[Python] Improve usability of pc.map_lookup / MapLookupOptions

[jorisvandenbossche]

Trying to use pc.map_lookup today, I noticed some minor usability issues that could be improved:

arr = pa.array([{'a': 'a1', 'b': 'b1'}, {'a': 'a2', 'b': 'b2'}, {'a': 'a1', 'b': 'b3'}], 
               type=pa.map_(pa.string(), pa.string()))

1. Passing the key value as a python string instead of scalar errors:

# works fine
>>> pc.map_lookup(arr, pa.scalar("a"), "all")
<pyarrow.lib.ListArray object at 0x7f099d2fabc0>
[
  [
    "a1"
  ],
  [
    "a2"
  ],
  [
    "a1"
  ]
]

# gives confusing error
>>> pc.map_lookup(arr, "a", "all")
...
ArrowInvalid: map_lookup: query_key can't be empty.

If not passing a Scalar object, we should probably just pass the python value to pa.scalar(..) for convenience.

2. Allow to specify the occurence keyword, given that this is not necessarily obvious, it's nice to be able to specify the named argument:

>>> pc.map_lookup(arr, pa.scalar("a"), occurence="first")
...
TypeError: __init__() takes exactly 3 positional arguments (2 given)

I suppose we need to give it a default value for this, though (to make our auto-generation work, or we can look into improving the autogeneration of the signature)

[jorisvandenbossche]

2. Allow to specify the occurence keyword, given that this is not necessarily obvious, it's nice to be able to specify the named argument:

Actually, that errors only because I mis-spelled the keyword name. With the correct name it works:

>>> pc.map_lookup(arr, pa.scalar("a"), occurrence="all")
<pyarrow.lib.ListArray object at 0x7f099c96bd00>
[
  [
    "a1"
  ],
  [
    "a2"
  ],
  [
    "a1"
  ]
]

Wondering if we can provide a better error message in that case, but that might just be a Python/cython behaviour of first checking the positional keywords before the named ones)

[jorisvandenbossche]

Wondering if we can provide a better error message in that case, but that might just be a Python/cython behaviour of first checking the positional keywords before the named ones)

Yes, that's a cython issue -> cython/cython#1281

[R-JunmingChen]

1. Passing the key value as a python string instead of scalar errors:

So, we should only handle the issue one. I can make a quick implementation for it

[R-JunmingChen]

take