format() function in Python

Difficulty Level : Easy
Last Updated : 15 Oct, 2020

str.format() is one of the string formatting methods in Python3, which allows multiple substitutions and value formatting. This method lets us concatenate elements within a string through positional formatting.

Using a Single Formatter :

Formatters work by putting in one or more replacement fields and placeholders defined by a pair of curly braces { } into a string and calling the str.format(). The value we wish to put into the placeholders and concatenate with the string passed as parameters into the format function.

Syntax : { } .format(value)

Parameters :
(value) : Can be an integer, floating point numeric constant, string, characters or even variables.

Returntype : Returns a formatted string with the value passed as parameter in the placeholder position.

Code #1 : Simple demonstration of format().

# Python3 program to demonstarte 
# the str.format() method 
  
# using format option in a simple string 
print ("{}, A computer science portal for geeks."
                        .format("GeeksforGeeks")) 
  
# using format option for a 
# value stored in a variable 
str = "This article is written in {}"
print (str.format("Python")) 
  
# formatting a string using a numeric constant 
print ("Hello, I am {} years old !".format(18))  

Output :

GeeksforGeeks, A computer science portal for geeks.
This article is written in Python
Hello, I am  18 years old!

Using Multiple Formatters :

Multiple pairs of curly braces can be used while formatting the string. Let’s say if another variable substitution is needed in sentence, can be done by adding a second pair of curly braces and passing a second value into the method. Python will replace the placeholders by values in order.

Syntax : { } { } .format(value1, value2)

Parameters :
(value1, value2) : Can be integers, floating point numeric constants, strings, characters and even variables. Only difference is, the number of values passed as parameters in format() method must be equal to the number of placeholders created in the string.

Errors and Exceptions :
IndexError : Occurs when string has an extra placeholder and we didn’t pass any value for it in the format() method. Python usually assigns the placeholders with default index in order like 0, 1, 2, 3…. to acces the values passed as parameters. So when it encounters a placeholder whose index doesn’t have any value passed inside as parameter, it throws IndexError.

Code #2 :

# Python program demonstrating Index error 
  
# Number of placeholders are four but 
# there are only three values passed 
  
# parameters in format function. 
my_string = "{}, is a {} {} science portal for {}"
  
print (my_string.format("GeeksforGeeks", "computer", "geeks")) 

Output :

Traceback (most recent call last):
  File "/home/949ca7b5b7e26575871639f03193d1b3.py", line 2, in 
    print (my_string.format("GeeksforGeeks", "computer", "geeks"))
IndexError: tuple index out of range

Code #3 : Formatters with multiple place holders.

# Python program using multiple place  
# holders to demonstrate str.format() method  
  
# Multiple placeholders in format() function 
my_string = "{}, is a {} science portal for {}"
print (my_string.format("GeeksforGeeks", "computer", "geeks")) 
  
# different datatypes can be used in formatting 
print ("Hi ! My name is {} and I am {} years old"
                            .format("User", 19)) 
  
# The values passed as parameters 
# are replaced in order of their entry 
print ("This is {} {} {} {}"
       .format("one", "two", "three", "four")) 

Output :

GeeksforGeeks, is a computer science portal for geeks
Hi! My name is User and I am 19 years old
This is one two three four

Formatters with Positional and Keyword Arguments :

When placeholders { } are empty, Python will replace the values passed through str.format() in order.

The values that exist within the str.format() method are essentially tuple data types and each individual value contained in the tuple can be called by its index number, which starts with the index number 0. These index numbers can be passed into the curly braces that serve as the placeholders in the original string.

Syntax : {0} {1}.format(positional_argument, keyword_argument)

Parameters : (positional_argument, keyword_argument)

Positional_argument can be integers, floating point numeric constants, strings, characters and even variables.
Keyword_argument is essentially a variable storing some value, which is passed as parameter.

Code #4 :

# To demonstrate the use of formatters 
# with positional key arguments. 
  
# Positional arguments 
# are placed in order 
print("{0} love {1}!!".format("GeeksforGeeks", 
                                    "Geeks")) 
  
# Reverse the index numbers with the 
# parameters of the placeholders 
print("{1} love {0}!!".format("GeeksforGeeks", 
                                    "Geeks")) 
  
  
print("Every {} should know the use of {} {} programming and {}"
    .format("programmer", "Open", "Source", "Operating Systems")) 
  
  
# Use the index numbers of the 
# values to change the order that 
# they appear in the string 
print("Every {3} should know the use of {2} {1} programming and {0}"
        .format("programmer", "Open", "Source", "Operating Systems")) 
  
  
# Keyword arguments are called 
# by their keyword name 
print("{gfg} is a {0} science portal for {1}"
.format("computer", "geeks", gfg ="GeeksforGeeks")) 

Output :

GeeksforGeeks love Geeks!!
Geeks love GeeksforGeeks!!
Every programmer should know the use of Open Source programming and Operating Systems
Every Operating Systems should know the use of Source Open programming and programmer
GeeksforGeeks is a computer science portal for geeks

Type Specifying :

More parameters can be included within the curly braces of our syntax. Use the format code syntax {field_name:conversion}, where field_name specifies the index number of the argument to the str.format() method, and conversion refers to the conversion code of the data type.

s – strings
d – decimal integers (base-10)
f – floating point display
c – character
b – binary
o – octal
x – hexadecimal with lowercase letters after 9
X – hexadecimal with uppercase letters after 9
e – exponent notation

Syntax :
String {field_name:conversion} Example.format(value)

Errors and Exceptions :
ValueError : Error occurs during type conversion in this method.

Code #5 :

# Demonstrate ValueError while 
# doing forced type-conversions 
  
# When explicitly converted floating-point 
# values to decimal with base-10 by 'd'  
# type conversion we encounter Value-Error. 
print("The temperature today is {0:d} degrees outside !"
                                        .format(35.567)) 
  
# Instead write this to avoid value-errors 
''' print("The temperature today is {0:.0f} degrees outside !" 
                                            .format(35.567))'''

Output :

Traceback (most recent call last):
  File "/home/9daca03d1c7a94e7fb5fb326dcb6d242.py", line 5, in 
    print("The temperature today is {0:d} degrees outside!".format(35.567))
ValueError: Unknown format code 'd' for object of type 'float'

Code #6 :

# Convert base-10 decimal integers  
# to floating point numeric constants 
print ("This site is {0:f}% securely {1}!!". 
                    format(100, "encrypted")) 
  
# To limit the precision 
print ("My average of this {0} was {1:.2f}%"
            .format("semester", 78.234876)) 
  
# For no decimal places 
print ("My average of this {0} was {1:.0f}%"
            .format("semester", 78.234876)) 
  
# Convert an integer to its binary or 
# with other different converted bases. 
print("The {0} of 100 is {1:b}"
        .format("binary", 100)) 
          
print("The {0} of 100 is {1:o}"
        .format("octal", 100)) 

Output :

This site is 100.000000% securely encrypted!!
My average of this semester was 78.23%
My average of this semester was 78%
The binary of 100 is 1100100
The octal of 100 is 144

Padding Substitutions or Generating Spaces :

Code #7 :

By default strings are left-justified within the field, and numbers are right-justified. We can modify this by placing an alignment code just following the colon.

<   :  left-align text in the field
^   :  center text in the field
>   :  right-align text in the field

# To demonstrate spacing when  
# strings are passed as parameters 
print("{0:4}, is the computer science portal for {1:8}!"
                        .format("GeeksforGeeks", "geeks")) 
  
# To demonstrate spacing when numeric 
# constants are passed as parameters. 
print("It is {0:5} degrees outside !"
                        .format(40)) 
  
# To demonstrate both string and numeric 
# constants passed as parameters 
print("{0:4} was founded in {1:16}!"
    .format("GeeksforGeeks", 2009)) 
  
  
# To demonstrate aligning of spaces 
print("{0:^16} was founded in {1:<4}!"
        .format("GeeksforGeeks", 2009)) 
  
print("{:*^20s}".format("Geeks")) 

Output :

GeeksforGeeks, is the computer science portal for geeks   !
It is    40 degrees outside!
GeeksforGeeks was founded in             2009!
 GeeksforGeeks   was founded in 2009 !
*******Geeks********

Applications :

Formatters are generally used to Organize Data. Formatters can be seen in their best light when they are being used to organize a lot of data in a visual way. If we are showing databases to users, using formaters to increase field size and modify alignment can make the output more readable.

Code #8 : To demonstrate organization of large data

# which prints out i, i ^ 2, i ^ 3, 
#  i ^ 4 in the given range 
  
# Function prints out values 
# in an unorganized manner 
def unorganized(a, b): 
    for i in range (a, b): 
        print ( i, i**2, i**3, i**4 ) 
  
# Function prints the organized set of values 
def organized(a, b): 
    for i in range (a, b): 
  
        # Using formatters to give 6  
        # spaces to each set of values 
        print("{:6d} {:6d} {:6d} {:6d}"
        .format(i, i ** 2, i ** 3, i ** 4)) 
  
# Driver Code 
n1 = int(input("Enter lower range :-\n")) 
n2 = int(input("Enter upper range :-\n")) 
  
print("------Before Using Formatters-------") 
  
# Calling function without formatters 
unorganized(n1, n2) 
  
print() 
print("-------After Using Formatters---------") 
print() 
  
# Calling function that contains 
# formatters to organize the data 
organized(n1, n2) 

Output :

Enter lower range :-
3
Enter upper range :-
10
------Before Using Formatters-------
3 9 27 81
4 16 64 256
5 25 125 625
6 36 216 1296
7 49 343 2401
8 64 512 4096
9 81 729 6561

-------After Using Formatters---------

     3      9     27     81
     4     16     64    256
     5     25    125    625
     6     36    216   1296
     7     49    343   2401
     8     64    512   4096
     9     81    729   6561

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