CS105: Introduction to Python

Inheritance: Logic Gates and Circuits

Our final section will introduce another important aspect of object-oriented programming. Inheritance is the ability for one class to be related to another class in much the same way that people can be related to one another. Children inherit characteristics from their parents. Similarly, Python child classes can inherit characteristic data and behavior from a parent class. These classes are often referred to as subclasses and superclasses.

Figure 8 shows the built-in Python collections and their relationships to one another. We call a relationship structure such as this an inheritance hierarchy. For example, the list is a child of the sequential collection. In this case, we call the list the child and the sequence the parent (or subclass list and superclass sequence). This is often referred to as an IS-A Relationship (the list IS-A sequential collection). This implies that lists inherit important characteristics from sequences, namely the ordering of the underlying data and operations such as concatenation, repetition, and indexing.

Lists, tuples, and strings are all types of sequential collections. They all inherit common data organization and operations. However, each of them is distinct based on whether the data is homogeneous and whether the collection is immutable. The children all gain from their parents but distinguish themselves by adding additional characteristics.

By organizing classes in this hierarchical fashion, object-oriented programming languages allow previously written code to be extended to meet the needs of a new situation. In addition, by organizing data in this hierarchical manner, we can better understand the relationships that exist. We can be more efficient in building our abstract representations.

To explore this idea further, we will construct a simulation, an application to simulate digital circuits. The basic building block for this simulation will be the logic gate. These electronic switches represent boolean algebra relationships between their input and their output. In general, gates have a single output line. The value of the output is dependent on the values given on the input lines.

AND gates have two input lines, each of which can be either 0 or 1 (representing False or True, repectively). If both of the input lines have the value 1, the resulting output is 1. However, if either or both of the input lines is 0, the result is 0. OR gates also have two input lines and produce a 1 if one or both of the input values is a 1. In the case where both input lines are 0, the result is 0.

NOT gates differ from the other two gates in that they only have a single input line. The output value is simply the opposite of the input value. If 0 appears on the input, 1 is produced on the output. Similarly, 1 produces 0. Figure 9 shows how each of these gates is typically represented. Each gate also has a truth table of values showing the input-to-output mapping that is performed by the gate.

By combining these gates in various patterns and then applying a set of input values, we can build circuits that have logical functions. Figure 10 shows a circuit consisting of two AND gates, one OR gate, and a single NOT gate. The output lines from the two AND gates feed directly into the OR gate, and the resulting output from the OR gate is given to the NOT gate. If we apply a set of input values to the four input lines (two for each AND gate), the values are processed and a result appears at the output of the NOT gate. Figure 10 also shows an example with values.

In order to implement a circuit, we will first build a representation for logic gates. Logic gates are easily organized into a class inheritance hierarchy as shown in Figure 11. At the top of the hierarchy, the LogicGate class represents the most general characteristics of logic gates: namely, a label for the gate and an output line. The next level of subclasses breaks the logic gates into two families, those that have one input line and those that have two. Below that, the specific logic functions of each appear.

We can now start to implement the classes by starting with the most general, LogicGate. As noted earlier, each gate has a label for identification and a single output line. In addition, we need methods to allow a user of a gate to ask the gate for its label.

The other behavior that every logic gate needs is the ability to know its output value. This will require that the gate perform the appropriate logic based on the current input. In order to produce output, the gate needs to know specifically what that logic is. This means calling a method to perform the logic computation. The complete class is shown in Listing 8.

Listing 8

class LogicGate:

    def __init__(self,n):
        self.label = n
        self.output = None

    def getLabel(self):
        return self.label

    def getOutput(self):
        self.output = self.performGateLogic()
        return self.output

At this point, we will not implement the performGateLogic function. The reason for this is that we do not know how each gate will perform its own logic operation. Those details will be included by each individual gate that is added to the hierarchy. This is a very powerful idea in object-oriented programming. We are writing a method that will use code that does not exist yet. The parameter self is a reference to the actual gate object invoking the method. Any new logic gate that gets added to the hierarchy will simply need to implement the performGateLogic function and it will be used at the appropriate time. Once done, the gate can provide its output value. This ability to extend a hierarchy that currently exists and provide the specific functions that the hierarchy needs to use the new class is extremely important for reusing existing code.

We categorized the logic gates based on the number of input lines. The AND gate has two input lines. The OR gate also has two input lines. NOT gates have one input line. The BinaryGate class will be a subclass of LogicGate and will add two input lines. The UnaryGate class will also subclass LogicGate but will have only a single input line. In computer circuit design, these lines are sometimes called "pins" so we will use that terminology in our implementation.

Listing 9

class BinaryGate(LogicGate):

    def __init__(self,n):
        LogicGate.__init__(self,n)

        self.pinA = None
        self.pinB = None

    def getPinA(self):
        return int(input("Enter Pin A input for gate "+ self.getLabel()+"-->"))

    def getPinB(self):
        return int(input("Enter Pin B input for gate "+ self.getLabel()+"-->"))

Listing 10

class UnaryGate(LogicGate):

    def __init__(self,n):
        LogicGate.__init__(self,n)

        self.pin = None

    def getPin(self):
        return int(input("Enter Pin input for gate "+ self.getLabel()+"-->"))

Listing 9 and Listing 10 implement these two classes. The constructors in both of these classes start with an explicit call to the constructor of the parent class using the parent's __init__ method. When creating an instance of the BinaryGate class, we first want to initialize any data items that are inherited from LogicGate. In this case, that means the label for the gate. The constructor then goes on to add the two input lines (pinA and pinB). This is a very common pattern that you should always use when building class hierarchies. Child class constructors need to call parent class constructors and then move on to their own distinguishing data.

Python also has a function called super which can be used in place of explicitly naming the parent class. This is a more general mechanism, and is widely used, especially when a class has more than one parent. But, this is not something we are going to discuss in this introduction. For example in our example above LogicGate.__init__(self,n) could be replaced with super(UnaryGate,self).__init__(n) .

The only behavior that the BinaryGate class adds is the ability to get the values from the two input lines. Since these values come from some external place, we will simply ask the user via an input statement to provide them. The same implementation occurs for the UnaryGate class except that there is only one input line.

Now that we have a general class for gates depending on the number of input lines, we can build specific gates that have unique behavior. For example, the AndGate class will be a subclass of BinaryGate since AND gates have two input lines. As before, the first line of the constructor calls upon the parent class constructor ( BinaryGate), which in turn calls its parent class constructor (LogicGate ). Note that the AndGate class does not provide any new data since it inherits two input lines, one output line, and a label.

Listing 11

class AndGate(BinaryGate):

    def __init__(self,n):
        super(AndGate,self).__init__(n)

    def performGateLogic(self):

        a = self.getPinA()
        b = self.getPinB()
        if a==1 and b==1:
            return 1
        else:
            return 0

The only thing AndGate needs to add is the specific behavior that performs the boolean operation that was described earlier. This is the place where we can provide the performGateLogic method. For an AND gate, this method first must get the two input values and then only return 1 if both input values are 1. The complete class is shown in Listing 11.

We can show the AndGate class in action by creating an instance and asking it to compute its output. The following session shows an AndGate object, g1, that has an internal label "G1". When we invoke the getOutput method, the object must first call its performGateLogic method which in turn queries the two input lines. Once the values are provided, the correct output is shown.

>>> g1 = AndGate("G1")
>>> g1.getOutput()
Enter Pin A input for gate G1-->1
Enter Pin B input for gate G1-->0
0

The same development can be done for OR gates and NOT gates. The OrGate class will also be a subclass of BinaryGate and the NotGate class will extend the UnaryGate class. Both of these classes will need to provide their own performGateLogic functions, as this is their specific behavior.

We can use a single gate by first constructing an instance of one of the gate classes and then asking the gate for its output (which will in turn need inputs to be provided). For example:

>>> g2 = OrGate("G2")
>>> g2.getOutput()
Enter Pin A input for gate G2-->1
Enter Pin B input for gate G2-->1
1
>>> g2.getOutput()
Enter Pin A input for gate G2-->0
Enter Pin B input for gate G2-->0
0
>>> g3 = NotGate("G3")
>>> g3.getOutput()
Enter Pin input for gate G3-->0
1

Now that we have the basic gates working, we can turn our attention to building circuits. In order to create a circuit, we need to connect gates together, the output of one flowing into the input of another. To do this, we will implement a new class called Connector.

The Connector class will not reside in the gate hierarchy. It will, however, use the gate hierarchy in that each connector will have two gates, one on either end (see Figure 12). This relationship is very important in object-oriented programming. It is called the HAS-A Relationship. Recall earlier that we used the phrase "IS-A Relationship" to say that a child class is related to a parent class, for example UnaryGate IS-A LogicGate.

Now, with the Connector class, we say that a Connector HAS-A LogicGate meaning that connectors will have instances of the LogicGate class within them but are not part of the hierarchy. When designing classes, it is very important to distinguish between those that have the IS-A relationship (which requires inheritance) and those that have HAS-A relationships (with no inheritance).

Listing 12 shows the Connector class. The two gate instances within each connector object will be referred to as the fromgate and the togate, recognizing that data values will "flow" from the output of one gate into an input line of the next. The call to setNextPin is very important for making connections (see Listing 13). We need to add this method to our gate classes so that each togate can choose the proper input line for the connection.

Listing 12

class Connector:

    def __init__(self, fgate, tgate):
        self.fromgate = fgate
        self.togate = tgate

        tgate.setNextPin(self)

    def getFrom(self):
        return self.fromgate

    def getTo(self):
        return self.togate

In the BinaryGate class, for gates with two possible input lines, the connector must be connected to only one line. If both of them are available, we will choose pinA by default. If pinA is already connected, then we will choose pinB . It is not possible to connect to a gate with no available input lines.

Listing 13

def setNextPin(self,source):
    if self.pinA == None:
        self.pinA = source
    else:
        if self.pinB == None:
            self.pinB = source
        else:
           raise RuntimeError("Error: NO EMPTY PINS")

Now it is possible to get input from two places: externally, as before, and from the output of a gate that is connected to that input line. This requires a change to the getPinA and getPinB methods (see Listing 14). If the input line is not connected to anything ( None ), then ask the user externally as before. However, if there is a connection, the connection is accessed and fromgate's output value is retrieved. This in turn causes that gate to process its logic. This continues until all input is available and the final output value becomes the required input for the gate in question. In a sense, the circuit works backwards to find the input necessary to finally produce output.

Listing 14

def getPinA(self):
    if self.pinA == None:
        return input("Enter Pin A input for gate " + self.getLabel()+"-->")
    else:
        return self.pinA.getFrom().getOutput()
        

The following fragment constructs the circuit shown earlier in the section:

>>> g1 = AndGate("G1")
>>> g2 = AndGate("G2")
>>> g3 = OrGate("G3")
>>> g4 = NotGate("G4")
>>> c1 = Connector(g1,g3)
>>> c2 = Connector(g2,g3)
>>> c3 = Connector(g3,g4)

The outputs from the two AND gates (g1 and g2 ) are connected to the OR gate (g3) and that output is connected to the NOT gate (g4 ). The output from the NOT gate is the output of the entire circuit. For example:

>>> g4.getOutput()
Pin A input for gate G1-->0
Pin B input for gate G1-->1
Pin A input for gate G2-->1
Pin B input for gate G2-->1
0

Source: Runestone Academy, https://runestone.academy/runestone/books/published/pythonds/Introduction/ObjectOrientedProgramminginPythonDefiningClasses.html#inheritance-logic-gates-and-circuits
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