Add example on ignoring warnings from the commnd line

Author: gjbex

source-code/error-handling/README.md

@@ -8,3 +8,5 @@ Illustrations of error handling.
    ways ot handle errors.
 1. `data.txt`: data file to use as input.
 1. `data_not_ok.txt`: incorrect data file to use as input.
+1. `warnings`: illustrations of using the `warmings` module
+   for error handling.

source-code/error-handling/warnings/README.md

@@ -0,0 +1,27 @@
+# Warnings
+
+Python supports warnings and errors at runtime.  This can provide
+a useful error handling mechanism for your code.  You can use the
+`warnings` module to issue warnings and errors.
+
+
+## What is it?
+
+1. `will_warn.py`: script that issues a DeprecationWarning, intended
+   to illustrate how to suppress it at runtime.
+
+
+## How to use it?
+
+You can suppress the warning from the command line in two ways:
+  * using the `-W` option to the Python interpreter, and
+  * setting the `PYTHONWARNINGS` environment variable.
+
+```bash
+$ python -Wi::DeprecationWarning will_warn.py
+```
+
+```bash
+$ export PYTHONWARNINGS="ignore::DeprecationWarning"
+$ python will_warn.py
+```

source-code/error-handling/warnings/will_warn.py

@@ -0,0 +1,24 @@
+#!/usr/bin/env python
+
+import warnings
+
+
+class MyClass:
+
+    def __init__(self, value):
+        self._value = value
+
+    @property
+    def value(self):
+        return self._value
+
+    def get_value(self):
+        warnings.warn("get_value is deprecated. Use the value property",
+                      DeprecationWarning)
+        return self._value
+
+
+if __name__ == '__main__':
+    obj = MyClass(42)
+    print(obj.get_value())
+    print(obj.value)