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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.

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

person = {
    'first': 'Tobin',
    'age':20
}

print('{0} is {1} years old'.format(
    person['first'],
    person['age'])
)
# Output: Tobin is 20 years old

person = {
    'first': 'Tobin',
    'last': 'Brown',
    'age':20
}

# Bad: we have to change the replacement fields within
# our string, once we add new values
print('{0} {1} is {2} years old'.format(
    person['first'],
    person['last'],
    person['age'])
)  # bad
# Output: Tobin Brown is 20 years old

Best practice

By using the dictionary keys in the string we are formatting, the code is much more readable and explicit.

person = {
    'first': 'Tobin',
    'age':20
}

print('{first} is {age} years old'.format(**person))
# Output: Tobin is 20 years old

person = {
    'first':'Tobin',
    'last': 'Brown',
    'age':20
}
print('{first} {last} is {age} years old'.format(**person))
# Output: Tobin Brown is 20 years old

Going even further, the same result can be achieved with your own objects by using obj.__dict__.

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)
# Output: Tobin Brown is 20 years old

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