No need to check or require a symmetrical sparse matrix if the kNN s precomputed

umap/umap_.py

@@ -2480,18 +2480,19 @@ def fit(self, X, y=None, force_all_finite=True, **kwargs):
             print(ts(), "Construct fuzzy simplicial set")
 
         if self.metric == "precomputed" and self._sparse_data:
-            # For sparse precomputed distance matrices, we just argsort the rows to find
-            # nearest neighbors. To make this easier, we expect matrices that are
-            # symmetrical (so we can find neighbors by looking at rows in isolation,
-            # rather than also having to consider that sample's column too).
-            # print("Computing KNNs for sparse precomputed distances...")
-            if sparse_tril(X).getnnz() != sparse_triu(X).getnnz():
-                raise ValueError(
-                    "Sparse precomputed distance matrices should be symmetrical!"
-                )
             if not np.all(X.diagonal() == 0):
                 raise ValueError("Non-zero distances from samples to themselves!")
             if self.knn_dists is None:
+                # For sparse precomputed distance matrices, we just argsort the rows
+                # to find nearest neighbors. To make this easier, we expect matrices
+                # that are symmetrical (so we can find neighbors by looking at rows
+                # in isolation, rather than also having to consider that sample's 
+                # column too). 
+                # print("Computing KNNs for sparse precomputed distances...")
+                if sparse_tril(X).getnnz() != sparse_triu(X).getnnz():
+                    raise ValueError(
+                        "Sparse precomputed distance matrices should be symmetrical!"
+                    )
                 self._knn_indices = np.zeros((X.shape[0], self.n_neighbors), dtype=int)
                 self._knn_dists = np.zeros(self._knn_indices.shape, dtype=float)
                 for row_id in range(X.shape[0]):