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Custom formatting for a class

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Note:

Everything below applies to the str.format method, as well as the format function. In the text below, the two are interchangeable.

For every value which is passed to the format function, Python looks for a __format__ method for that argument. Your own custom class can therefore have their own __format__ method to determine how the format function will display and format your class and it’s attributes.

This is different than the __str__ method, as in the __format__ method you can take into account the formatting language, including alignment, field width etc, and even (if you wish) implement your own format specifiers, and your own formatting language extensions.1 

object.__format__(self, format_spec)

For example : 

# Example in Python 2 - but can be easily applied to Python 3

class Example(object):
    def __init__(self,a,b,c):
        self.a, self.b, self.c = a,b,c

    def __format__(self, format_spec):
        """ Implement special semantics for the 's' format specifier """
        # Reject anything that isn't an s
        if format_spec[-1] != 's':
            raise ValueError('{} format specifier not understood for this object', format_spec[:-1])

        # Output in this example will be (<a>,<b>,<c>)
        raw = "(" + ",".join([str(self.a), str(self.b), str(self.c)]) + ")"
        # Honor the format language by using the inbuilt string format
        # Since we know the original format_spec ends in an 's' 
        # we can take advantage of the str.format method with a 
        # string argument we constructed above
        return "{r:{f}}".format( r=raw, f=format_spec )

inst = Example(1,2,3)
print "{0:>20s}".format( inst )
# out :              (1,2,3)
# Note how the right align and field width of 20 has been honored.

Note:

If your custom class does not have a custom __format__ method and an instance of the class is passed to the format function, Python2 will always use the return value of the __str__ method or __repr__ method to determine what to print (and if neither exist then the default repr will be used), and you will need to use the s format specifier to format this. With Python3, to pass your custom class to the format function, you will need define __format__ method on your custom class.
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String Formatting:
* String Formatting
* Basics of String Formatting
* Alignment and padding
* Format literals f-string
* Float formatting
* String formatting with datetime
* Named placeholders
* Formatting Numerical Values
* Nested formatting
* Format using Getitem and Getattr
* Padding and truncating strings combined
* Custom formatting for a class
Table Of Contents
0 Getting Started
1 List comprehension
2 Filter
3 List
4 Functions
5 Decorators
6 Math module
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11 Sorting Minimum and Maximium
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13 Variable Scope and Binding
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35 Writing C extensions
36 Unit Testing
37 Regular Expressions
38 Bitwise Operators
39 Incompatibilities moving from Python 2 to Python 3
40 Virtual environments
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42 Tuple
43 Context Managers with Statement
44 Hidden Features
45 Enum
46 String Formatting
47 Conditionals
48 Complex math
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50 The __name__ special variable
51 Check if path exists
52 Networking
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54 Print Function
55 os.path module
56 Creating Python packages
57 Parsing Command Line Arguments
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60 setup.py
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63 Itertools Module
64 Recursion
65 Boolean Operators
66 dis module
67 Type Hints
68 pip PyPI Package Manager
69 locale module
70 Exceptions
71 Web scraping
72 deque module
73 Distributing self-contained applications
74 Property Objects
75 Overloading
76 Debugging
77 Reading and Writing CSV
78 Dynamic code execution with exec and eval
79 PyInstaller - Distributing Python Code
80 Iterables and Iterators
81 Data Visualization
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83 args and kwargs
84 functools module
85 Garbage Collection
86 Indentation
87 Security and Cryptograhy
88 Pickle data serialization
89 urllib
90 Binary Data
91 Python and Excel
92 Idioms
93 Method Overriding
94 Difference between a module and a package
95 Data Serialization
96 Python Concurrency
97 RabbitMQ using AMQPStorm
98 PostgreSQL
99 Descriptor
100 Common Pitfalls
101 Multiprocessing
102 Creating temporary files with tempfile
103 Working with ZIP files
104 Stack
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106 User-Defined Methods
107 Working around Global Interpreter Lock
108 Deployment using conda
109 Logging
110 Processes and Threads
111 os module
112 Comments and documentation
113 Database Access
114 Python HTTP Server
115 Alternatives to switch statement from other languages
116 List destructuring
117 Accessing Python source code and bytecode
118 Mixins
119 Attribute Access
120 ArcPy
121 Python Anti-Patterns
122 Plugin and Extension Classes
123 Websockets
124 Immutable data types
125 String representations of class
126 Arrays
127 Operator Precedence
128 Polymorphism
129 Alternative Python implementations
130 List Comprehensions
131 Web Server Gateway Intrerface WSGI
132 2to3 tool
133 Abstract Syntax Tree
134 Abstract Base Classes
135 Unicode
136 ssh in Python
137 Serial Communication with pyserial
138 Neo4j
139 Performance optimization
140 Curses
141 Templates
142 pass statement
143 Testing with py.test
144 Date Formatting
145 heapq
146 tkinter
147 CLI subcommands
148 Defining functions with list arguments
149 SQLite3 module
150 Persistence with pickle
151 Connecting to SQL Server
152 Design Patterns
153 Multidimensional arrays
154 Audio
155 pyglet
156 queue module
157 ijson
158 webbrowser module
159 base64 module
160 Flask
161 Groupby
162 Sockets and Message Encryption / Decryption
163 pygame
164 Input Subset and Output External Data Files using Pandas
165 hashlib
166 Gzip
167 ctypes
168 Creating a Windows Service
169 Mutable vs. Immutable
170 configparser
171 Common Exceptions
172 Optical Character Recognition OCR
173 Python Data Types
174 Partial functions
175 Generating graphs
176 Unzipping Files
177 Functional Programming
178 Python Virtual Environment - virtualenv
179 sys module
180 virtual environment with virtualenvwrapper
181 virtualenvwrapper on Windows
182 Python Requests Post
183 Plotting with Matplotlib
184 Python Lex-Yacc
185 pyaudio
186 shelve
187 pip and PyPI Package Manager
188 Writing to CSV from String or List
189 Raise Custom Errors Exceptions
190 Using loops within functions
191 Contributors