When you call an object's constructor method, you'll be allocating memory to hold the information about that object and the method will return an instance variable - a reference with which you can later refer back to the object.

That's good as far as it goes - but there are times when you'll have data from which you want to construct an object but the object type is hidden within the data. For example, I could have a file of data record like
Estate Agent, 01380 724040, 2500
and
Graham, 600
where the lines with three comma separated fields are customers, and the lines with two comma separate fields are team members. And you want to construct two different types of object, depending on the data.

The first possible answer is to have your application program work out from each data line which particular object type you need to build for each piece of data but that is messy as it means that the data-specific code that should be common to all uses of the class has to migrate outside the class to the application, with serious issues about code duplication, reuse and maintainability.

The preferable alternative is for you to provide a utility method within(Java, etc) or in association with (Python, etc) the class coding which takes the data record and constructs either a "customer" or a "team member" record, and returns whichever instance variable type it finds to be appropriate.

I have an example of this principle in the Intermediate Objects in Python module from our Python Programming Course.

The "control case" - the example which shows the decision made in the application code without the utility method reads as follows:
operation.append(customer("Local Council","01225 776655",10000))
operation.append(customer("Estate Agent","01380 724040",2500))
operation.append(employee("Charlie",20))
operation.append(employee("Graham",600))
which is modified to read as follows when you use utility method calls:
operation.append(fing("Local Council, 01225 776655, 10000"))
operation.append(fing("Estate Agent, 01380 724040, 2500"))
operation.append(fing("Charlie, 20"))
operation.append(fing("Graham, 600"))
Which looks a subtle enough change when the data is a simple example, but becomes significant when the number of object types increases and the data is read from file.

Here's the code of the utility method, which will be stored and maintained with the classes:
def fing(about):
  bits = about.split(", ")
  if len(bits) == 3:
    return customer(bits[0],bits[1],int(bits[2]))
  return employee(bits[0],int(bits[1]))

See control example - source and example with utility method - source
(written 2008-05-17)

 Y112 - Python - Objects - Intermediate
  [296] Using a Python dictionary as a holder of object attributes - (2005-04-30)
  [383] Overloading of operators on standard objects in Python - (2005-07-19)
  [477] Class, static and unbound variables - (2005-10-25)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [903] Pieces of Python - (2006-10-23)
  [964] Practical polymorphism in action - (2006-12-04)
  [1146] __new__ v __init__ - python constructor alternatives? - (2007-04-14)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1517] Python - formatting objects - (2008-01-24)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
  [1819] Calling base class constructors - (2008-10-03)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2409] TypeError: super() argument 1 must be type, not classobj (Python) - (2009-09-18)
  [2485] How do I set up a constant in Python? - (2009-10-31)
  [2693] Methods that run on classes (static methods) in Python - (2010-03-25)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2720] Multiple inheritance in Python - complete working example - (2010-04-14)
  [2722] Mixins example in Python - (2010-04-14)
  [2764] Python decorators - your own, staticmethod and classmethod - (2010-05-14)
  [2785] The Light bulb moment when people see how Object Orientation works in real use - (2010-05-28)
  [2889] Should Python classes each be in their own file? - (2010-07-27)
  [2905] Defining static methods in Python - (2010-08-05)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [3002] A list of special method and attribute names in Python - (2010-10-17)
  [3442] A demonstration of how many Python facilities work together - (2011-09-16)
  [3472] Static variables in functions - and better ways using objects - (2011-10-10)
  [3524] Metaclasses (Python) and Metatables (Lua) - (2011-11-17)
  [3796] Backquote, backtic, str and repr in Python - conversion object to string - (2012-07-05)
  [3887] Inheritance, Composition and Associated objects - when to use which - Python example - (2012-10-10)
  [4028] Really Simple Class and Inheritance example in Python - (2013-03-04)
  [4094] Python Properties - how and why - (2013-05-18)
  [4344] Python base and inherited classes, test harness and unit testing - new examples - (2014-12-07)
  [4356] Object factories in C++, Python, PHP and Perl - (2014-12-19)
  [4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)
  [4410] A good example of recursion - a real use in Python - (2015-02-01)
  [4449] Spike solution, refactoring into encapsulated object methods - good design practise - (2015-03-05)
  [4450] Deciding whether to use parameters, conditional statements or subclasses - (2015-03-05)
  [4541] Setting up and tearing down with the Python with keyword - (2015-10-16)
  [4649] Object and Static methods - what is the difference; example in Python 3 - (2016-02-17)
  [4717] with in Python - examples of use, and of defining your own context - (2016-11-02)
  [4718] Defining an object that is a modified standard type in Python - (2016-11-02)
  [4719] Nesting decorators - (2016-11-02)

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