added dict keys antipattern

Author: Brobin

docs/readability/not_using_dict_keys_when_formatting_strings.rst

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+
+Not using dict keys when formatting strings
+===========================================
+
+When formatting a string with values from a dictionary, you can use the dictionary keys instead of explicity defining all of the format parameters. Consider this dictionary that stores the name and age of a person.
+
+
+.. code:: python
+
+    person = {'first':'Tobin', 'age':20}
+
+
+Anti-pattern
+------------
+
+Here is an example of formatting the string with values from the person. This is bad! If we added another key-value pair to the person dictionary, we would have to change the string and the format arguments
+
+.. code:: python
+
+	person = {'first':'Tobin', 'age':20}
+	print('{0} is {1} years old'.format(person['first'], person['age']))  # bad
+
+	person = {'first':'Tobin', 'last': 'Brown', 'age':20}
+	print('{0} {1} is {2} years old'.format(person['first'], person['last'], person['age']))  # bad
+
+
+Best practice
+-------------
+
+By using the dictionary keys in the string we are formatting, the code is much more readable and explicit.
+
+.. code:: python
+
+	person = {'first':'Tobin', 'age':20}
+    print('{first} is {age} years old'.format(**person))
+
+    person = {'first':'Tobin', 'last': 'Brown', 'age':20}
+    print('{first} {last} is {age} years old'.format(**person))
+
+
+Going even further, the same result can be achieved with your own objects by using ``obj.__dict__``.
+
+.. code:: python
+
+	class Person(object):
+
+		def __init__(self, first, last, age):
+			self.first = first
+			self.last = last
+			self.age = age
+
+		def __str__(self):
+			return '{first} {last} is {age} years old'.format(**self.__dict__)
+
+
+	person = Person('Tobin', 'Brown', 20)
+	print(person)