Reduce complexity of 'sklearn_train_linear_model' (#1375)

Summary:
Pull Request resolved: #1375

Reduce complexity of 'sklearn_train_linear_model'

Reviewed By: jsawruk

Differential Revision: D64438317

fbshipit-source-id: aa99f2ec9d9a0b349a423fc0a37e9b21b2a0ff39

captum/_utils/models/linear_model/train.py

@@ -1,6 +1,8 @@
 # pyre-strict
 import time
 import warnings
+from functools import reduce
+from types import ModuleType
 from typing import Any, Callable, cast, Dict, List, Optional, Tuple
 
 import torch
@@ -282,14 +284,38 @@ def forward(self, x):
         return (x - self.mean) / (self.std + self.eps)
 
 
+def _import_sklearn() -> ModuleType:
+    try:
+        import sklearn
+        import sklearn.linear_model
+        import sklearn.svm
+    except ImportError:
+        raise ValueError("sklearn is not available. Please install sklearn >= 0.23")
+
+    if not sklearn.__version__ >= "0.23.0":
+        warnings.warn(
+            "Must have sklearn version 0.23.0 or higher to use "
+            "sample_weight in Lasso regression.",
+            stacklevel=1,
+        )
+    return sklearn
+
+
+def _import_numpy() -> ModuleType:
+    try:
+        import numpy
+    except ImportError:
+        raise ValueError("numpy is not available. Please install numpy.")
+    return numpy
+
+
 def sklearn_train_linear_model(
     model: LinearModel,
     dataloader: DataLoader,
     construct_kwargs: Dict[str, Any],
     sklearn_trainer: str = "Lasso",
     norm_input: bool = False,
-    # pyre-fixme[2]: Parameter must be annotated.
-    **fit_kwargs,
+    **fit_kwargs: Any,
 ) -> Dict[str, float]:
     r"""
     Alternative method to train with sklearn. This does introduce some slight
@@ -318,26 +344,9 @@ def sklearn_train_linear_model(
         fit_kwargs
             Other arguments to send to `sklearn_trainer`'s `.fit` method
     """
-    from functools import reduce
-
-    try:
-        import numpy as np
-    except ImportError:
-        raise ValueError("numpy is not available. Please install numpy.")
-
-    try:
-        import sklearn
-        import sklearn.linear_model
-        import sklearn.svm
-    except ImportError:
-        raise ValueError("sklearn is not available. Please install sklearn >= 0.23")
-
-    if not sklearn.__version__ >= "0.23.0":
-        warnings.warn(
-            "Must have sklearn version 0.23.0 or higher to use "
-            "sample_weight in Lasso regression.",
-            stacklevel=1,
-        )
+    # Lazy imports
+    np = _import_numpy()
+    sklearn = _import_sklearn()
 
     num_batches = 0
     xs, ys, ws = [], [], []
@@ -369,8 +378,8 @@ def sklearn_train_linear_model(
 
     t1 = time.time()
     # pyre-fixme[29]: `str` is not a function.
-    sklearn_model = reduce(
-        lambda val, el: getattr(val, el), [sklearn] + sklearn_trainer.split(".")
+    sklearn_model = reduce(  # type: ignore
+        lambda val, el: getattr(val, el), [sklearn] + sklearn_trainer.split(".")  # type: ignore  # noqa: E501
     )(**construct_kwargs)
     try:
         sklearn_model.fit(x, y, sample_weight=w, **fit_kwargs)