10.4 Time Class with Properties for Data Access

• Properties can control the manner in which they get and modify an object’s data—assuming programmers follow conventions
• For robust date and time manipulation capabilities, see Python's datetime module

10.4.1 Test-Driving Class Time

Before we look at class Time’s definition, let’s demonstrate its capabilities

from timewithproperties import Time

Creating a Time Object

• Create a Time object
• Class Time’s __init__ method has hour, minute and second parameters, each with a default argument value of 0

wake_up = Time(hour=6, minute=30)

Displaying a Time Object

• Class Time defines two methods that produce string representations of Time object
• When you evaluate a variable in IPython, it calls the object’s __repr__ special method to produce a string representation of the object

wake_up

Time(hour=6, minute=30, second=0)

• The __str__ special method is called when an object is converted to a string, such as when you output the object with print

print(wake_up)

6:30:00 AM

Getting an Attribute Via a Property

• Class Time provides hour, minute and second properties
  • Provide the convenience of data attributes for getting and modifying an object’s data
  • Implemented as methods, so they may contain additional logic

wake_up.hour

6

• Appears to simply get an hour data attribute’s value
• A ctually a call to an hour method that returns the value of an _hour data attribute

Setting the Time

• Time method set_time method provides hour, minute and second parameters, each with a default of 0

wake_up.set_time(hour=7, minute=45)

wake_up

Time(hour=7, minute=45, second=0)

Setting an Attribute via a Property

• Class Time also supports setting the hour, minute and second values individually via its properties

wake_up.hour = 6

wake_up

Time(hour=6, minute=45, second=0)

• Appears to simply assign a value to a data attribute
• Actually a call to an hour method that takes 6 as an argument, validates the value, then assigns it to a corresponding data attribute named _hour

Attempting to Set an Invalid Value

To prove that class Time’s properties validate the values you assign to them, let’s try to assign an invalid value to the hour property, which results in a ValueError:

wake_up.hour = 100

------------------------------------------------------------------------
ValueError                             Traceback (most recent call last)
<ipython-input-10-1fce0716ef14> in <module>
----> 1 wake_up.hour = 100

~/Dropbox/books/2019/Python/PyCDS_JupyterSlides/ch10/timewithproperties.py in hour(self, hour)
     20         """Set the hour."""
     21         if not (0 <= hour < 24):
---> 22             raise ValueError(f'Hour ({hour}) must be 0-23')
     23 
     24         self._hour = hour

ValueError: Hour (100) must be 0-23

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