Python Mini-Course

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Lesson 27Classes and Functions

• Python Mini-Course
• University of Oklahoma
• Department of Psychology

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Lesson objectives

1. Create functions that operate on objects
2. Differentiate between pure functions and
   modifiers
3. Use argument checking in functions

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The Time class

• Throughout this lesson, we will be using the
  custom Time class
• See time.py
• In IDLE, run the time.py script to define the
  Time class and functions
• We can now use these at the command line

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Creating time objects

• time1 Time(1,0,0)
• print_time(time1)
• time2 Time(0,30,5)
• print_time(time2)

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Passing by reference

• A variable name is a pointer to the memory
  location where data are stored
• When you pass an argument to a function, you are
  passing in the pointer
• If the function changes the value of an argument,
  it changes the data in the memory location
• This is then seen outside the function

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Modifier functions

• Modifiers can (and usually do) change the value
  of arguments that are passed into the function
• See the increment_time() function

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Modifier functions Example

• increment_time(time1, time2)
• print_time(time1)
• print_time(time2)

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Pure functions

• Pure functions do not change the value of any
  arguments
• See the add_time() function

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Pure functions Example

• time3 add_time(time1, time2)
• print_time(time1)
• print_time(time2)
• print_time(time3)

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Argument checking

• Try this
• time4 Time(0,90,0)
• increment_time(time4, time1)
• increment_time(time4, 30)

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Argument checking

• Two problems
• The Time class allows invalid times (gt60 minutes
  or seconds)
• The increment_time() function is designed to
  accept two time object, but the wrong data types
  were passed to it

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Argument checking

• Solutions
• Check the formatting of the time
• Check the type for the input argument

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The valid_time() function

• def valid_time(t)
• validity True
• All values must be at least zero
• if t.hours lt 0 or t.minutes lt 0 or t.seconds
  lt 0
• validity False
• Minutes and seconds must be base 60
• if t.minutes gt 60 or t.seconds gt 60
• validity False
• return validity

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Adding argument checking

• def increment_time(t1, t2)
• """
• Given two Time objects t1 and t2, add t2 to
  t1
• """
• Check the input arguments
• if type(t1) ! Time or type(t2) ! Time
• raise AttributeError, \
• 'invalid argument passed to
  increment_time'
• if not valid_time(t1) or not valid_time(t2)
• raise ValueError, 'invalid Time object in
  increment_time'
• Add the times
• t1.hour t2.hour
• t1.minute t2.minute
• t1.second t2.second