Python String format_map() Method

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The Python String format_map() method is used to is used to perform string formatting with the help of a mapping object, such as a dictionary.

It is similar to the format() method but instead of passing arguments directly, it takes a single mapping object as its argument. This mapping object contains "key-value" pairs where the "keys" correspond to the placeholders in the string, and the "values" represent the replacements for those placeholders.

Syntax

Following is the basic syntax of the Python String format_map() method −

string.format_map(mapping)

Parameters

This method accepts a mapping object (such as a dictionary) that contains key-value pairs as a parameter.

Return Value

The method returns a formatted string with the placeholders replaced by their corresponding values from the mapping. It does not modify the original string.

Example 1

In the following example, we are formatting a string using a dictionary person with keys 'name' and 'age'. The format_map() method replaces placeholders "{name}" and "{age}" with the corresponding values from the dictionary −

person = {'name': 'Alice', 'age': 25}
result = "Name: {name}, Age: {age}".format_map(person)
print(result)  

Output

The output obtained is as follows −

Name: Alice, Age: 25

Example 2

Here, we access nested values in the dictionary "student". The format_map() method replaces placeholders "{details[name]}" and "{details[age]}" with the corresponding nested values −

student = {'details': {'name': 'Bob', 'age': 20}}
result = "Name: {details[name]}, Age: {details[age]}".format_map(student)
print(result)  

Output

Following is the output of the above code −

Name: Bob, Age: 20

Example 3

In this example, we format a string using attributes of an object "person" dynamically. The vars() function converts the object's attributes into a dictionary, which is then used by format_map() method to replace placeholders −

class Person:
    pass

person = Person()
person.name = 'John'
person.age = 30

result = "Name: {name}, Age: {age}".format_map(vars(person))
print(result)  

Output

The result produced is as shown below −

Name: John, Age: 30

Example 4

In here, we define a custom dictionary class "CustomDict" where missing keys are handled by the __missing__() method. If a key is missing in the dictionary data, it returns a custom string instead of raising a KeyError −

class CustomDict(dict):
    def __missing__(self, key):
        return f'Unknown: {key}'

data = CustomDict({'name': 'Alice'})
result = "Name: {name}, Age: {age}".format_map(data)
print(result)  

Output

We get the output as shown below −

Name: Alice, Age: Unknown: age