MAINT Improve pipeline's validation readability (#1066)

Co-authored-by: Guillaume Lemaitre <guillaume@probabl.ai>

imblearn/pipeline.py

@@ -163,21 +163,20 @@ def _validate_steps(self):
         for t in transformers:
             if t is None or t == "passthrough":
                 continue
-            if not (
-                hasattr(t, "fit")
-                or hasattr(t, "fit_transform")
-                or hasattr(t, "fit_resample")
-            ) or not (hasattr(t, "transform") or hasattr(t, "fit_resample")):
+
+            is_transfomer = hasattr(t, "fit") and hasattr(t, "transform")
+            is_sampler = hasattr(t, "fit_resample")
+            is_not_transfomer_or_sampler = not (is_transfomer or is_sampler)
+
+            if is_not_transfomer_or_sampler:
                 raise TypeError(
                     "All intermediate steps of the chain should "
                     "be estimators that implement fit and transform or "
                     "fit_resample (but not both) or be a string 'passthrough' "
                     "'%s' (type %s) doesn't)" % (t, type(t))
                 )
 
-            if hasattr(t, "fit_resample") and (
-                hasattr(t, "fit_transform") or hasattr(t, "transform")
-            ):
+            if is_transfomer and is_sampler:
                 raise TypeError(
                     "All intermediate steps of the chain should "
                     "be estimators that implement fit and transform or "