From 5f064d15fdcd9a4b4f4fd49e04d7fe5f0a76bdf8 Mon Sep 17 00:00:00 2001
From: Nils Werner <nils.werner@gmail.com>
Date: Wed, 10 Jan 2018 18:06:19 +0100
Subject: [PATCH 1/8] Implemented __array_interface__

---
 sparse/core.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/sparse/core.py b/sparse/core.py
index ecf59639..33cac726 100644
--- a/sparse/core.py
+++ b/sparse/core.py
@@ -488,6 +488,14 @@ def nbytes(self):
         """
         return self.data.nbytes + self.coords.nbytes
 
+    @property
+    def __array_interface__(self):
+        return {
+            'shape': self.shape,
+            'data': self.todense(),
+            'typestr': self.dtype.str,
+        }
+
     def __len__(self):
         """
         Get "length" of array, which is by definition the size of the first

From 25885be36c16f9661b254f52f509c139ded142a5 Mon Sep 17 00:00:00 2001
From: Nils Werner <nils.werner@gmail.com>
Date: Wed, 10 Jan 2018 21:04:59 +0100
Subject: [PATCH 2/8] __array__ instead of __array_interface__ seems like the
 way to go

---
 sparse/core.py | 31 ++++++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/sparse/core.py b/sparse/core.py
index 33cac726..77459702 100644
--- a/sparse/core.py
+++ b/sparse/core.py
@@ -488,13 +488,30 @@ def nbytes(self):
         """
         return self.data.nbytes + self.coords.nbytes
 
-    @property
-    def __array_interface__(self):
-        return {
-            'shape': self.shape,
-            'data': self.todense(),
-            'typestr': self.dtype.str,
-        }
+    def __array__(self, dtype=None):
+        """
+        Helper function to speed up :code:`np.array(x)` conversion
+
+        Parameters
+        ----------
+        dtype: type
+            Datatype of dense array. The sparse array is cast before densification.
+
+        Returns
+        -------
+        array_like
+            The dense array.
+
+        See Also
+        --------
+        numpy.ndarray.__array__ : Numpy equivalent function.
+
+        """
+        if dtype is not None:
+            tmp = self.astype(dtype)
+        else:
+            tmp = self
+        return tmp.todense()
 
     def __len__(self):
         """

From 8a1a4c230693351427451c6d71cb6b72f6a41352 Mon Sep 17 00:00:00 2001
From: Nils Werner <nils.werner@gmail.com>
Date: Thu, 11 Jan 2018 09:23:53 +0100
Subject: [PATCH 3/8] Simplified __array__ if a bit

---
 sparse/core.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/sparse/core.py b/sparse/core.py
index 77459702..19da8914 100644
--- a/sparse/core.py
+++ b/sparse/core.py
@@ -508,10 +508,9 @@ def __array__(self, dtype=None):
 
         """
         if dtype is not None:
-            tmp = self.astype(dtype)
+            return self.astype(dtype).todense()
         else:
-            tmp = self
-        return tmp.todense()
+            return self.todense()
 
     def __len__(self):
         """

From 916e0168b610d791896e2ebd92d910981c4db357 Mon Sep 17 00:00:00 2001
From: Nils Werner <nils.werner@gmail.com>
Date: Thu, 11 Jan 2018 09:24:16 +0100
Subject: [PATCH 4/8] Test comparing np.array(x) and x.todense()

---
 sparse/tests/test_core.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/sparse/tests/test_core.py b/sparse/tests/test_core.py
index 94800a26..9c751aec 100644
--- a/sparse/tests/test_core.py
+++ b/sparse/tests/test_core.py
@@ -887,3 +887,10 @@ def test_scalar_shape_construction():
 def test_len():
     s = sparse.random((20, 30, 40))
     assert len(s) == 20
+
+
+def test_numpy_todense():
+    # make sure that .astype(int) is never all zeros
+    s = sparse.random((20, 30, 40)) * 100
+    assert np.allclose(np.array(s, dtype=int), s.astype(int).todense())
+    assert np.allclose(np.array(s), s.todense())

From 120bf82e3701ee59082c18f3535e8c9747a8eaa5 Mon Sep 17 00:00:00 2001
From: Nils Werner <nils.werner@gmail.com>
Date: Thu, 11 Jan 2018 10:36:24 +0100
Subject: [PATCH 5/8] Use maybe_densify in __array__

---
 sparse/core.py | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/sparse/core.py b/sparse/core.py
index 19da8914..7ce0cad8 100644
--- a/sparse/core.py
+++ b/sparse/core.py
@@ -508,9 +508,9 @@ def __array__(self, dtype=None):
 
         """
         if dtype is not None:
-            return self.astype(dtype).todense()
+            return self.astype(dtype).maybe_densify()
         else:
-            return self.todense()
+            return self.maybe_densify()
 
     def __len__(self):
         """
@@ -2406,7 +2406,8 @@ def maybe_densify(self, allowed_nnz=1000, allowed_fraction=0.25):
             return self.todense()
         else:
             raise NotImplementedError("Operation would require converting "
-                                      "large sparse array to dense")
+                                      "large sparse array to dense. Use "
+                                      ".todense() to force densification.")
 
 
 def tensordot(a, b, axes=2):

From f615b795fa298a0ee58124ff5ad3ad23802537a5 Mon Sep 17 00:00:00 2001
From: Nils Werner <nils.werner@gmail.com>
Date: Mon, 15 Jan 2018 11:24:02 +0100
Subject: [PATCH 6/8] __array__ returns NotImplemented

---
 sparse/core.py            | 17 ++++++++---------
 sparse/tests/test_core.py | 10 ++++++++--
 2 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/sparse/core.py b/sparse/core.py
index 7ce0cad8..79a44316 100644
--- a/sparse/core.py
+++ b/sparse/core.py
@@ -490,27 +490,26 @@ def nbytes(self):
 
     def __array__(self, dtype=None):
         """
-        Helper function to speed up :code:`np.array(x)` conversion
+        Helper function that gets called during :code:`np.array(x)` conversion.
+        We deliberately return :code:`NotImplemented` to prevent accidental
+        densification.
 
         Parameters
         ----------
         dtype: type
-            Datatype of dense array. The sparse array is cast before densification.
+            Datatype requested by :code:`np.array(x)`. Has no effect on
+            output.
 
         Returns
         -------
-        array_like
-            The dense array.
-
+        NotImplemented
+            We do not implement this function, so this is what we return.
         See Also
         --------
         numpy.ndarray.__array__ : Numpy equivalent function.
 
         """
-        if dtype is not None:
-            return self.astype(dtype).maybe_densify()
-        else:
-            return self.maybe_densify()
+        return NotImplemented
 
     def __len__(self):
         """
diff --git a/sparse/tests/test_core.py b/sparse/tests/test_core.py
index 9c751aec..b9e7513b 100644
--- a/sparse/tests/test_core.py
+++ b/sparse/tests/test_core.py
@@ -892,5 +892,11 @@ def test_len():
 def test_numpy_todense():
     # make sure that .astype(int) is never all zeros
     s = sparse.random((20, 30, 40)) * 100
-    assert np.allclose(np.array(s, dtype=int), s.astype(int).todense())
-    assert np.allclose(np.array(s), s.todense())
+    with pytest.raises(ValueError):
+        assert np.array(s)
+
+    with pytest.raises(ValueError):
+        assert np.array(s, dtype=int)
+
+    with pytest.raises(ValueError):
+        assert np.allclose(s, s.todense())

From 7a9b597ad68c198d2ca7d38c9ee0c476dfd4e9c8 Mon Sep 17 00:00:00 2001
From: Nils Werner <nils.werner@gmail.com>
Date: Mon, 15 Jan 2018 16:30:09 +0100
Subject: [PATCH 7/8] Fix docstring

---
 sparse/core.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/sparse/core.py b/sparse/core.py
index 79a44316..d67a7e8d 100644
--- a/sparse/core.py
+++ b/sparse/core.py
@@ -2397,7 +2397,8 @@ def maybe_densify(self, allowed_nnz=1000, allowed_fraction=0.25):
         >>> s.maybe_densify(allowed_nnz=5, allowed_fraction=0.25)
         Traceback (most recent call last):
             ...
-        NotImplementedError: Operation would require converting large sparse array to dense
+        NotImplementedError: Operation would require converting large sparse \
+array to dense. Use .todense() to force densification.
         """
         elements = np.prod(self.shape)
 

From 13f7841060cbbacb3a2ab8ee17099099c69076c5 Mon Sep 17 00:00:00 2001
From: Nils Werner <nils.werner@gmail.com>
Date: Mon, 15 Jan 2018 16:56:16 +0100
Subject: [PATCH 8/8] Docs

---
 docs/generated/sparse.COO.__array__.rst | 6 ++++++
 docs/generated/sparse.COO.rst           | 1 +
 sparse/core.py                          | 4 +++-
 3 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 docs/generated/sparse.COO.__array__.rst

diff --git a/docs/generated/sparse.COO.__array__.rst b/docs/generated/sparse.COO.__array__.rst
new file mode 100644
index 00000000..d2ff7ba3
--- /dev/null
+++ b/docs/generated/sparse.COO.__array__.rst
@@ -0,0 +1,6 @@
+COO\.\_\_array\_\_
+==================
+
+.. currentmodule:: sparse
+
+.. automethod:: COO.__array__
diff --git a/docs/generated/sparse.COO.rst b/docs/generated/sparse.COO.rst
index 99fcb719..21826cfb 100644
--- a/docs/generated/sparse.COO.rst
+++ b/docs/generated/sparse.COO.rst
@@ -68,6 +68,7 @@ COO
       COO.to_scipy_sparse
       COO.tocsc
       COO.tocsr
+      COO.__array__
 
    .. rubric:: :doc:`Other operations <../user_manual/operations/other>`
    .. autosummary::
diff --git a/sparse/core.py b/sparse/core.py
index d67a7e8d..47a98fdf 100644
--- a/sparse/core.py
+++ b/sparse/core.py
@@ -503,7 +503,9 @@ def __array__(self, dtype=None):
         Returns
         -------
         NotImplemented
-            We do not implement this function, so this is what we return.
+            We do not implement this function, so we return
+            :obj:`NotImplemented`.
+
         See Also
         --------
         numpy.ndarray.__array__ : Numpy equivalent function.