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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
When to check an object type - Python isinstance example

I'll encourage you not to look at an object in a Python program - or in any other OO langauge - you find out what type it is ... at least until you've thought about why you're doing it.

If you need to, you can look - you have the type function, you have the __class__.__name__ variable and you have isinstance.

Isinstance lets you check the hierarcy of an object - so I can use it to ask if an object is of a particular type, or a type that's based on it.

Here's a look to see if something's a list:
  if isinstance(meal,list):
and in use in a program [here]. I'm not thrilled with the example - but is shows you the syntax. I would much rather you used polymorphism (several methods of the same name in order to decide internally how to print the data) rather than a branch chain based on the type.

Where I will encourage you to use isinstance is upon entry to a function that's called from outside your area of control - for example within the API of a function, method, module or package that you've written for others to use, and you want to check they're passing you sensible parameters. See [here] for an example.
(written 2016-11-03)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y106 - Object Oriented Python
  [477] Class, static and unbound variables - (2005-10-25)
  [834] Python makes University Challenge - (2006-08-15)
  [900] Python - function v method - (2006-10-20)
  [1306] Python class rattling around - (2007-08-16)
  [1348] Screw it or Glue it? Access to Object variables - a warning - (2007-09-12)
  [1925] Introduction to Object Oriented Programming - (2008-12-06)
  [2017] Python - a truly dynamic language - (2009-01-30)
  [2169] When should I use OO techniques? - (2009-05-11)
  [2604] Tips for writing a test program (Ruby / Python / Java) - (2010-01-29)
  [3085] Object Oriented Programming for Structured Programmers - conversion training - (2010-12-14)
  [3399] From fish, loaves and apples to money, plastic cards and BACS (Perl references explained) - (2011-08-20)
  [3436] Moving from scripting to Object Orientation in Python - (2011-09-13)
  [3673] Object oriented or structured - a comparison in Python. Also writing clean regular expressions - (2012-03-26)
  [3878] From Structured to Object Oriented Programming. - (2012-10-02)
  [3947] this or self - what are they, and what is the difference? (Python) - (2012-12-08)
  [4021] Spike solution, refactored and reusable, Python - Example - (2013-02-28)
  [4028] Really Simple Class and Inheritance example in Python - (2013-03-04)
  [4129] Simple OO demonstration in C++, comparison to Python - (2013-07-01)
  [4448] What is the difference between a function and a method? - (2015-03-04)
  [4591] From single block to structure and object oriented programming - (2015-12-02)
  [4650] Why populate object with values as you construct them? - (2016-02-18)


Back to
Celebrating our diversity rather than discriminating for it
Previous and next
or
Horse's mouth home
Forward to
Embedding more complex code into a named block
Some other Articles
Writing Python 2 such that Python 3 is easy
From and Import in Python - where is the module loaded from?
Conditional operators in Python
Embedding more complex code into a named block
When to check an object type - Python isinstance example
Celebrating our diversity rather than discriminating for it
Nesting decorators
Defining an object that is a modified standard type in Python
with in Python - examples of use, and of defining your own context
Profiling your Python program
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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