19.7. Case Study: Structured Postal Addresses

Postal addresses are interesting things. Every country has its own format for postal addresses, and sometimes one country can have multiple address formats.

Postal addresses generally consist of a few standard elements: the recipient name; street address; city or locality; state or province; and a postal code. However, other elements are often included, such as neighborhood, district, post office identifier, and so on. For example, the following is

Mr. Abe Jones
Acme Corporation
123 Somewhere Ln
Greenville, SC 29609
USA

This same address would be written as follows for delivery to the Netherlands (in the example, the street, city, and state are unchanged, even though they do not exist in the Netherlands):

Acme Corporation
Mr. Abe Jones
Somewhere Ln 123
29609 SC Greenville
NETHERLANDS

Addresses in Ireland are complex, having up to 12 parts (such as building name and number, primary and secondary thoroughfare, primary and secondary locality, town, county, …) plus an Eircode, a unique identifier assigned to each of the ~2 million addresses in Ireland. For example, Abe might live at the following address (English translation is given in parentheses, and would be omitted):

Abe Jones
Cnoc na Sceiche (The Hill of the Thorn)
Leac an Anfa (The Flagstone of the Storm)
Cathair na Mart (The City of the Beeves)
Co. Mhaigh Eo (The County of the Plain of the Yews)
A65 F4E2
IRELAND

(One would think that since each address has its own unique Eircode, it ought to be possible to address mail to Abe Jones, A65 F4E2, IRELAND. On second thought, perhaps that is not such a great idea. Can you imagine the practical concerns with such a scheme?)

19.7.1. Storing Postal Addresses

Suppose we want to write a contact management application. Among other things, the application stores names and addresses. What would be the best way to design a class that holds the information for an address? One approach would be to store the parts of the address that are consistent, such as the recipient name and the country, in instance variables, and store the rest of the address as a list of address lines:

This approach treats an address as a collection of unstructured bits of information. If we want to look up an address, we can search by full name or country, but if we want to find all addresses in Greenville, or all addresses in zip code 29609, we can’t do it very easily, since information such as city and zip code is mashed together in an unstructured address line along with the state abbreviation.

An approach that stores addresses as structured pieces of information might look like this:

Now, if we have a list of StructuredAddress objects and we want to find all of the ones that hold addresses in Greenville, we can do it much more easily:

for addr in addrList:
    if addr.city == 'Greenville':
        addr.display()

19.7.2. Storing International Addresses

But now we have another problem. Our StructuredAddress works fine for U.S. addresses, but not for those of other countries. Suppose we want to handle Irish and Italian addresses. We might enhance the display() method to handle these with appropriate logic:

def display(self):
    print(self.recipient)

    if self.country == 'USA':
        print(self.street)
        print(self.city + ", " + self.state + "  " + self.postalCode)
    elif self.country == 'IRELAND':
        print(self.postalCode)
    elif self.country == 'ITALY':
        print(self.street)
        print(self.postalCode + ' ' + self.city + ' ' + self.state)
    else:
        pass

    print(self.country)

This example works for Italian addresses because they conveniently have the same elements as U.S. addresses (just displayed in a slightly different order). For Irish addresses, we ignore the complicated address format and assume that the Irish post office will get mail to the recipient because of Ireland’s unique Eircode scheme. But what if we wanted to include the additional elements of Irish addresses? We might create additional instance variables for those elements in our StructuredAddress class. However, you can probably see that approach will quickly become unwieldy.

19.7.3. Inheritance Applied

Let’s apply inheritance to the problem of managing structured postal addresses. We will define a base class that contains the attributes in common to all postal addresses: recipient and country.

This class isn’t very useful by itself; relatively few people in the world could receive mail addressed to them using only their name and country. But it does establish two methods to perform functionality we want all addresses to perform: display themselves, and check whether the required information is present and of an appropriate length.

Next, we build on BasePostalAddress by creating a separate class for each country that inherits from it:

19.7.4. A List of Addresses

Now, let’s construct a list containing both US and Irish addresses, and display them using a loop:

Normally, if a program iterates over a list that contains different types of objects, it has to be careful about making assumptions about the methods and operations that it can invoke on the different objects in the list, since an attempt to invoke a method or apply an operator to an object that does not support the method or operator will result in a runtime error. In this case, we know that all of the objects in the list inherit from BasePostalAddress. It is safe to invoke any methods defined in BasePostalAddress, since all children of BasePostalAddress are guaranteed to contain those methods. Programs that use inheritance often contain loops like this.

Notice something else. As the loop iterates over the list, each time the display() method is invoked, the computer will execute the one that is defined for the specific object referenced by addr. The first time through the loop, addr references an IrishPostalAddress, so the display() method for Irish addresses is invoked. The second time through the loop, the display() method in USPostalAddress is invoked. This behavior—where the computer always executes the method that is defined for the object being referenced—is called polymorphism. Python exhibits this behavior whether or not the objects in question utilize inheritance, but languages like Java and C++, polymorphism is available only through inheritance.

19.7.5. Using isinstance

Let’s try something else with our list of addresses. Suppose we wanted to display all addresses with a given city. We might write some code like this:

for addr in addrList:
    if addr.city == 'Greenville':
        addr.display()

However, we would get into trouble on the first iteration of the loop. The first address is an Irish address, which does not have a city attribute. Python would raise an error. We want to perform this test only for US addresses.

In this case, since all addresses have a country attribute, we could write the loop this way:

for addr in addrList:
    if addr.country == 'USA' and addr.city == 'Greenville':
        addr.display()

Another way to test the address is to find out if the object belongs to a specific class. Python provides the isinstance() function for this purpose. isinstance() is designed for situations where you want to access a field or invoke a method on an object, but you want to do so only if the object provides the needed functionality. Given an object obj and a class cls, isinstance(obj, cls) returns True if obj is an instance of cls (or a subclass of cls), and False if it is not. Here is how we might use it in our loop:

for addr in addrList:
    if isinstance(addr, USPostalAddress) and addr.city == 'Greenville':
        addr.display()

In this version of the code, the city attribute will be tested only if addr references an instance of USPostalAddress, or a child of USPostalAddress (which would also have a city attribute).

Now that you’ve learned about isinstance(), you should know that, like inheritance itself, isinstance() should be used sparingly. Code that invokes isinstance() is often performing work on an object that the object should be designed to do itself, and is not utilizing inheritance and polymorphism to its full potential.

To make this loop better utilize inheritance and polymorphism, we need a way to test each address to see if it is in a given city. Let’s add a method to BasePostalAddress for this purpose. It will return a boolean indicating whether the address is in a certain city.

class BasePostalAddress:

    ...

    def isInCity(self, city):
        return False

BasePostalAddresses do not have a city attribute, so they just return False. USPostalAddresses do have a city, so we’ll override this method for that class:

class USPostalAddress:

    ...

    def isInCity(self, city):
        return self.city == city

Now, we rewrite our loop to use isInCity() to perform the test:

for addr in addrList:
    if addr.isInCity('Greenville'):
        addr.display()

Notice how we’ve eliminated the isinstance() test. Also, notice how this test works for IrishPostalAddress objects, even though we didn’t define isInCity() for IrishPostalAddress, since IrishPostalAddress inherits its version from BasePostalAddress.

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