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Setting up and tearing down with the Python with keyword

Python's with operator lets you set up and tear down an object for use in a bock of code - typically useful for a file handle / resource accessor. Here's an example of it in use:

  with day() as today:
    print ("Today it is " + today)


It's set up using the __enter__ method for the object, and released / torn down using the __exit__ method. Here's the setting up of my "day" class for this example:

  class day:
    def __enter__(self):
      daymeal = ("Beef Salad","Curried Beef","Sausages","Pasties","Fish","Sandwiches","Roast Beef")
      self.food = daymeal[datetime.datetime.today().weekday()]
      print ("serving " + self.food)
      return self.food.upper()
    def __exit__(self ,type, value, traceback):
      print ("cleaning up from eating " + self.food)


Python's in operator when used with an if lets you check whether any value in a collection (list or tuple) is equal to the gieven value - i.e. check if a value's in a list, and with the use of a not you can check if a value's missing (not present):

  stuff = [20,"fifty",70,66]
  if 55 in stuff: print (1)
  if not 55 in stuff: print (3)


Python's in operator when used with a for iterates through a collection - placing a refernce to each item in the collection in turn into a target variable so that you can step through them in a loop:

  for value in stuff:
    print ("Value is {}".format(value))


Complete program showing all of these: [here].

Sample output:

  WomanWithCat:flask grahamellis$ python3 shin
  2
  3
  serving Fish
  Today it is FISH
  cleaning up from eating Fish
  None
  None
  None
  Value is 20
  Value is fifty
  Value is 70
  Value is 66
  WomanWithCat:flask grahamellis$


Next Python courses start on 15th December, and further courses follow every 2 months next year. See [here] for a full schedule. "Learning to program in Python" is for newcomers to programming; "Python Programming" for those with prior experience in another language.
(written 2015-10-16)

 
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)
  [4713] Equality (in Python) - (2016-10-30)
  [4545] Method, Class, Module, Package - how to they relate in Python? - (2015-10-17)
  [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)

Y112 - Python - Objects - Intermediate
  [4719] Nesting decorators - (2016-11-02)
  [4718] Defining an object that is a modified standard type in Python - (2016-11-02)
  [4717] with in Python - examples of use, and of defining your own context - (2016-11-02)
  [4649] Object and Static methods - what is the difference; example in Python 3 - (2016-02-17)
  [4450] Deciding whether to use parameters, conditional statements or subclasses - (2015-03-05)
  [4449] Spike solution, refactoring into encapsulated object methods - good design practise - (2015-03-05)
  [4410] A good example of recursion - a real use in Python - (2015-02-01)
  [4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)
  [4356] Object factories in C++, Python, PHP and Perl - (2014-12-19)
  [4344] Python base and inherited classes, test harness and unit testing - new examples - (2014-12-07)
  [4094] Python Properties - how and why - (2013-05-18)
  [4028] Really Simple Class and Inheritance example in Python - (2013-03-04)
  [3887] Inheritance, Composition and Associated objects - when to use which - Python example - (2012-10-10)
  [3796] Backquote, backtic, str and repr in Python - conversion object to string - (2012-07-05)
  [3524] Metaclasses (Python) and Metatables (Lua) - (2011-11-17)
  [3472] Static variables in functions - and better ways using objects - (2011-10-10)
  [3442] A demonstration of how many Python facilities work together - (2011-09-16)
  [3002] A list of special method and attribute names in Python - (2010-10-17)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [2905] Defining static methods in Python - (2010-08-05)
  [2889] Should Python classes each be in their own file? - (2010-07-27)
  [2785] The Light bulb moment when people see how Object Orientation works in real use - (2010-05-28)
  [2764] Python decorators - your own, staticmethod and classmethod - (2010-05-14)
  [2722] Mixins example in Python - (2010-04-14)
  [2720] Multiple inheritance in Python - complete working example - (2010-04-14)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2693] Methods that run on classes (static methods) in Python - (2010-03-25)
  [2485] How do I set up a constant in Python? - (2009-10-31)
  [2409] TypeError: super() argument 1 must be type, not classobj (Python) - (2009-09-18)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [1819] Calling base class constructors - (2008-10-03)
  [1644] Using a utility method to construct objects of different types - Python - (2008-05-17)
  [1517] Python - formatting objects - (2008-01-24)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1146] __new__ v __init__ - python constructor alternatives? - (2007-04-14)
  [964] Practical polymorphism in action - (2006-12-04)
  [903] Pieces of Python - (2006-10-23)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [477] Class, static and unbound variables - (2005-10-25)
  [383] Overloading of operators on standard objects in Python - (2005-07-19)
  [296] Using a Python dictionary as a holder of object attributes - (2005-04-30)


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Previous and next
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Forward to
The principle of mocking - and the Python Mock package
Some other Articles
RSpec - Ruby testing (stand alone example / no cucumber)
Saturday morning at Well House Manor
The principle of mocking - and the Python Mock package
Setting up and tearing down with the Python with keyword
Unittest of a Flask application including forms
TransWilts public transport corridor
Flask and unittest - hello web app test world
example of SQLite using a local database file through SQLalchemy
Json load from URL, recursive display, Python 3.4
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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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