Essential Books

collections.Counter

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Counter is a dict sub class that allows you to easily count objects. It has utility methods for working with the frequencies of the objects that you are counting. 

import collections
counts = collections.Counter([1,2,3])

the above code creates an object, counts, which has the frequencies of all the elements passed to the constructor. This example has the value Counter({1: 1, 2: 1, 3: 1})

Constructor examples

Letter Counter 

>>> collections.Counter('Happy Birthday')
Counter({'a': 2, 'p': 2, 'y': 2, 'i': 1, 'r': 1, 'B': 1, ' ': 1, 'H': 1, 'd': 1, 'h': 1, 't': 1})

Word Counter 

>>> collections.Counter('I am Sam Sam I am That Sam-I-am That Sam-I-am! I do not like that Sam-I-am'.split())
Counter({'I': 3, 'Sam': 2, 'Sam-I-am': 2, 'That': 2, 'am': 2, 'do': 1, 'Sam-I-am!': 1, 'that': 1, 'not': 1, 'like': 1})

Recipes 

>>> c = collections.Counter({'a': 4, 'b': 2, 'c': -2, 'd': 0})

Get count of individual element 

>>> c['a']
4

Set count of individual element 

>>> c['c'] = -3
>>> c
Counter({'a': 4, 'b': 2, 'd': 0, 'c': -3})

Get total number of elements in counter (4 + 2 + 0 - 3) 

>>> sum(c.itervalues())  # negative numbers are counted!
3

Get elements (only those with positive counter are kept) 

>>> list(c.elements())
['a', 'a', 'a', 'a', 'b', 'b']

Remove keys with 0 or negative value 

>>> c - collections.Counter()
Counter({'a': 4, 'b': 2})

Remove everything 

>>> c.clear()
>>> c
Counter()

Add remove individual elements 

>>> c.update({'a': 3, 'b':3})
>>> c.update({'a': 2, 'c':2})  # adds to existing, sets if they don't exist
>>> c
Counter({'a': 5, 'b': 3, 'c': 2})
>>> c.subtract({'a': 3, 'b': 3, 'c': 3})  # subtracts (negative values are allowed)
>>> c
Counter({'a': 2, 'b': 0, 'c': -1})
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Collections module:
* Collections module
* collections.Counter
* collections.defaultdict
* collections.OrderedDict
* collections.namedtuple
* collections.deque
* collections.ChainMap
Table Of Contents
0 Getting Started
1 List comprehension
2 Filter
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6 Math module
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18 Metaclasses
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39 Incompatibilities moving from Python 2 to Python 3
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68 pip PyPI Package Manager
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72 deque module
73 Distributing self-contained applications
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79 PyInstaller - Distributing Python Code
80 Iterables and Iterators
81 Data Visualization
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83 args and kwargs
84 functools module
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89 urllib
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91 Python and Excel
92 Idioms
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94 Difference between a module and a package
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98 PostgreSQL
99 Descriptor
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102 Creating temporary files with tempfile
103 Working with ZIP files
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108 Deployment using conda
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116 List destructuring
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118 Mixins
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121 Python Anti-Patterns
122 Plugin and Extension Classes
123 Websockets
124 Immutable data types
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129 Alternative Python implementations
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131 Web Server Gateway Intrerface WSGI
132 2to3 tool
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135 Unicode
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138 Neo4j
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149 SQLite3 module
150 Persistence with pickle
151 Connecting to SQL Server
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153 Multidimensional arrays
154 Audio
155 pyglet
156 queue module
157 ijson
158 webbrowser module
159 base64 module
160 Flask
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162 Sockets and Message Encryption / Decryption
163 pygame
164 Input Subset and Output External Data Files using Pandas
165 hashlib
166 Gzip
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168 Creating a Windows Service
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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