diff --git a/neuralmonkey/runners/base_runner.py b/neuralmonkey/runners/base_runner.py
index bc60ea1a2..8075563c2 100644
--- a/neuralmonkey/runners/base_runner.py
+++ b/neuralmonkey/runners/base_runner.py
@@ -4,7 +4,6 @@
 import numpy as np
 import tensorflow as tf
 
-from neuralmonkey.logging import notice
 from neuralmonkey.model.model_part import GenericModelPart
 from neuralmonkey.model.feedable import Feedable
 from neuralmonkey.model.parameterized import Parameterized
@@ -25,7 +24,7 @@ class ExecutionResult(NamedTuple(
          ("scalar_summaries", tf.Summary),
          ("histogram_summaries", tf.Summary),
          ("image_summaries", tf.Summary)])):
-    """A data structure that represents a result of a graph execution.
+    """A data structure that represents the result of a graph execution.
 
     The goal of each runner is to populate this structure and set it as its
     ``self._result``.
@@ -40,8 +39,41 @@ class ExecutionResult(NamedTuple(
 
 
 class GraphExecutor(GenericModelPart):
+    """The abstract parent class of all graph executors.
+
+    In Neural Monkey, a graph executor is an object that retrieves tensors
+    from the computational graph. The two major groups of graph executors are
+    trainers and runners.
+
+    Each graph executor is an instance of `GenericModelPart` class, which means
+    it has parameterized and feedable dependencies which reference the model
+    part objects needed to be created in order to compute the tensors of
+    interest (called "fetches").
+
+    Every graph executor has a method called `get_executable`, which returns
+    an `GraphExecutor.Executable` instance, which specifies what tensors to
+    execute and collects results from the session execution.
+    """
 
     class Executable(Generic[Executor]):
+        """Abstract base class for executables.
+
+        Executables are objects associated with the graph executors. Each
+        executable has two main functions: `next_to_execute` and
+        `collect_results`. These functions are called in a loop, until
+        the executable's result has been set.
+
+        To make use of Mypy's type checking, the executables are generic and
+        are parameterized by the type of their graph executor. Since Python
+        does not know the concept of nested classes, each executable receives
+        the instance of the graph executor through its constructor.
+
+        When subclassing `GraphExecutor`, it is also necessary to subclass
+        the `Executable` class and name it `Executable`, so it overrides the
+        definition of this class. Following this guideline, the default
+        implementation of the `get_executable` function on the graph executor
+        will work without the need of overriding it.
+        """
 
         def __init__(self,
                      executor: Executor,
@@ -110,6 +142,12 @@ def parameterizeds(self) -> Set[Parameterized]:
 
 
 class BaseRunner(GraphExecutor, Generic[MP]):
+    """Base class for runners.
+
+    Runners are graph executors that retrieve tensors from the model without
+    changing the model parameters. Each runner has a top-level model part it
+    relates to.
+    """
 
     # pylint: disable=too-few-public-methods
     # Pylint issue here: https://github.com/PyCQA/pylint/issues/2607
@@ -130,12 +168,9 @@ def __init__(self,
                  decoder: MP) -> None:
         GraphExecutor.__init__(self, {decoder})
         self.output_series = output_series
+        # TODO(tf-data) rename decoder to something more general
         self.decoder = decoder
 
-        if not hasattr(decoder, "data_id"):
-            notice("Top-level decoder {} does not have the 'data_id' attribute"
-                   .format(decoder))
-
     @property
     def decoder_data_id(self) -> Optional[str]:
         return getattr(self.decoder, "data_id", None)