Adds sklearn version check for ColumnTransformer import

dask_ml/compose/__init__.py

@@ -2,6 +2,15 @@
 
 These estimators are useful for working with heterogenous tabular data.
 """
-from ._column_transformer import ColumnTransformer, make_column_transformer
+from .._compat import SK_VERSION
+from packaging.version import parse
 
-__all__ = ["ColumnTransformer", "make_column_transformer"]
+__all__ = []
+
+if SK_VERSION >= parse("0.20.0.dev0"):
+    from ._column_transformer import ColumnTransformer, make_column_transformer  # noqa
+
+    __all__.extend(["ColumnTransformer", "make_column_transformer"])
+
+del SK_VERSION
+del parse

dask_ml/compose/_column_transformer.py

@@ -2,9 +2,16 @@
 import dask.dataframe as dd
 import numpy as np
 import pandas as pd
-import sklearn.compose
+from packaging import version
 from scipy import sparse
-from sklearn.compose._column_transformer import _get_transformer_list
+
+from .._compat import SK_VERSION
+
+if SK_VERSION >= version.Version("0.20.0dev0"):
+    import sklearn.compose
+    from sklearn.compose._column_transformer import _get_transformer_list
+else:
+    raise ImportError("scikit-learn >= 0.20 required from ColumnTransformer")
 
 
 class ColumnTransformer(sklearn.compose.ColumnTransformer):
@@ -23,6 +30,10 @@ class ColumnTransformer(sklearn.compose.ColumnTransformer):
 
     .. versionadded:: 0.9.0
 
+    .. note::
+
+       This requires scikit-learn 0.20.0 or newer.
+
     Parameters
     ----------
     transformers : list of tuples