If you make a fish pie for your friends, you'll catch all the fish you need before you start the preparation and cooking work, and you'll do the cooking all at once.

If you run a fish pie factory, you'll have a steady flow of fish arriving while your production line is running. Receiving all the fish you need before you start production each week would give you a major storage problem (inside) or an issue with some rathe keen gulls (outside).

In Python, you can iterate through a list - and that's fine for the smaller cases. But in cases where you have a substantial amount of data, you should read / request / work it out on a "just in time" basis, using a generator function.

Here's an example piece of code to show you how ...

  # Getting all the fish at once
  
  def getvals(top):
  stuff = []
    vat = 0
    while vat < top:
      print "caught mackrel no",vat
      stuff.append(vat)
      vat += 1
    return stuff
  
  # Just in time
  
  def xgetvals(top):
    vat = 0
    while vat < top:
      print "caught mackrel no",vat
      yield vat
      vat += 1
  
  ending = 15
  
  for value in getvals(ending):
    print "Added to pie",value
  
  for value in xgetvals(ending):
    print "Added to pie",value

(written 2011-09-27, updated 2011-09-28)

 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)
  [1790] Sharing variables with functions, but keeping them local too - Python - (2008-09-09)
  [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)
  [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)

Back to On this day ... one PHP script with three uses

Previous and next or Horse's mouth home

Forward to Flying Colours!

Busy weekend of contrasts.
Hangers, luggage and possessions
From flowchart to program - code design for the newcomer
Flying Colours!
Catching the fishes first?
On this day ... one PHP script with three uses
Away to train - but still around by video for Melksham meetings
Stepping stones - early coding, and writing re-usable code quickly
MySQL, MySQLi, PDO or something else - how best to talk to databases from PHP
Your PHP website - how to factor and refactor to reduce growing pains