Python String format() Method

08 Jul, 2022
Python format() function has been introduced for handling complex string formatting more efficiently. 
Python3
txt = "I have {an:.2f} Ruppes!"print(txt.format(an = 4))
Output:
I have 4.00 Ruppes!
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,...). Here we will try to understand how to Format A String That Contains Curly Braces In Python. 
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. 
In this example, we will use the string bracket notation program to demonstrate the str.format() method.
Python3
# using format option in a simple stringprint("{}, A computer science portal for geeks."      .format("GeeksforGeeks")) # using format option for a# value stored in a variablestr = "This article is written in {}"print(str.format("Python")) # formatting a string using a numeric constantprint("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!
Multiple pairs of curly braces can be used while formatting the string in Python. Let’s say another variable substitution is needed in the sentence, which 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 access 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. 
Python program using multiple placeholders to demonstrate str.format() method.
Python3
# Multiple placeholders in format() functionmy_string = "{}, is a {} science portal for {}"print(my_string.format("GeeksforGeeks", "computer", "geeks")) # different datatypes can be used in formattingprint("Hi ! My name is {} and I am {} years old"      .format("User", 19)) # The values passed as parameters# are replaced in order of their entryprint("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
Python program demonstrating Index error number of placeholders is four but there are only three values passed.
Python3
# parameters in format function.my_string = "{}, is a {} {} science portal for {}" print(my_string.format("GeeksforGeeks", "computer", "geeks"))
Output : 
IndexError: tuple index out of range
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.
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.
Example:
To demonstrate the use of formatters with positional key arguments.
Python3
# Positional arguments# are placed in orderprint("{0} love {1}!!".format("GeeksforGeeks",                              "Geeks")) # Reverse the index numbers with the# parameters of the placeholdersprint("{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 stringprint("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 nameprint("{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
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.
Python3
print("%20s" % ('geeksforgeeks', ))print("%-20s" % ('Interngeeks', ))print("%.5s" % ('Interngeeks', ))
Output:
geeksforgeeks
Interngeeks         
Inter
Python3
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.
Python3
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 number
o – octal number
%x – hexadecimal with lowercase letters after 9
%X– hexadecimal with uppercase letters after 9
e – exponent notation
You can also specify formatting symbols. The only change is using a 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. 
Python3
print("This site is {0:f}% securely {1}!!".      format(100, "encrypted")) # To limit the precisionprint("My average of this {0} was {1:.2f}%"      .format("semester", 78.234876)) # For no decimal placesprint("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
Demonstrate ValueError while doing forced type-conversions
Python3
# 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 : 
ValueError: Unknown format code 'd' for object of type 'float'
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 parametersprint("{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 parametersprint("{0:4} was founded in {1:16}!"      .format("GeeksforGeeks", 2009))  # To demonstrate aligning of spacesprint("{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********
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 formatters to increase field size and modify alignment can make the output more readable.
Python3
# which prints out i, i ^ 2, i ^ 3,#  i ^ 4 in the given range # Function prints out values# in an unorganized mannerdef unorganized(a, b):    for i in range(a, b):        print(i, i**2, i**3, i**4) # Function prints the organized set of valuesdef 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 Coden1 = int(input("Enter lower range :-\n"))n2 = int(input("Enter upper range :-\n")) print("------Before Using Formatters-------") # Calling function without formattersunorganized(n1, n2) print()print("-------After Using Formatters---------")print() # Calling function that contains# formatters to organize the dataorganized(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
Using a dictionary to unpack values into the placeholders in the string that needs to be formatted. We basically use ** to unpack the values. This method can be useful in string substitution while preparing an SQL query.
Python3
introduction = 'My name is {first_name} {middle_name} {last_name} AKA the {aka}.'full_name = {    'first_name': 'Tony',    'middle_name': 'Howard',    'last_name': 'Stark',    'aka': 'Iron Man',} # Notice the use of "**" operator to unpack the values.print(introduction.format(**full_name))
Output:
My name is Tony Howard Stark AKA the Iron Man.
Given a list of float values, the task is to truncate all float values to 2-decimal digits. Let’s see the different methods to do the task.
Python3
# Python code to truncate float# values to 2 decimal digits.   # List initializationInput = [100.7689454, 17.232999, 60.98867, 300.83748789]   # Using formatOutput = ['{:.2f}'.format(elem) for elem in Input]   # Print outputprint(Output)
Output:
['100.77', '17.23', '60.99', '300.84']
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