Repository: creare-com/podpac
Issue: Interpolation Specification no JSON serializable

**Description**
When I try to specify different interpolation methods for different dimensions, I get an error: 

```python
C:\\Anaconda3\\envs\\podpac\\lib\\json\\encoder.py in iterencode(self, o, _one_shot)
    255                 self.key_separator, self.item_separator, self.sort_keys,
    256                 self.skipkeys, _one_shot)
--> 257         return _iterencode(o, 0)
    258 
    259 def _make_iterencode(markers, _default, _encoder, _indent, _floatstr,

TypeError: keys must be str, int, float, bool or None, not tuple
```

**Steps to Reproduce**
```python
import podpac
from podpac.datalib.terraintiles import TerrainTiles
from podpac import clinspace, Coordinates
coords = Coordinates([clinspace(4800000, 4900000, 256), clinspace(-8500000, -8400000, 256)],
                     dims=['lat', 'lon'], crs='epsg:3857')

interp = {
        'method': 'nearest',
        'interpolators': [podpac.interpolators.ScipyGrid]
}            
interpolation = {
    ('lat', 'lon'): interp,
    ('time',): {'method': 'bilinear'}
}
node_near_tol = TerrainTiles(interpolation=interpolation, zoom=4) 
o = node_near_tol.eval(coords)
o.plot()
node_near_tol.sources[0].interpolators  
```
The problem is that tuple keys are not JSON-serializable. 

**Proposed Fix**
Let's change the specification to something like this instead: 

```python
interp = {
        'method': 'nearest',
        'interpolators': [podpac.interpolators.ScipyGrid, NearestNeighbor, ...],
        'dims': ['lat', 'lon'], 
}            
interpolation = [
    interp, 
    {'method': 'bilinear', 
    'dims': ['time']}
]
```
That is, interpolation can now be a `str`, `dict`, or `list` of `dicts`. 



Changed files:
podpac/core/utils.py, podpac/core/data/test/test_interpolate.py, podpac/core/data/interpolation.py, podpac/core/data/datasource.py

diff --git a/podpac/core/data/datasource.py b/podpac/core/data/datasource.py
index bb1bcd16..cab0d03e 100644
--- a/podpac/core/data/datasource.py
+++ b/podpac/core/data/datasource.py
@@ -99,26 +99,24 @@ DATA_DOC = {
         :attr:`podpac.data.INTERPOLATION_SHORTCUTS`. The interpolation method associated
         with this string will be applied to all dimensions at the same time.
 
-        If input is a dict, the dict must contain one of two definitions:
-
-        1. A dictionary which contains the key ``'method'`` defining the interpolation method name.
-           If the interpolation method is not one of :attr:`podpac.data.INTERPOLATION_SHORTCUTS`, a
-           second key ``'interpolators'`` must be defined with a list of
-           :class:`podpac.interpolators.Interpolator` classes to use in order of uages.
-           The dictionary may contain an option ``'params'`` key which contains a dict of parameters to pass along to
-           the :class:`podpac.interpolators.Interpolator` classes associated with the interpolation method.
-           This interpolation definition will be applied to all dimensions.
-        2. A dictionary containing an ordered set of keys defining dimensions and values
-           defining the interpolation method to use with the dimensions.
-           The key must be a string or tuple of dimension names (i.e. ``'time'`` or ``('lat', 'lon')`` ).
-           The value can either be a string matching one of the interpolation shortcuts defined in
-           :attr:`podpac.data.INTERPOLATION_SHORTCUTS` or a dictionary meeting the previous requirements
-           (1). If the dictionary does not contain a key for all unstacked dimensions of the source coordinates, the
-           :attr:`podpac.data.INTERPOLATION_DEFAULT` value will be used.
-           All dimension keys must be unstacked even if the underlying coordinate dimensions are stacked.
-           Any extra dimensions included but not found in the source coordinates will be ignored.
-
-        If input is a :class:`podpac.data.Interpolation` class, this interpolation
+        If input is a dict or list of dict, the dict or dict elements must adhere to the following format:
+
+        The key ``'method'`` defining the interpolation method name.
+        If the interpolation method is not one of :attr:`podpac.data.INTERPOLATION_SHORTCUTS`, a
+        second key ``'interpolators'`` must be defined with a list of
+        :class:`podpac.interpolators.Interpolator` classes to use in order of uages.
+        The dictionary may contain an option ``'params'`` key which contains a dict of parameters to pass along to
+        the :class:`podpac.interpolators.Interpolator` classes associated with the interpolation method.
+        
+        The dict may contain the key ``'dims'`` which specifies dimension names (i.e. ``'time'`` or ``('lat', 'lon')`` ). 
+        If the dictionary does not contain a key for all unstacked dimensions of the source coordinates, the
+        :attr:`podpac.data.INTERPOLATION_DEFAULT` value will be used.
+        All dimension keys must be unstacked even if the underlying coordinate dimensions are stacked.
+        Any extra dimensions included but not found in the source coordinates will be ignored.
+
+        The dict may contain a key ``'params'`` that can be used to configure the :class:`podpac.interpolators.Interpolator` classes associated with the interpolation method.
+
+        If input is a :class:`podpac.data.Interpolation` class, this Interpolation
         class will be used without modification.
         """,
 }
diff --git a/podpac/core/data/interpolation.py b/podpac/core/data/interpolation.py
index f02b495b..55255a59 100644
--- a/podpac/core/data/interpolation.py
+++ b/podpac/core/data/interpolation.py
@@ -85,7 +85,7 @@ def interpolation_trait(default_value=INTERPOLATION_DEFAULT, allow_none=True, **
         Union trait for an interpolation definition
     """
     return tl.Union(
-        [tl.Dict(), tl.Enum(INTERPOLATION_METHODS), tl.Instance(Interpolation)],
+        [tl.Dict(), tl.List(), tl.Enum(INTERPOLATION_METHODS), tl.Instance(Interpolation)],
         allow_none=allow_none,
         default_value=default_value,
         **kwargs
@@ -116,7 +116,7 @@ class Interpolation(object):
 
     def __init__(self, definition=INTERPOLATION_DEFAULT):
 
-        self.definition = definition
+        self.definition = deepcopy(definition)
         self.config = OrderedDict()
 
         # if definition is None, set to default
@@ -126,37 +126,34 @@ class Interpolation(object):
             self.definition = INTERPOLATION_DEFAULT
 
         # set each dim to interpolator definition
-        if isinstance(definition, dict):
+        if isinstance(definition, (dict, list)):
 
-            # covert input to an ordered dict to preserve order of dimensions
-            definition = OrderedDict(definition)
+            # convert dict to list
+            if isinstance(definition, dict):
+                definition = [definition]
 
-            for key in iter(definition):
+            for interp_definition in definition:
 
-                # if dict is a default definition, skip the rest of the handling
-                if not isinstance(key, tuple):
-                    if key in ["method", "params", "interpolators"]:
-                        method = self._parse_interpolation_method(definition)
-                        self._set_interpolation_method(("default",), method)
-                        break
+                # get interpolation method dict
+                method = self._parse_interpolation_method(interp_definition)
 
-                # if key is not a tuple, convert it to one and set it to the udims key
-                if not isinstance(key, tuple):
-                    udims = (key,)
+                # specify dims
+                if "dims" in interp_definition:
+                    if isinstance(interp_definition["dims"], list):
+                        interp_definition["dims"].sort()  # make sure the dims are always in the same order
+                        udims = tuple(interp_definition["dims"])
+                    else:
+                        raise TypeError('The "dims" key of an interpolation definition must be a list')
                 else:
-                    udims = key
+                    udims = ("default",)
 
                 # make sure udims are not already specified in config
                 for config_dims in iter(self.config):
                     if set(config_dims) & set(udims):
                         raise InterpolationException(
                             'Dimensions "{}" cannot be defined '.format(udims)
-                            + "multiple times in interpolation definition {}".format(definition)
+                            + "multiple times in interpolation definition {}".format(interp_definition)
                         )
-
-                # get interpolation method
-                method = self._parse_interpolation_method(definition[key])
-
                 # add all udims to definition
                 self._set_interpolation_method(udims, method)
 
@@ -173,7 +170,7 @@ class Interpolation(object):
         else:
             raise TypeError(
                 '"{}" is not a valid interpolation definition type. '.format(definition)
-                + "Interpolation definiton must be a string or dict"
+                + "Interpolation definiton must be a string or list of dicts"
             )
 
         # make sure ('default',) is always the last entry in config dictionary
diff --git a/podpac/core/data/test/test_interpolate.py b/podpac/core/data/test/test_interpolate.py
index df17d787..8536e94f 100644
--- a/podpac/core/data/test/test_interpolate.py
+++ b/podpac/core/data/test/test_interpolate.py
@@ -64,20 +64,16 @@ class TestInterpolation(object):
 
     def test_dict_definition(self):
 
-        # should handle a default definition without any dimensions specified as keys
+        # should handle a default definition without any dimensions
         interp = Interpolation({"method": "nearest", "params": {"spatial_tolerance": 1}})
         assert isinstance(interp.config[("default",)], dict)
         assert interp.config[("default",)]["method"] == "nearest"
         assert isinstance(interp.config[("default",)]["interpolators"][0], Interpolator)
         assert interp.config[("default",)]["params"] == {"spatial_tolerance": 1}
 
-        # should throw an error on _parse_interpolation_method(definition)
-        # if definition is not in INTERPOLATION_METHODS
-        with pytest.raises(InterpolationException):
-            Interpolation({("lat", "lon"): "test"})
-
         # handle string methods
-        interp = Interpolation({("lat", "lon"): "nearest"})
+        interp = Interpolation({"method": "nearest", "dims": ["lat", "lon"]})
+        print (interp.config)
         assert isinstance(interp.config[("lat", "lon")], dict)
         assert interp.config[("lat", "lon")]["method"] == "nearest"
         assert isinstance(interp.config[("default",)]["interpolators"][0], Interpolator)
@@ -87,34 +83,38 @@ class TestInterpolation(object):
 
         # should throw an error if method is not in dict
         with pytest.raises(InterpolationException):
-            Interpolation({("lat", "lon"): {"test": "test"}})
+            Interpolation([{"test": "test", "dims": ["lat", "lon"]}])
 
         # should throw an error if method is not a string
         with pytest.raises(InterpolationException):
-            Interpolation({("lat", "lon"): {"method": 5}})
+            Interpolation([{"method": 5, "dims": ["lat", "lon"]}])
 
         # should throw an error if method is not one of the INTERPOLATION_METHODS and no interpolators defined
         with pytest.raises(InterpolationException):
-            Interpolation({("lat", "lon"): {"method": "myinter"}})
+            Interpolation([{"method": "myinter", "dims": ["lat", "lon"]}])
 
         # should throw an error if params is not a dict
         with pytest.raises(TypeError):
-            Interpolation({("lat", "lon"): {"method": "nearest", "params": "test"}})
+            Interpolation([{"method": "nearest", "dims": ["lat", "lon"], "params": "test"}])
 
         # should throw an error if interpolators is not a list
         with pytest.raises(TypeError):
-            Interpolation({("lat", "lon"): {"method": "nearest", "interpolators": "test"}})
+            Interpolation([{"method": "nearest", "interpolators": "test", "dims": ["lat", "lon"]}])
 
         # should throw an error if interpolators are not Interpolator classes
         with pytest.raises(TypeError):
-            Interpolation({("lat", "lon"): {"method": "nearest", "interpolators": [NearestNeighbor, "test"]}})
+            Interpolation([{"method": "nearest", "interpolators": [NearestNeighbor, "test"], "dims": ["lat", "lon"]}])
 
         # should throw an error if dimension is defined twice
         with pytest.raises(InterpolationException):
-            Interpolation({("lat", "lon"): "nearest", "lat": "bilinear"})
+            Interpolation([{"method": "nearest", "dims": ["lat", "lon"]}, {"method": "bilinear", "dims": ["lat"]}])
+
+        # should throw an error if dimension is not a list
+        with pytest.raises(TypeError):
+            Interpolation([{"method": "nearest", "dims": "lat"}])
 
         # should handle standard INTEPROLATION_SHORTCUTS
-        interp = Interpolation({("lat", "lon"): {"method": "nearest"}})
+        interp = Interpolation([{"method": "nearest", "dims": ["lat", "lon"]}])
         assert isinstance(interp.config[("lat", "lon")], dict)
         assert interp.config[("lat", "lon")]["method"] == "nearest"
         assert isinstance(interp.config[("lat", "lon")]["interpolators"][0], Interpolator)
@@ -123,35 +123,47 @@ class TestInterpolation(object):
         # should not allow custom methods if interpolators can't support
         with pytest.raises(InterpolatorException):
             interp = Interpolation(
-                {("lat", "lon"): {"method": "myinter", "interpolators": [NearestNeighbor, NearestPreview]}}
+                [{"method": "myinter", "interpolators": [NearestNeighbor, NearestPreview], "dims": ["lat", "lon"]}]
             )
 
         # should allow custom methods if interpolators can support
         class MyInterp(Interpolator):
             methods_supported = ["myinter"]
 
-        interp = Interpolation({("lat", "lon"): {"method": "myinter", "interpolators": [MyInterp]}})
+        interp = Interpolation([{"method": "myinter", "interpolators": [MyInterp], "dims": ["lat", "lon"]}])
         assert interp.config[("lat", "lon")]["method"] == "myinter"
         assert isinstance(interp.config[("lat", "lon")]["interpolators"][0], MyInterp)
 
         # should allow params to be set
         interp = Interpolation(
-            {("lat", "lon"): {"method": "myinter", "interpolators": [MyInterp], "params": {"spatial_tolerance": 5}}}
+            [
+                {
+                    "method": "myinter",
+                    "interpolators": [MyInterp],
+                    "params": {"spatial_tolerance": 5},
+                    "dims": ["lat", "lon"],
+                }
+            ]
         )
+
         assert interp.config[("lat", "lon")]["params"] == {"spatial_tolerance": 5}
 
         # set default equal to empty tuple
-        interp = Interpolation({"lat": "bilinear"})
+        interp = Interpolation([{"method": "bilinear", "dims": ["lat"]}])
         assert interp.config[("default",)]["method"] == INTERPOLATION_DEFAULT
 
         # use default with override if not all dimensions are supplied
-        interp = Interpolation({"lat": "bilinear", "default": "nearest"})
+        interp = Interpolation([{"method": "bilinear", "dims": ["lat"]}, "nearest"])
         assert interp.config[("default",)]["method"] == "nearest"
 
         # make sure default is always the last key in the ordered config dict
-        interp = Interpolation({"default": "nearest", "lat": "bilinear"})
+        interp = Interpolation(["nearest", {"method": "bilinear", "dims": ["lat"]}])
         assert list(interp.config.keys())[-1] == ("default",)
 
+        # should sort the dims keys
+        interp = Interpolation(["nearest", {"method": "bilinear", "dims": ["lon", "lat"]}])
+        assert interp.config[("lat", "lon")]["method"] == "bilinear"
+
     def test_init_interpolators(self):
 
         # should set method
@@ -159,16 +171,16 @@ class TestInterpolation(object):
         assert interp.config[("default",)]["interpolators"][0].method == "nearest"
 
         # Interpolation init should init all interpolators in the list
-        interp = Interpolation({"default": {"method": "nearest", "params": {"spatial_tolerance": 1}}})
+        interp = Interpolation([{"method": "nearest", "params": {"spatial_tolerance": 1}}])
         assert interp.config[("default",)]["interpolators"][0].spatial_tolerance == 1
 
         # should throw TraitErrors defined by Interpolator
         with pytest.raises(tl.TraitError):
-            Interpolation({"default": {"method": "nearest", "params": {"spatial_tolerance": "tol"}}})
+            Interpolation([{"method": "nearest", "params": {"spatial_tolerance": "tol"}}])
 
         # should not allow undefined params
         with pytest.warns(DeprecationWarning):  # eventually, Traitlets will raise an exception here
-            interp = Interpolation({"default": {"method": "nearest", "params": {"myarg": 1}}})
+            interp = Interpolation([{"method": "nearest", "params": {"myarg": 1}}])
         with pytest.raises(AttributeError):
             assert interp.config[("default",)]["interpolators"][0].myarg == "tol"
 
@@ -212,10 +224,10 @@ class TestInterpolation(object):
         # set up a strange interpolation definition
         # we want to interpolate (lat, lon) first, then after (time, alt)
         interp = Interpolation(
-            {
-                ("lat", "lon"): {"method": "myinterp", "interpolators": [LatLon, TimeLat]},
-                ("time", "alt"): {"method": "myinterp", "interpolators": [TimeLat, Lon]},
-            }
+            [
+                {"method": "myinterp", "interpolators": [LatLon, TimeLat], "dims": ["lat", "lon"]},
+                {"method": "myinterp", "interpolators": [TimeLat, Lon], "dims": ["time", "alt"]},
+            ]
         )
 
         # default = 'nearest', which will return NearestPreview for can_select
@@ -271,10 +283,10 @@ class TestInterpolation(object):
         # set up a strange interpolation definition
         # we want to interpolate (lat, lon) first, then after (time, alt)
         interp = Interpolation(
-            {
-                ("lat", "lon"): {"method": "myinterp", "interpolators": [LatLon, TimeLat]},
-                ("time", "alt"): {"method": "myinterp", "interpolators": [TimeLat, Lon]},
-            }
+            [
+                {"method": "myinterp", "interpolators": [LatLon, TimeLat], "dims": ["lat", "lon"]},
+                {"method": "myinterp", "interpolators": [TimeLat, Lon], "dims": ["time", "alt"]},
+            ]
         )
 
         coords, cidx = interp.select_coordinates(srccoords, [], reqcoords)
@@ -301,7 +313,7 @@ class TestInterpolation(object):
             np.zeros(srcdata.shape), coords=[reqcoords[c].coordinates for c in reqcoords], dims=reqcoords.dims
         )
 
-        interp = Interpolation({("lat", "lon"): {"method": "myinterp", "interpolators": [TestInterp]}})
+        interp = Interpolation({"method": "myinterp", "interpolators": [TestInterp], "dims": ["lat", "lon"]})
         outdata = interp.interpolate(srccoords, srcdata, reqcoords, outdata)
 
         assert np.all(outdata == srcdata)
@@ -322,7 +334,7 @@ class TestInterpolation(object):
             np.zeros(srcdata.shape), coords=[reqcoords[c].coordinates for c in reqcoords], dims=reqcoords.dims
         )
 
-        interp = Interpolation({("lat", "lon"): {"method": "myinterp", "interpolators": [TestFakeInterp]}})
+        interp = Interpolation({"method": "myinterp", "interpolators": [TestFakeInterp], "dims": ["lat", "lon"]})
         outdata = interp.interpolate(srccoords, srcdata, reqcoords, outdata)
 
         assert np.all(outdata == srcdata)
@@ -387,7 +399,9 @@ class TestInterpolators(object):
             reqcoords = Coordinates([[-0.5, 1.5, 3.5], [0.5, 2.5, 4.5]], dims=["lat", "lon"])
             srccoords = Coordinates([[0, 1, 2, 3, 4, 5], [0, 1, 2, 3, 4, 5]], dims=["lat", "lon"])
 
-            interp = Interpolation({"lat": "nearest_preview", "lon": "nearest_preview"})
+            interp = Interpolation(
+                [{"method": "nearest_preview", "dims": ["lat"]}, {"method": "nearest_preview", "dims": ["lon"]}]
+            )
 
             srccoords, srccoords_index = srccoords.intersect(reqcoords, outer=True, return_indices=True)
             coords, cidx = interp.select_coordinates(srccoords, srccoords_index, reqcoords)
@@ -481,14 +495,14 @@ class TestInterpolators(object):
             node = MockArrayDataSource(
                 source=source,
                 native_coordinates=coords_src,
-                interpolation={"default": {"method": "nearest", "params": {"spatial_tolerance": 1.1}}},
+                interpolation={"method": "nearest", "params": {"spatial_tolerance": 1.1}},
             )
 
             coords_dst = Coordinates([[1, 1.2, 1.5, 5, 9]], dims=["lat"])
             output = node.eval(coords_dst)
 
-            print(output)
-            print(source)
+            print (output)
+            print (source)
             assert isinstance(output, UnitsDataArray)
             assert np.all(output.lat.values == coords_dst.coords["lat"])
             assert output.values[0] == source[0] and np.isnan(output.values[1]) and output.values[2] == source[1]
@@ -504,10 +518,8 @@ class TestInterpolators(object):
                 source=source,
                 native_coordinates=coords_src,
                 interpolation={
-                    "default": {
-                        "method": "nearest",
-                        "params": {"spatial_tolerance": 1.1, "time_tolerance": np.timedelta64(1, "D")},
-                    }
+                    "method": "nearest",
+                    "params": {"spatial_tolerance": 1.1, "time_tolerance": np.timedelta64(1, "D")},
                 },
             )
 
@@ -601,7 +613,7 @@ class TestInterpolators(object):
 
             assert isinstance(output, UnitsDataArray)
             assert np.all(output.lat.values == coords_dst.coords["lat"])
-            print(output)
+            print (output)
             assert output.data[0, 0] == 0.0
             assert output.data[0, 3] == 3.0
             assert output.data[1, 3] == 8.0
diff --git a/podpac/core/utils.py b/podpac/core/utils.py
index 6811de09..bb0929ed 100644
--- a/podpac/core/utils.py
+++ b/podpac/core/utils.py
@@ -233,6 +233,9 @@ class JSONEncoder(json.JSONEncoder):
         elif isinstance(obj, podpac.core.style.Style):
             return obj.definition
 
+        elif isinstance(obj, podpac.data.Interpolation):
+            return obj.definition
+
         # pint Units
         elif isinstance(obj, podpac.core.units.ureg.Unit):
             return str(obj)