Repository: fatiando/verde (python)

Issue:
Allow Spline to take a region argument
Right now, `verde.Spline` uses the data region to create a regular grid of forces (if `spacing` is provided). The problem with this is that the model will change between runs of a kfold cross-validation because the data changed. A way around that is allow `Spline` to take a `region` argument that fixes this region.

Simply checking if the data region already exists is not good because there is the implicit assumption that it is set to the first data-set that we fitted. And there is no way to change that via `set_params`. Having an explicit parameter is better. 

Pull request:
Functions for cross-validation
Implements the `cross_val_score` and `train_test_split` functions 
with a similar interface as scikit-learn but that can act on our 
multi-component spatial data with optional weights.
`cross_val_score` can take a `dask.distributed.Client` and submit
tasks to it instead of doing the actual calculations. In this case,
the scores returned are futures and can be used in other distributed
calculations. If no client is given, then the computations are done
serially. Created a `DummyClient` that just executes the function
immediately to simplify the code.

Adds the `region` option for `verde.Spline` so that it can be used with
`cross_val_score`. Otherwise, model region would change for 
different data folds.

Fixes #55 

Patch:
diff --git a/.gitignore b/.gitignore
index 2a61bde..609124d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,3 +17,5 @@ doc/api/backreferences
 .ipynb_checkpoints
 *.egg-info
 MANIFEST
+dask-worker-space
+.coverage.*
diff --git a/.pylintrc b/.pylintrc
index 2064e80..7bfc216 100644
--- a/.pylintrc
+++ b/.pylintrc
@@ -240,7 +240,7 @@ function-name-hint=(([a-z][a-z0-9_]{2,50})|(_[a-z0-9_]*))$
 function-rgx=(([a-z][a-z0-9_]{2,50})|(_[a-z0-9_]*))$
 
 # Good variable names which should always be accepted, separated by a comma
-good-names=i,j,k,w,e,s,n,x,y,z
+good-names=i,j,k,w,e,s,n,x,y,z,cv
 
 # Include a hint for the correct naming format with invalid-name
 include-naming-hint=no
diff --git a/Makefile b/Makefile
index 97fc983..0419305 100644
--- a/Makefile
+++ b/Makefile
@@ -41,5 +41,6 @@ check: pep8 lint
 
 clean:
 	find . -name "*.pyc" -exec rm -v {} \;
+	find . -name ".coverage.*" -exec rm -v {} \;
 	rm -rvf build dist MANIFEST *.egg-info __pycache__ .coverage .cache .pytest_cache
-	rm -rvf $(TESTDIR)
+	rm -rvf $(TESTDIR) dask-worker-space
diff --git a/examples/spline.py b/examples/spline.py
index 3af0bfa..f88682a 100644
--- a/examples/spline.py
+++ b/examples/spline.py
@@ -40,9 +40,18 @@ chain = vd.Chain([
     ('spline', vd.Spline(damping=1e-10))])
 print(chain)
 
-# Weights need to 1/uncertainty**2 for the error propagation in BlockMean to
-# work
-chain.fit(projection(*coordinates), data.velocity_up, weights=1/data.std_up**2)
+# Split the data into a training and testing set. We'll use the training set to
+# grid the data and the testing set to validate our spline model. Weights need
+# to 1/uncertainty**2 for the error propagation in BlockMean to work.
+train, test = vd.train_test_split(projection(*coordinates), data.velocity_up,
+                                  weights=1/data.std_up**2, random_state=0)
+# Fit the model on the training set
+chain.fit(*train)
+# And calculate an R^2 score coefficient on the testing set. The best possible
+# score (perfect prediction) is 1. This can tell us how good our spline is at
+# predicting data that was not in the input dataset.
+score = chain.score(*test)
+print("\nScore: {:.3f}".format(score))
 
 # Create a grid of the vertical velocity and mask it to only show points close
 # to the actual data.
diff --git a/verde/__init__.py b/verde/__init__.py
index e3fe520..9b1fa65 100644
--- a/verde/__init__.py
+++ b/verde/__init__.py
@@ -15,6 +15,7 @@ from .scipygridder import ScipyGridder
 from .trend import Trend, trend_jacobian, VectorTrend
 from .chain import Chain
 from .spline import Spline, spline_jacobian
+from .model_selection import cross_val_score, train_test_split
 
 
 # Get the version number through versioneer
diff --git a/verde/base.py b/verde/base.py
index 01af1cc..4907b68 100644
--- a/verde/base.py
+++ b/verde/base.py
@@ -562,11 +562,11 @@ def check_fit_input(coordinates, data, weights, ravel=True):
 
     """
     data = check_data(data)
+    weights = check_data(weights)
     if any(i.shape != j.shape for i in coordinates for j in data):
         raise ValueError(
             "Coordinate and data arrays must have the same shape.")
-    if weights is not None:
-        weights = check_data(weights)
+    if any(w is not None for w in weights):
         if len(weights) != len(data):
             raise ValueError(
                 "Number of data '{}' and weights '{}' must be equal."
diff --git a/verde/chain.py b/verde/chain.py
index 27f3971..73c66ce 100644
--- a/verde/chain.py
+++ b/verde/chain.py
@@ -69,11 +69,14 @@ class Chain(BaseGridder):
         coordinates : tuple of arrays
             Arrays with the coordinates of each data point. Should be in the
             following order: (easting, northing, vertical, ...).
-        data : array
-            The data values of each data point.
-        weights : None or array
-            If not None, then the weights assigned to each data point.
-            Typically, this should be 1 over the data uncertainty squared.
+        data : array or tuple of arrays
+            the data values of each data point. if the data has more than one
+            component, *data* must be a tuple of arrays (one for each
+            component).
+        weights : none or array or tuple of arrays
+            if not none, then the weights assigned to each data point. if more
+            than one data component is provided, you must provide a weights
+            array for each data component (if not none).
 
         Returns
         -------
diff --git a/verde/model_selection.py b/verde/model_selection.py
new file mode 100644
index 0000000..fd8eda4
--- /dev/null
+++ b/verde/model_selection.py
@@ -0,0 +1,234 @@
+"""
+Functions for automating model selection through cross-validation.
+
+Supports using a dask.distributed.Client object for parallelism. The
+DummyClient is used as a serial version of the parallel client.
+"""
+import numpy as np
+from sklearn.model_selection import KFold, ShuffleSplit
+
+from .base import check_fit_input
+
+
+class DummyClient():  # pylint: disable=no-self-use,too-few-public-methods
+    """
+    Dummy client to mimic a dask.distributed.Client for immediate local
+    execution.
+
+    >>> client = DummyClient()
+    >>> client.submit(sum, (1, 2, 3))
+    6
+
+    """
+
+    def submit(self, function, *args, **kwargs):
+        "Execute function with the given arguments and return its output"
+        return function(*args, **kwargs)
+
+
+def train_test_split(coordinates, data, weights=None, **kwargs):
+    r"""
+    Split a dataset into a training and a testing set for cross-validation.
+
+    Similar to :func:`sklearn.model_selection.train_test_split` but is tuned to
+    work on multi-component spatial data with optional weights.
+
+    Extra keyword arguments will be passed to
+    :class:`sklearn.model_selection.ShuffleSplit`, except for ``n_splits``
+    which is always 1.
+
+    Parameters
+    ----------
+    coordinates : tuple of arrays
+        Arrays with the coordinates of each data point. Should be in the
+        following order: (easting, northing, vertical, ...).
+    data : array or tuple of arrays
+        the data values of each data point. If the data has more than one
+        component, *data* must be a tuple of arrays (one for each component).
+    weights : none or array or tuple of arrays
+        if not none, then the weights assigned to each data point. If more than
+        one data component is provided, you must provide a weights array for
+        each data component (if not none).
+
+    Returns
+    -------
+    train, test : tuples
+        Each is a tuple = (coordinates, data, weights) generated by separating
+        the input values randomly.
+
+    Examples
+    --------
+
+    >>> import numpy as np
+    >>> # Split 2-component data with weights
+    >>> data = (np.array([1, 3, 5, 7]), np.array([0, 2, 4, 6]))
+    >>> coordinates = (np.arange(4), np.arange(-4, 0))
+    >>> weights = (np.array([1, 1, 2, 1]), np.array([1, 2, 1, 1]))
+    >>> train, test = train_test_split(coordinates, data, weights,
+    ...                                random_state=0)
+    >>> print("Coordinates:", train[0], test[0], sep='\n  ')
+    Coordinates:
+      (array([3, 1, 0]), array([-1, -3, -4]))
+      (array([2]), array([-2]))
+    >>> print("Data:", train[1], test[1], sep='\n  ')
+    Data:
+      (array([7, 3, 1]), array([6, 2, 0]))
+      (array([5]), array([4]))
+    >>> print("Weights:", train[2], test[2], sep='\n  ')
+    Weights:
+      (array([1, 1, 1]), array([1, 2, 1]))
+      (array([2]), array([1]))
+    >>> # Split single component data without weights
+    >>> train, test = train_test_split(coordinates, data[0], None,
+    ...                                random_state=0)
+    >>> print("Coordinates:", train[0], test[0], sep='\n  ')
+    Coordinates:
+      (array([3, 1, 0]), array([-1, -3, -4]))
+      (array([2]), array([-2]))
+    >>> print("Data:", train[1], test[1], sep='\n  ')
+    Data:
+      (array([7, 3, 1]),)
+      (array([5]),)
+    >>> print("Weights:", train[2], test[2], sep='\n  ')
+    Weights:
+      (None,)
+      (None,)
+
+    """
+    args = check_fit_input(coordinates, data, weights, ravel=False)
+    ndata = args[1][0].size
+    indices = np.arange(ndata)
+    split = next(ShuffleSplit(n_splits=1, **kwargs).split(indices))
+    train, test = (tuple(select(i, index) for i in args) for index in split)
+    return train, test
+
+
+def cross_val_score(estimator, coordinates, data, weights=None, cv=None,
+                    client=None):
+    """
+    Score an estimator/gridder using cross-validation.
+
+    Similar to :func:`sklearn.model_selection.cross_val_score` but modified to
+    accept spatial multi-component data with weights.
+
+    By default, will use :class:`sklearn.model_selection.KFold` to split the
+    dataset. Another cross-validation class can be passed in through the *cv*
+    argument.
+
+    Can optionally run in parallel using `dask <https://dask.pydata.org/>`__.
+    To do this, pass in a :class:`dask.distributed.Client` as the *client*
+    argument. Tasks in this function will be submitted to the dask cluster,
+    which can be local. In this case, the resulting scores won't be the actual
+    values but :class:`dask.distributed.Future` objects. Call their
+    ``.result()`` methods to get back the values or pass them along to other
+    dask computations.
+
+    Parameters
+    ----------
+    estimator : verde gridder
+        Any verde gridder class that has the ``fit`` and ``score`` methods.
+    coordinates : tuple of arrays
+        Arrays with the coordinates of each data point. Should be in the
+        following order: (easting, northing, vertical, ...).
+    data : array or tuple of arrays
+        the data values of each data point. If the data has more than one
+        component, *data* must be a tuple of arrays (one for each component).
+    weights : none or array or tuple of arrays
+        if not none, then the weights assigned to each data point. If more than
+        one data component is provided, you must provide a weights array for
+        each data component (if not none).
+    cv : None or cross-validation generator
+        Any scikit-learn cross-validation generator. Defaults to
+        :class:`sklearn.model_selection.KFold`.
+    client : None or dask.distributed.Client
+        If None, then computations are run serially. Otherwise, should be a
+        dask ``Client`` object. It will be used to dispatch computations to the
+        dask cluster.
+
+    Returns
+    -------
+    scores : list
+        List of scores for each split of the cross-validation generator. If
+        *client* is not None, then the scores will be futures.
+
+    Examples
+    --------
+
+    >>> from verde import grid_coordinates, Trend
+    >>> coords = grid_coordinates((0, 10, -10, -5), spacing=0.1)
+    >>> data = 10 - coords[0] + 0.5*coords[1]
+    >>> # A linear trend should perfectly predict this data
+    >>> scores = cross_val_score(Trend(degree=1), coords, data)
+    >>> print(', '.join(['{:.2f}'.format(score) for score in scores]))
+    1.00, 1.00, 1.00
+
+    >>> # To run parallel, we need to create a dask.distributed Client. It will
+    >>> # create a local cluster if no arguments are given so we can run the
+    >>> # scoring on a single machine.
+    >>> from dask.distributed import Client
+    >>> client = Client()
+    >>> # The scoring will now only submit tasks to our local cluster
+    >>> scores = cross_val_score(Trend(degree=1), coords, data, client=client)
+    >>> # The scores are not the actual values but Futures
+    >>> type(scores[0])
+    <class 'distributed.client.Future'>
+    >>> # We need to call .result() to get back the actual value
+    >>> print('{:.2f}'.format(scores[0].result()))
+    1.00
+
+    """
+    coordinates, data, weights = check_fit_input(coordinates, data, weights,
+                                                 ravel=False)
+    if client is None:
+        client = DummyClient()
+    if cv is None:
+        cv = KFold(shuffle=True, random_state=0)
+    ndata = data[0].size
+    args = (coordinates, data, weights)
+    scores = []
+    for train, test in cv.split(np.arange(ndata)):
+        train_data, test_data = (tuple(select(i, index) for i in args)
+                                 for index in (train, test))
+        score = client.submit(fit_score, estimator, train_data, test_data)
+        scores.append(score)
+    return scores
+
+
+def fit_score(estimator, train_data, test_data):
+    """
+    Fit an estimator on the training data and then score it on the testing data
+    """
+    estimator.fit(*train_data)
+    return estimator.score(*test_data)
+
+
+def select(arrays, index):
+    """
+    Index each array in a tuple of arrays.
+
+    If the arrays tuple contains a ``None``, the entire tuple will be returned
+    as is.
+
+    Parameters
+    ----------
+    arrays : tuple of arrays
+    index : array
+        An array of indices to select from arrays.
+
+    Returns
+    -------
+    indexed_arrays : tuple of arrays
+
+    Examples
+    --------
+
+    >>> import numpy as np
+    >>> select((np.arange(5), np.arange(-3, 2, 1)), [1, 3])
+    (array([1, 3]), array([-2,  0]))
+    >>> select((None, None, None, None), [1, 2])
+    (None, None, None, None)
+
+    """
+    if arrays is None or any(i is None for i in arrays):
+        return arrays
+    return tuple(i.ravel()[index] for i in arrays)
diff --git a/verde/spline.py b/verde/spline.py
index 4c723fa..6706065 100644
--- a/verde/spline.py
+++ b/verde/spline.py
@@ -41,7 +41,8 @@ class Spline(BaseGridder):
 
     Approximate (and more stable) solutions can be obtained by applying damping
     regularization to smooth the estimated forces (and interpolated values) or
-    by distributing the forces on a regular grid instead of at the data points.
+    by distributing the forces on a regular grid instead (using arguments
+    *spacing*, *shape*, and/or *region*).
 
     Data weights can be used during fitting but only have an any effect when
     using the approximate solutions.
@@ -66,6 +67,10 @@ class Spline(BaseGridder):
     spacing : None or float or tuple
         If not None, then should be the spacing of the regular grid of forces.
         See :func:`verde.grid_coordinates` for details.
+    region : None or tuple
+        If not None, then the boundaries (``[W, E, S, N]``) used to generate a
+        regular grid of forces. If None is given, then will use the boundaries
+        of data given to the first call to :meth:`~verde.Spline.fit`.
 
     Attributes
     ----------
@@ -84,11 +89,13 @@ class Spline(BaseGridder):
 
     """
 
-    def __init__(self, fudge=1e-5, damping=None, shape=None, spacing=None):
+    def __init__(self, fudge=1e-5, damping=None, shape=None, spacing=None,
+                 region=None):
         self.damping = damping
         self.shape = shape
         self.spacing = spacing
         self.fudge = fudge
+        self.region = region
 
     def fit(self, coordinates, data, weights=None):
         """
@@ -132,7 +139,12 @@ class Spline(BaseGridder):
         if self.shape is None and self.spacing is None:
             self.force_coords_ = tuple(i.copy() for i in coordinates[:2])
         else:
-            self.force_coords_ = grid_coordinates(self.region_,
+            # Set the force grid region to the data region if none is given.
+            # This will only happen once so the model won't change during later
+            # fits.
+            if self.region is None:
+                self.region = self.region_
+            self.force_coords_ = grid_coordinates(self.region,
                                                   shape=self.shape,
                                                   spacing=self.spacing)
         jac = spline_jacobian(coordinates[:2], self.force_coords_, self.fudge)


Test patch:
diff --git a/verde/tests/test_base.py b/verde/tests/test_base.py
index ed1b712..7e24eb5 100644
--- a/verde/tests/test_base.py
+++ b/verde/tests/test_base.py
@@ -173,6 +173,21 @@ def test_basegridder_projection():
                         angular*coordinates_true[0][0, :] + linear)
 
 
+def test_check_fit_input():
+    "Make sure no exceptions are raised for standard cases"
+    size = 20
+    data = np.arange(size)
+    coords = (np.arange(size), np.arange(size))
+    weights = np.arange(size)
+    check_fit_input(coords, data, None)
+    check_fit_input(coords, data, weights)
+    check_fit_input(coords, (data, data), None)
+    check_fit_input(coords, (data, data), (weights, weights))
+    check_fit_input(coords, (data, data), (None, None))
+    check_fit_input(coords, (data,), (None,))
+    check_fit_input(coords, (data,), (weights,))
+
+
 def test_check_fit_input_fails_coordinates():
     "Test the failing conditions for check_fit_input"
     coords = (np.arange(20), np.arange(20))
diff --git a/verde/tests/test_spline.py b/verde/tests/test_spline.py
index e52f63d..1587ce3 100644
--- a/verde/tests/test_spline.py
+++ b/verde/tests/test_spline.py
@@ -69,6 +69,8 @@ def test_spline_grid():
     spline = Spline(shape=(35, 35)).fit(coords, data.scalars)
     npt.assert_allclose(spline.score(coords, data.scalars), 1, rtol=1e-2)
     assert spline.force_.size == 35**2
+    npt.assert_allclose(spline.force_coords_[0].min(), data.easting.min())
+    npt.assert_allclose(spline.force_coords_[1].min(), data.northing.min())
     shape = (3, 3)
     region = (2700, 3200, -7700, -7200)
     # Tolerance needs to be kind of high to allow for error due to small