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Equality (in Python)

"Equality" ... lovely subject. We're all equal in the eyes of many laws these days - man or workman, able bodied or not, young or old, left or right handed, any colour or religion. But just because we're equal doesn't mean we're the same.

In Python, the == operator returns True if the two parameters passed to it have an equal value, whether or not the two objects are the same object. Whereas the is operator only returns True if the parameters and one and the same object (i.e. if the id function returns the same value on both of the). Example program [here].

  >>> a = [2,3,4]
  >>> b = [2,3,4]
  >>> a == b
  True
  >>> a is b
  False
  >>> c = a
  >>> a == c
  True
  >>> a is c
  True
  >>>


Note that many simple immutable objects (such as integers) are only held once on the heap, and you may be surprised to get back a True response from is when you compare two variables derived in different ways. That's not the case for mutable objects such as types you're likely to create for yourself, or for lists and dicts.

You can override the == operator in your own classes by specifying a __eq__ method.

Want to know more? Read [[about our Python courses]] where we cover this and much more!
(written 2016-10-30)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y103 - Python - Conditionals and Loops
  [4723] Conditional operators in Python - (2016-11-05)
  [4545] Method, Class, Module, Package - how to they relate in Python? - (2015-10-17)
  [4541] Setting up and tearing down with the Python with keyword - (2015-10-16)
  [4402] Finding sum, minimum, maximum and average in Python (and Ruby) - (2015-01-19)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [4210] If elif elif elif - multiway selection in Python - (2013-11-16)
  [4092] Identity in Python - (2013-05-17)
  [3895] Flowchart to program - learning to program with Well House - (2012-10-14)
  [3762] Learning to program - the if statement. Python. - (2012-06-12)
  [3620] Finding the total, average, minimum and maximum in a program - (2012-02-22)
  [3558] Python or Lua - which should I use / learn? - (2011-12-21)
  [3439] Python for loops - applying a temporary second name to the same object - (2011-09-14)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [3200] How a for loop works Java, Perl and other languages - (2011-03-12)
  [3083] Python - fresh examples from recent courses - (2010-12-11)
  [2899] Groupsave tickets - 3 or 4 train tickets for the price of 2 - (2010-08-02)
  [2778] Learning to program in Python 2 ... and / or in Python 3 - (2010-05-24)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1201] No switch in Python - (2007-05-23)
  [909] Python is like a narrowboat - (2006-10-30)
  [835] Python - when to use the in operator - (2006-08-16)
  [788] New - Conditional expressions in Python 2.5 - (2006-07-01)
  [668] Python - block insets help with documentation - (2006-04-04)
  [657] The ternary operator in Python - (2006-03-25)
  [353] Wimbledon Neck - (2005-06-20)
  [299] What - no switch or case statement? - (2005-05-03)

Y199 - Python - Fast Start
  [4712] A reminder of the key issues to consider in moving from Python 2 to Python 3 - (2016-10-30)
  [4709] Some gems from Intermediate Python - (2016-10-30)
  [4707] Some gems from an introduction to Python - (2016-10-29)
  [3519] Python - current versions and implementations (CPython, Jython, IronPython etc) - (2011-11-13)
  [2017] Python - a truly dynamic language - (2009-01-30)


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