   -0.5%     
==========================================
  Files           13       13             
  Lines         1431     1459     +28     
==========================================
+ Hits          1392     1412     +20     
- Misses          39       47      +8

Impacted Files	Coverage Δ
jmespath/functions.py	99.51% <100%> (+0.02%)	:arrow_up:
jmespath/compat.py	79.66% <100%> (-10.34%)	:arrow_down:

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[jamesls]

The main reason to_number defined nulls instead of type errors was so you could use it as a succinct way to filter out non-numeric values when combined with a projection. For example:

>>> jmespath.search('sum([*].a.to_number(@))', [{"a": []}, {"a": "1"}, {"a": 2}, {"a": None}])
3

Those conditions don't really apply with to_object because it's not intended to be a generic conversion function. It expects a specific structure in its input arguments, and will error out if it doesn't get the structure it expects. That's why I'm hesitant for the name to suggest it's a generic function. I feel something like from_pairs would clarify its expectations more explicitly. What do you think?

[jstewmon]

Ok, I see your point. I think either from_pairs or from_items are good names. I like the latter because it expresses that this function is the complement of items.

Do you like from_items, or do you prefer from_pairs?

[jstewmon]

@jamesls I had forgotten about this PR, but someone just mentioned to me that it is linked in a StackOverflow answer, so I thought I'd ping you to see if you are still interested in accepting this PR and ask if you have any more feedback?

I rebased on the latest develop branch.

[curtisforrester]

Bump. This might have been nice for a use-case yesterday.