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Case insensitive string comparisons

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Comparing string in a case insensitive way seems like something that’s trivial, but it’s not. This section only considers unicode strings (the default in Python 3). Note that Python 2 may have subtle weaknesses relative to Python 3 - the later’s unicode handling is much more complete.

The first thing to note it that case-removing conversions in unicode aren’t trivial. There is text for which text.lower() != text.upper().lower(), such as "ß": 

>>> "ß".lower()
'ß'

>>> "ß".upper().lower()
'ss'

But let’s say you wanted to caselessly compare "BUSSE" and "Buße". Heck, you probably also want to compare "BUSSE" and "BUẞE" equal - that’s the newer capital form. The recommended way is to use casefold: 

>>> help(str.casefold)
"""
Help on method_descriptor:

casefold(...)
      S.casefold() -> str
    
     Return a version of S suitable for caseless comparisons.
"""

Do not just use lower. If casefold is not available, doing .upper().lower() helps (but only somewhat).

Then you should consider accents. If your font renderer is good, you probably think "ê" == "ê" - but it doesn’t: 

>>> "ê" == "ê"
False

This is because they are actually 

>>> import unicodedata

>>> [unicodedata.name(char) for char in "ê"]
['LATIN SMALL LETTER E WITH CIRCUMFLEX']
>>> [unicodedata.name(char) for char in "ê"]
['LATIN SMALL LETTER E', 'COMBINING CIRCUMFLEX ACCENT']

The simplest way to deal with this is unicodedata.normalize. You probably want to use NFKD normalization, but feel free to check the documentation. Then one does 

>>> unicodedata.normalize("NFKD", "ê") == unicodedata.normalize("NFKD", "ê")
True

To finish up, here this is expressed in functions: 

import unicodedata

def normalize_caseless(text):
    return unicodedata.normalize("NFKD", text.casefold())

def caseless_equal(left, right):
    return normalize_caseless(left) == normalize_caseless(right)
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String Methods:
* String Methods
* Changing the capitalization of a string
* str.translate Translating characters in a string
* str.format and f-strings Format values into a string
* String modules useful constants
* Replace all occurrences of one substring with another substring
* Reversing a string
* Testing what a string is composed of
* Split a string based on a delimiter into a list of strings
* Stripping unwanted leading/trailing characters from a string
* String Contains
* Join a list of strings into one string
* Counting number of times a substring appears in a string
* Case insensitive string comparisons
* Test the starting and ending characters of a string
* Justify strings
* Conversion between str or bytes data and unicode characters
Table Of Contents
0 Getting Started
1 List comprehension
2 Filter
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4 Functions
5 Decorators
6 Math module
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79 PyInstaller - Distributing Python Code
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98 PostgreSQL
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116 List destructuring
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118 Mixins
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120 ArcPy
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122 Plugin and Extension Classes
123 Websockets
124 Immutable data types
125 String representations of class
126 Arrays
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131 Web Server Gateway Intrerface WSGI
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134 Abstract Base Classes
135 Unicode
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149 SQLite3 module
150 Persistence with pickle
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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
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165 hashlib
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179 sys module
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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