Python String format_map() Method

The format_map() method in Python is used to format a string using a dictionary or any mapping object. It's similar to str.format(), but instead of passing variables as keyword arguments, you pass a dictionary.

Syntax

string.format_map(mapping)

Parameters:

• mapping – A dictionary or mapping object containing keys that match the placeholders in the string.

Returns:

• A formatted string where placeholders are replaced by corresponding values from the mapping.

Example 1: Basic Usage with Dictionary

person = {"name": "Alice", "age": 30}
text = "Name: {name}, Age: {age}".format_map(person)
print(text)

Name: Alice, Age: 30

Example 2: Using with Custom Mapping

class DefaultDict(dict):
    def __missing__(self, key):
        return 'N/A'

info = DefaultDict(name="Bob")
print("Name: {name}, City: {city}".format_map(info))

Name: Bob, City: N/A

This technique is useful when some keys might be missing in the dictionary.

Difference Between format() and format_map()

• format() uses keyword arguments: "{name}".format(name="John")
• format_map() uses a dictionary: "{name}".format_map({"name": "John"})
• format_map() cannot take multiple types of arguments like format()

Common Mistakes

• Passing a non-dictionary object will raise a TypeError.
• If the key is missing in the mapping, it raises a KeyError unless handled using __missing__.

Use Cases

• When formatting templates with predefined values.
• Building strings from JSON or config files.
• Useful in logging or templating systems where data is dictionary-based.

Interview Tip

Use format_map() when formatting strings from dynamic dictionaries. It's helpful when you can't hardcode keys into the function call.

Summary

• format_map() formats a string using a dictionary.
• It is similar to format() but takes a single mapping object.
• Use custom mappings to handle missing keys gracefully.

Practice Problem

Create a program that takes user input (name and score) and formats the output using format_map().

data = {"name": input("Enter your name: "), "score": input("Enter your score: ")}
template = "Player: {name} | Score: {score}"
print(template.format_map(data))