Python String format_map()

Python string format_map() function returns a formatted version of the string using substitutions from the mapping provided. The substitution arguments are present in the string and identified by curly braces ({ }).

Python String format_map()

This function was introduced in Python 3.2. It’s syntax is:

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str.format_map(mapping)

Let’s look at some examples of format_map() function.

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s = 'My name is {name} and I am a {job_title}'

my_dict = {'name': 'Pankaj', 'job_title': 'Software Engineer'}

print(s.format_map(my_dict))
print(s.format(**my_dict))

Output:

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My name is Pankaj and I am a Software Engineer
My name is Pankaj and I am a Software Engineer

What if the mapping contains more keys than we actually need in the formatting?

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my_dict = {'name': 'Meghna', 'job_title': 'Writer', 'company': 'JournalDev'}
print(s.format_map(my_dict))
print(s.format(**my_dict))

Output:

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My name is Meghna and I am a Writer
My name is Meghna and I am a Writer

What if the mapping has missing keys?

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my_dict = {'name': 'Pankaj'}

print(s.format_map(my_dict))

Output: KeyError: 'job_title'

Let’s see the output if we use string format() function.

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print(s.format(**my_dict))

Output: KeyError: 'job_title'

format_map() vs format()

Python string format_map() function is very similar to format() function. The only difference is that the mappings are used directly and not copied to a dictionary. This is useful when the mapping is a dict subclass.

If you look at the above examples, the format() and format_map() function behavior is exactly the same. Let’s see how it differs when the mapping is a subclass of dict.

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class MyDict(dict):
    def __missing__(self, key):
        return '#Not Found#'


s = 'My name is {name} and I am a {job_title}'
my_dict = MyDict(name='Pankaj')
print(my_dict)
print(s.format_map(my_dict))

Output: My name is Pankaj and I am a #Not Found#

So when the key is missing, __missing__ function is called and the output is used to replace the substitution.

Let’s see what happens if we try to use format() function here.

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print(s.format(**my_dict))

Output: KeyError: 'job_title'

String format() function is throwing an error because it’s copying the provided mappings to a new dict object. So the subclass where we have implemented __missing__ function is never used and hence the error.

Conclusion

Python String format_map() function is useful when we want to use a dict subclass for mapping purposes. Since the mapping is used directly, the functions from the subclass get called whereas if we use format() function then mappings are copied to a new dict object and our subclass functions will not be called.

Reference: Official Documentation

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