szha commented on a change in pull request #9869: Exception handling 
documentation
URL: https://github.com/apache/incubator-mxnet/pull/9869#discussion_r170358355
 
 

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 File path: docs/tutorials/basic/exception_handling.md
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+# Exception Handling in MXNet
+
+This tutorial explains the exception handling support in MXNet, 
+and provides examples on how to throw and handle exceptions when in a 
multithreaded context.
+Although, the examples are in Python, they can be easily extended to MXNet
+language bindings.
+
+MXNet exceptions can be thrown from two areas:
+- MXNet main thread. For eg. Infershape and InferType.
+- Spawned threads:
+    * By dependency engine for operator execution in parallel
+    * By the iterators, during the data loading, text parsing phase etc.
+
+In the first case, the exception is thrown and can be handled in the main 
thread.
+In the second case, the exception is thrown in a spawned thread, caught and 
transported to the
+main thread, where it is rethrown. This tutorial will give more explanation 
and examples on how 
+to handle exceptions for the second case.
+
+## Prerequisites 
+
+To complete this tutorial, we need:
+- MXNet 
[7b24137](https://github.com/apache/incubator-mxnet/commit/7b24137ed45df605defa4ce72ec91554f6e445f0).
 See Instructions in [Setup and 
Installation](http://mxnet.io/install/index.html).
+
+## Exception Handling for Iterators
+
+The below example shows how to handle exceptions for iterators. In this 
example, 
+we populate a data and a labels file with lesser number of labels compared to 
the 
+number of samples. This should throw an exception. 
+
+The CSVIter uses the PrefetcherIter for loading and parsing data. 
+The PrefetcherIter spawns a producer thread in the background which prefetches
+the data while the main thread consumes the data. The exception is thrown in 
the spawned
+producer thread during the prefetching, when the label is not found 
corresponding to a specific sample. 
+
+The exception is transported to the main thread, where it is rethrown when 
Next is 
+called as part of the following line: `for batch in iter(data_train)`.
+
+In general, Exception may be rethrown as part of `Next` and `BeforeFirst` 
calls which correspond to `reset()` and `next()` methods in `MXDataIter` for 
Python language bindings.
+
+```python
+import os
+import mxnet as mx
+
+cwd = os.getcwd()
+data_path = os.path.join(cwd, "data.csv")
+label_path = os.path.join(cwd, "label.csv")
+
+with open(data_path, "w") as fout:
+    fout.write("1,2,3,4,5,6,7,8,9,10\n")
+    fout.write("1,2,3,4,5,6,7,8,9,10\n")
+    fout.write("1,2,3,4,5,6,7,8,9,10\n")
+    fout.write("1,2,3,4,5,6,7,8,9,10\n")
+    fout.write("1,2,3,4,5,6,7,8,9,10\n")
+    fout.write("1,2,3,4,5,6,7,8,9,10\n")
+    fout.write("1,2,3,4,5,6,7,8,9,10\n")
+    fout.write("1,2,3,4,5,6,7,8,9,10\n")
+
+with open(label_path, "w") as fout:
+    fout.write("label1")
+    fout.write("label2")
+    fout.write("label3")
+    fout.write("label1")
+    fout.write("label2")
+    fout.write("label3")
+    fout.write("label3")
 
 Review comment:
   for loop?

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