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format() function in Python
  • Difficulty Level : Easy
  • Last Updated : 08 Feb, 2021

With Python 3.0, the format() method has been introduced for handling complex string formatting more efficiently. This method of the built-in string class provides

functionality for complex variable substitutions and value formatting. This new formatting technique is regarded as more elegant. The general syntax of format()

method is: string.format(var1, var2,…)
 

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




# 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 the 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 with 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 : 
 

Python3




# 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 placeholders. 
 

Python3




# 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

Formatting Strings using Escape Sequences:

You can use two or more specially designated characters within a string to format a string or perform a command. These characters are called escape sequences. An

Escape sequence in Python starts with a backslash (\). For example, \n is an escape sequence in which the common meaning of the letter n is literally escaped and given an alternative meaning – a new line.

Escape sequenceDescription     Example      
\nBreaks the string into a new lineprint(‘I designed this rhyme to explain in due time\nAll I know’)
\tAdds a horizontal tabprint(‘Time is a \tvaluable thing’)
\\Prints a backslashprint(‘Watch it fly by\\as the pendulum swings’)
\’   Prints a single quoteprint(‘It doesn\’t even matter how hard you try’)
\”    Prints a double quoteprint(‘It is so\”unreal\”‘)
\amakes a sound like a bellprint(‘\a’) 

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 :
 

Python3




# 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 – string conversion via str() prior to formatting

example:  

1)print(“%20s” % (‘geeksforgeeks’, ))

output- geeksforgeeks

2)print(“%-20s” % (‘Interngeeks’, ))

output- Interngeeks

3)print(“%.5s” % (‘Interngeeks’, ))

output- Inter

%c– character 

example:  

type=’bug’ 

result=’troubling’  

print(‘I wondered why the program was %s me. Then it dawned on me it was a %s .’ %(result, type ))

output-I wondered why the program was me troubling me. Then it dawned on me it was a bug.

%i– signed decimal integer

%d– signed decimal integer(base-10) 

example-

match=12000

site=’amazon’

print(“%s is so useful. I tried to look up mobile and they had a nice one that cost %d rupees.” % (site, match))

output- amazon is so useful. I tried to look up mobiles and they had a nice one that cost 12000 rupees”)

%u unsigned decimal integer 
%o octal integer 
f – floating point display 
b – binary 
o – octal 
%x – hexadecimal with lowercase letters after 9 
%X– hexadecimal with uppercase letters after 9 
e – exponent notation

You can also specify formatting symbols. Only change is using colon (:) instead of %. For example, instead of %s use {:s} and instead of %d use (:d}

 

Syntax : 
String {field_name:conversion} Example.format(value)
Errors and Exceptions : 
ValueError : Error occurs during type conversion in this method. 

Code #5 : 
 

Python3




# 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 : 
 

Python3




# 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

 

Python3




# 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 
 

Python3




# 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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