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| Listing 1 . Multiple objects from the same class. |
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#Manufacture a 1-button radio
radio01 = Radio()## Code omitted for brevity
radio01.playStation(1)## Code omitted for brevity
#Manufacture another 1-button radioradio02 = Radio()
## Code omitted for brevityradio02.playStation(1) |
The code in Listing 1 instantiates two objects of the class Radio . References to those objects are stored in the variables named radio01 and radio02 .
Once a variable contains a reference to an object, the name of that variable can be used along with the dot operator to call a method on that specific object.
The code in Listing 1 uses the references to the two objects to call a method named playStation on each of the objects. Even though the name of the method is the same in both cases and the value of the parameter passed to themethod is the same in both cases, the result of calling the method on one object is different from the result of calling the method on the other object. This isbecause the data stored in one object is different from the data stored in the other object.
The behavior of the method is the same in both cases. However, because the method operates on different data while executing each call, the results are different. Thisis evidenced by the output shown in Figure 2 .
Each object occupies its own chunk of memory and stores its instance variables within that chunk of memory. However (probably for reasons of economy) , each object does not have its own copy of the methods defined in the class from which the object is instantiated. Instead, all objectsinstantiated from a given class share a common copy of each method defined in the class. This is where the word self comes into play. The word self is used to cause the program to behave as if each object has its own copy of each method.
Consider the variables named radio01 and radio02 in Listing 1 . Each of these variables contains a value that is returned by the process of instantiating an object. (I frequently refer to this value as a reference to the object.) The value stored in each of these variables identifies the chunk of memory occupied by one object. (The manner in which this identification is accomplished probably varies among differentimplementations of Python such as CPython, Jython, PyPy, etc. )
In any event, when you execute a statement that calls a common method such as the one shown in Listing 2 , the method must have a way of identifying the chunk of memory belonging to the object whose reference was used to make the call .
| Listing 2 . Call the playStation method on an object. |
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radio02.playStation(1) |
In other words, in the case shown in Listing 2 , the method must be able to find and use the chunk of memory identified by the contents of radio02 as opposed to the chunk of memory identified by the contents of radio01 . That is what self is all about.
Listing 3 shows an abbreviated version of the class named Radio .
| Listing 3 . Abbreviated version of the class named Radio. |
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class Radio(object):
## Code omitted for brevitydef __init__(self):self.stationNumber = [0,0,0]
## Code omitted for brevitydef playStation(self,index):
print("Playing " + self.stationNumber[index-1]) |
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