Adds context manager section to structure.rst

docs/writing/structure.rst

@@ -316,6 +316,75 @@ expensive function in a table and use them directly instead of recomputing
 them when they have already been computed. This is clearly not part
 of the function logic.
 
+Context Managers
+----------------
+
+A context manager is a Python object that provides extra contextual information
+to an action. This extra information takes the form of running a function upon
+initiating the context using the ``with`` statement as well as running a function
+upon completing all the code inside the ``with`` block. The most well known
+example of using a context manager is operating on a file:
+
+.. code-block:: python
+
+    with open('file.txt') as f:
+        contents = f.read()
+
+Anyone familiar with this pattern knows that invoking ``open`` in this fashion
+ensures that ``f``'s ``close`` method will be called at some point. This reduces
+a developer's cognitive load and makes code easier to read.
+
+There are two easy ways to implement this functionality yourself: using a class
+or using a generator. Let's implement the above functionality ourselves, starting
+with the class approach:
+
+.. code-block:: python
+
+    class CustomOpen(object):
+        def __init__(self, filename):
+          self.file = open(filename)
+
+        def __enter__(self):
+            return self.file
+
+        def __exit__(self, ctx_type, ctx_value, ctx_traceback):
+            self.file.close()
+
+    with CustomOpen('file') as f:
+        contents = f.read()
+
+This is just a regular Python object with two extra methods that are used
+by the ``with`` statement. CustomOpen is first instantiated and then its
+``__enter__`` method is called and whatever ``__enter__`` returns is assigned to
+``f`` in the ``as f`` part of the statement. When the contents of the ``with`` block
+is finished executing, the ``__exit__`` method is then called.
+
+And now the generator approach using Python's own 
+`contextlib <https://docs.python.org/2/library/contextlib.html>`_:
+
+.. code-block:: python
+
+    from contextlib import contextmanager
+
+    @contextmanager
+    def custom_open(filename):
+        f = open(filename)
+        yield f
+        f.close()
+
+    with custom_open('file') as f:
+        contents = f.read()
+
+This works in exactly the same way as the class example above, albeit it's
+more terse. The ``custom_open`` function executes until it reaches the ``yield``
+statement. It then gives control back to the ``with`` statement, which assigns
+whatever was ``yield``'ed to `f` in the ``as f`` portion.
+
+Since the two approaches appear the same, we should follow the Zen of Python
+to decide when to use which. The class approach might be better if there's
+a considerable amount of logic to encapsulate. The function approach
+might be better for situations where we're dealing with a simple action.
+
 Dynamic typing
 --------------