Python String format_map()

Python format_map() function is somewhat similar in functionality to the Python format() function. We use Python format_map() function when we want to map each key with its value and then the mapping is a dict subclass. Here, mapping can be viewed in the form of {key: value} i.e. in a Dictionary form. This article focuses more on the applicability of the Python format_map() string method along with its examples.

Definition

• The Python format_map() string function returns a formatted version of the string using substitutions based on the mapping specified. The substitution arguments are included in the string and are denoted by curly braces {}.
• Python format_map() returns a new string by concatenating the values in the user-supplied dict within the defined places. This approach uses the placeholder ({}) formatting style.
• This format_map() function is used to returns a dictionary key’s value.

Python format_map() Syntax

Python format_map() follows the below syntax:

                    

string.format_map(mapping)

                

format_map() Parameters

The format_map() accepts a single argument:

• mapping (dictionary) – A dictionary with key-value pair required for mapping

Return value from format_map()

The Python format_map() formats the given string and returns the key’s values of the input dictionary.

Example 1: How format_map works?

Example

                    

# Python program to illustrate format_map()
# mapping dictionary
stud_marks = {'x':'Phillip', 'y':89, 'z':'Maths'}
# format string
txt = '{x} scored {y} marks in {z}'
print(txt.format_map(stud_marks))
print('')

print('My {x} is {y} USD'.format_map({'x':'Salary', 'y':4500}))

                

Output

                    

Phillip scored 89 marks in Maths

My Salary is 4500 USD

                

Explanation

We have considered stud_marks as a mapping dictionary. Then we have declared a format string txt having the keys of the mapping dictionary under the format substitution (curly brackets). Now, we can substitute all occurrences of {x}, {y} and {z} with Phillip, 89 and Maths using the format_map() function.

Example 2: KeyError in format_map()

What if we add another extra format that does not exist on the mapping dictionary? Or what if we miss keys in the mapping dictionary? In such cases, the format_map() function throws a KeyError exception which displays the key that is extra added or missed in the mapping dictionary.

Example 1

                    

details = {'name':'Emily', 'lang1':'Java'}
# added extra format that does not exist in mapping dict
txt = '{name} loves {lang1} and {lang2}'
print(txt.format_map(details))

                

Output

                    

Traceback (most recent call last):
File "", line 4, in 
KeyError: 'lang2'

                

Example 2

                    

# contains 1 less key
details = {'name':'Emily'}
txt = '{name} loves {lang1}'
print(txt.format_map(details))

                

Output

                    

Traceback (most recent call last):
File "", line 4, in 
KeyError: 'lang1'

                

Example 3: How does format_map() work with dict subclass?

The format_map(mapping) function is more adaptable than format(**mapping) because it allows for missing keys.

Example

                    

# Python program to illustrate format_map()
class Coordinates(dict):
    def __missing__(self, key):
      return key


print('({x}, {y}, {z})'.format_map(Coordinates(x = '5')))

print('({x}, {y}, {z})'.format_map(Coordinates(y = '10')))

print('({x}, {y}, {z})'.format_map(Coordinates(x = '5', y = '10', z = '15')))

                

Output

                    

(5, y, z)
(x, 10, z)
(5, 10, 15)

                

Practical Applications of format_map()

Any practical application can take advantage of the format map() method.

Example

                    

# Program 1
def chk_msg(n):
    # input stored in variable details.
    details = {'name':'Alex', 'msg':n}
    # use of format_map() function
    print('{name} has {msg} new messages'.format_map(details))

chk_msg(25)
print('')

# Program 2
detail = { 'name':['Sean', 'David'],
               'profession':['Architect', 'Businessman'],
               'age':[34, 29] }

print('{name[0]} is an {profession[0]} and he'
      ' is {age[0]} years old'.format_map(detail))

print('{name[1]} is a {profession[1]} and he'
      ' is {age[1]} years old'.format_map(detail))

                

Output

                    

Alex has 25 new messages

Sean is an Architect and he is 34 years old
David is a Businessman and he is 29 years old

                

Difference between format_map() and format()

• The Python format() function performs an indirect substitution utilizing the method parameters by first generating a mapping dictionary and then conducting the substitution.
• In the instance of Python String format_map(), the substitution is performed directly using a mapping dictionary.
• The format_map() is significantly faster than format() because it does not create a new dictionary ().
• Also format map(), unlike format(), can use a dictionary subclass for mapping.