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Sharing variables with functions, but keeping them local too - Python

""" One of the big issues with any programming language is how in shares (scopes) variables between blocks of code. On one hand there's a desire to have variables easily accessible without a lot of passing around. On the other hand there's the need to keep information in vary different parts of you code apart, with a variable name you happen to have used twice not resulting in a memory location being shared between two pieces of code which are otherwise independent of each other.

With the notable exception of Perl, languages "scope" variables to the subroutine / procedure / method / proc / macro in which they are defined, unless you take other specific action such as declaring them global.

This piece of Python code - a rather messy example which resulted from a practical demonstration earlier today - shows examples of many facets of variable sharing to defined functions, and a couple of surprises too! """

# This first is a variable in the main code only.
# There is a different first in the function "one"
 
# second is in the main code
# As it is used READ ONLY in the function, it is shared there
 
first = 1
second = 14
 
def one():
   first = 4 + second
   first *= 2
   print first
   result = first+3
# Referring to a variable in the functions own name space
# is broadly equivalent to it being "static" - it is visible
# outside, and retained for next time the function is called
   one.latest = result
   return result, first, second
 
# A Lambda is a quick way of defining a function
two = lambda john: john + 2
# A function is an object, so you can easily give
# it another reference (i.e. a second name!)
three = two
 
print first
# A function returns one object - but if that object is a
# tuple, then it can be saved as multiple variables (objects)
zig, zag, zog = one()
print zig
print zag
print zog
 
print first
print second
 
# Swapping over two values in Python
# No need for a temporary variable
# (or rather - Python provides the temporary tuple!)
first, second = second, first
 
print first
print second
 
dis = two(first)
print dis
 
dat = three(dis)
print dat
 
print one.latest
 
""" Here are the results if running that!
1
36
39
36
14
1
14
14
1
16
18
39
"""


""" Using the ''' notation, the whole of this article is a working Python program which you can simply cut and paste and run for yourself!"""
(written 2008-09-09, updated 2008-09-11)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y105 - Python - Functions, Modules and Packages
  [96] Variable Scope - (2004-10-22)
  [105] Distance Learning - (2004-10-31)
  [294] Python generator functions, lambdas, and iterators - (2005-04-28)
  [303] Lambdas in Python - (2005-05-06)
  [308] Call by name v call by value - (2005-05-11)
  [340] Code and code maintainance efficiency - (2005-06-08)
  [386] What is a callback? - (2005-07-22)
  [418] Difference between import and from in Python - (2005-08-18)
  [561] Python's Generator functions - (2006-01-11)
  [668] Python - block insets help with documentation - (2006-04-04)
  [745] Python modules. The distribution, The Cheese Shop and the Vaults of Parnassus. - (2006-06-05)
  [749] Cottage industry or production line data handling methods - (2006-06-07)
  [775] Do not duplicate your code - (2006-06-23)
  [821] Dynamic functions and names - Python - (2006-08-03)
  [900] Python - function v method - (2006-10-20)
  [912] Recursion in Python - (2006-11-02)
  [913] Python - A list of methods - (2006-11-03)
  [949] Sludge off the mountain, and Python and PHP - (2006-11-27)
  [959] It's the 1st, not the 1nd 1rd or 1th. - (2006-12-01)
  [1134] Function / method parameters with * and ** in Python - (2007-04-04)
  [1163] A better alternative to cutting and pasting code - (2007-04-26)
  [1202] Returning multiple values from a function (Perl, PHP, Python) - (2007-05-24)
  [1464] Python Script - easy examples of lots of basics - (2007-12-08)
  [1784] Global - Tcl, PHP, Python - (2008-09-03)
  [1869] Anonymous functions (lambdas) and map in Python - (2008-11-04)
  [1870] What to do with a huge crop of apples - (2008-11-04)
  [1871] Optional and named parameters in Python - (2008-11-05)
  [1879] Dynamic code - Python - (2008-11-11)
  [2011] Conversion of OSI grid references to Eastings and Northings - (2009-01-28)
  [2439] Multiple returns from a function in Python - (2009-10-06)
  [2440] Optional parameters to Python functions - (2009-10-07)
  [2481] Sample code with errors in it on our web site - (2009-10-29)
  [2506] Good example of recursion in Python - analyse an RSS feed - (2009-11-18)
  [2520] Global and Enable - two misused words! - (2009-11-30)
  [2718] Python - access to variables in the outer scope - (2010-04-12)
  [2766] Optional and named parameters to Python functions/methods - (2010-05-15)
  [2878] Program for reliability and efficiency - do not duplicate, but rather share and re-use - (2010-07-19)
  [2929] Passing a variable number of parameters in to a function / method - (2010-08-20)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [2998] Using an exception to initialise a static variable in a Python function / method - (2010-10-13)
  [3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)
  [3280] Passing parameters to Python functions - the options you have - (2011-05-07)
  [3459] Catching the fishes first? - (2011-09-27)
  [3464] Passing optional and named parameters to python methods - (2011-10-04)
  [3472] Static variables in functions - and better ways using objects - (2011-10-10)
  [3474] Python Packages - groupings of modules. An introduction - (2011-10-11)
  [3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)
  [3695] Functions are first class variables in Lua and Python - (2012-04-13)
  [3766] Python timing - when to use a list, and when to use a generator - (2012-06-16)
  [3852] Static variables in Python? - (2012-08-29)
  [3885] Default local - a good choice by the author of Python - (2012-10-08)
  [3931] Optional positional and named parameters in Python - (2012-11-23)
  [3945] vargs in Python - how to call a method with unknown number of parameters - (2012-12-06)
  [4029] Exception, Lambda, Generator, Slice, Dict - examples in one Python program - (2013-03-04)
  [4161] Python varables - checking existance, and call by name or by value? - (2013-08-27)
  [4212] Python functions - an introduction to how they work - (2013-11-16)
  [4361] Multiple yields and no loops in a Python generator? - (2014-12-22)
  [4407] Python - even named code blocks are objects - (2015-01-28)
  [4410] A good example of recursion - a real use in Python - (2015-02-01)
  [4441] Reading command line parameters in Python - (2015-02-23)
  [4448] What is the difference between a function and a method? - (2015-03-04)
  [4645] What are callbacks? Why use them? An example in Python - (2016-02-11)
  [4662] Recursion in Python - the classic example - (2016-03-07)
  [4719] Nesting decorators - (2016-11-02)
  [4722] Embedding more complex code into a named block - (2016-11-04)
  [4724] From and Import in Python - where is the module loaded from? - (2016-11-06)


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The road ahead - Python 3
Some other Articles
Refactoring - a PHP demo becomes a production page
Which country does a search engine think you are located in?
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The road ahead - Python 3
Sharing variables with functions, but keeping them local too - Python
Looking for a value in a list - Python
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