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Testing what a string is composed of

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Python’s str type also features a number of methods that can be used to evaluate the contents of a string. These are str.isalpha, str.isdigit, str.isalnum, str.isspace. Capitalization can be tested with str.isupper, str.islower and str.istitle.

str.isalpha

str.isalpha takes no arguments and returns True if the all characters in a given string are alphabetic, for example: 

>>> "Hello World".isalpha()  # contains a space
False
>>> "Hello2World".isalpha()  # contains a number
False
>>> "HelloWorld!".isalpha()  # contains punctuation
False
>>> "HelloWorld".isalpha()
True

As an edge case, the empty string evaluates to False when used with "".isalpha().

str.isupper, str.islower, str.istitle

These methods test the capitalization in a given string.

str.isupper is a method that returns True if all characters in a given string are uppercase and False otherwise. 

>>> "HeLLO WORLD".isupper()
False
>>> "HELLO WORLD".isupper()
True
>>> "".isupper()
False

Conversely, str.islower is a method that returns True if all characters in a given string are lowercase and False otherwise. 

>>> "Hello world".islower()
False
>>> "hello world".islower()
True
>>> "".islower()
False

str.istitle returns True if the given string is title cased; that is, every word begins with an uppercase character followed by lowercase characters. 

>>> "hello world".istitle()
False
>>> "Hello world".istitle()
False
>>> "Hello World".istitle()
True
>>> "".istitle()
False

str.isdecimal, str.isdigit, str.isnumeric

str.isdecimal returns whether the string is a sequence of decimal digits, suitable for representing a decimal number.

str.isdigit includes digits not in a form suitable for representing a decimal number, such as superscript digits.

str.isnumeric includes any number values, even if not digits, such as values outside the range 0-9. 

isdecimal    isdigit   isnumeric

12345        True        True       True
១2߃໔5        True        True       True
①²³🄅₅       False       True       True
⑩⒓          False       False      True
Five         False       False      False

Bytestrings (bytes in Python 3, str in Python 2), only support isdigit, which only checks for basic ASCII digits.

As with str.isalpha, the empty string evaluates to False.

str.isalnum

This is a combination of str.isalpha and str.isnumeric, specifically it evaluates to True if all characters in the given string are alphanumeric, that is, they consist of alphabetic or numeric characters: 

>>> "Hello2World".isalnum()
True
>>> "HelloWorld".isalnum()
True
>>> "2016".isalnum()
True
>>> "Hello World".isalnum()  # contains whitespace
False

str.isspace

Evaluates to True if the string contains only whitespace characters. 

>>> "\t\r\n".isspace()
True
>>> " ".isspace()
True

Sometimes a string looks “empty” but we don’t know whether it’s because it contains just whitespace or no character at all 

>>> "".isspace()
False

To cover this case we need an additional test 

>>> my_str = ''
>>> my_str.isspace()
False
>>> my_str.isspace() or not my_str
True

But the shortest way to test if a string is empty or just contains whitespace characters is to use strip(with no arguments it removes all leading and trailing whitespace characters) 

>>> not my_str.strip()
True
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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
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79 PyInstaller - Distributing Python Code
80 Iterables and Iterators
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98 PostgreSQL
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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
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128 Polymorphism
129 Alternative Python implementations
130 List Comprehensions
131 Web Server Gateway Intrerface WSGI
132 2to3 tool
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135 Unicode
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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
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162 Sockets and Message Encryption / Decryption
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164 Input Subset and Output External Data Files using Pandas
165 hashlib
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170 configparser
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172 Optical Character Recognition OCR
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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
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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