Not using ``get()`` to return a default value from a dict

Summary

Rather than assigning a default value to a variable and then overwriting that variable if a certain key exists in a dict, just use the Python Standard Library built-in method ``dict.get(key[, default])``. If the key exists, then that value is returned. If it does not exist, then the default value specified in the second argument is returned.

Description

Frequently you will see a module create a variable, assign a default value to the variable, and then check a dict for a certain key. If the key exists, then the value of the key is copied into the value for the variable. While there is nothing wrong this, it is much more concise to use the built-in method ``dict.get(key[, default])`` from the Python Standard Library. If the key exists in the dict, then the value for that key is returned. If it does not exist, then the default value specified as the second argument to ``get()`` is returned. Note that the default value defaults to ``None`` if a second argument is not provided.

Examples

Not using ``get()`` to return a default value

The module below initializes a variable called ``data`` to an empty string. Then it checks if a certain key called ``message`` exists in a dict called ``dictionary``. If the key exists, then the value of that key is copied into the ``data`` variable.

Although there is nothing wrong with this code, it is verbose. Later on you see how you can express the same idea in a more concise manner by using ``dict.get(key[, default])``.

.. warning:: The code below is an example of an error. Using this code will create bugs in your programs!

.. code:: python

Solutions

Use ``dict.get(key[, default])`` to assign default values

The code below is functionally equivalent to the original code above, but this code is more concise.

When ``get()`` is called, Python checks if the specified key exists in the dict. If it does, then ``get()`` returns the value of that key. If the key does not exist, then ``get()`` returns the value specified in the second argument to ``get()``.

.. code:: python

References

- `Python Standard Library - dict.get <https://docs.python.org/2/library/stdtypes.html#dict.get>`_