Essential Books

Nonlocal Variables

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Python 3 added a new keyword called nonlocal. The nonlocal keyword adds a scope override to the inner scope. You can read all about it in PEP 3104. This is best illustrated with a couple of code examples. One of the most common examples is to create function that can increment: 

def counter():
    num = 0
    def incrementer():
        num += 1
        return num
    return incrementer

If you try running this code, you will receive an UnboundLocalError because the num variable is referenced before it is assigned in the innermost function. Let’s add nonlocal to the mix: 

def counter():
    num = 0
    def incrementer():
        nonlocal num
        num += 1
        return num
    return incrementer

c = counter()
c() # = 1
c() # = 2
c() # = 3

Basically nonlocal will allow you to assign to variables in an outer scope, but not a global scope. So you can’t use nonlocal in our counter function because then it would try to assign to a global scope. Give it a try and you will quickly get a SyntaxError. Instead you must use nonlocal in a nested function.

(Note that the functionality presented here is better implemented using generators.)

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Variable Scope and Binding:
* Variable Scope and Binding
* Nonlocal Variables
* Global Variables
* Local Variables
* Binding Occurrence
* Functions skip class scope when looking up names
* The del command
* Local vs Global Scope
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
12 Operator module
13 Variable Scope and Binding
14 Basic Input and Output
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79 PyInstaller - Distributing Python Code
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121 Python Anti-Patterns
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154 Audio
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156 queue module
157 ijson
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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
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168 Creating a Windows Service
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170 configparser
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172 Optical Character Recognition OCR
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177 Functional Programming
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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