0:08
Here is our class cash register with our special methods in it and eq, as well as,
get total, add, and prove. Down at the bottom, we create three cash
registers. And we print whether they are equal in
various ways. Let's print cr1, and well as cr2.
And see what Python produces for us. When we run it, we see output that looks
a lot like what we saw in the debugger for a cash register object.
0:40
We first printed what CR1 referred to, and that's a cash register object at a
particular memory address that ends in 550.
The object that CR2 refers to is a cash register object that a memory address
that ends in C90. This might be useful for debugging, but
it's not particularly useful for presenting information to a human.
1:16
Here's the header. Subfest equal the peremiter.
For example, let's go grab a cash register.
And then modify it so that all the values for loonies, toonies, fives, tens and
ones are different. Now we'll call this special method.
Now we need to decide what we want this output to look like.
We'll say the cash register has $160 in it.
We did a quick calculation off screen... And let's put in parenthesis how many
loonies there are, how many two toonies there are, and so on.
For the type contract, self refers to a cash register object, and this method
returns a string. In fact it returns a string
representation of this cash register. For the method body, well return the
string CashRegister followed by a colon, a space, and a dollar symbol followed by
the total amount of money in this cash register.
What comes next is the number of loonies. We'll start with one and then add the
number of toons and fives. Oops, I forget myself.
Then comes the number of tens, and finally twenties.
Last, we need a closing parenthesis. It's time to try running it and there's
an error. We see here that we have a type error.
We can't convert into object to Stir implicitly.
3:02
When we try again, we find that we've forgotten one INT.
The result of get_total. Run it once more, we see that we're
almost there. We just need one more space after the
total number of dollars. We'll add that space, run it, and it
looks like we're done but wow is this ugly.
We're going to introduce you to another String method that can help clean this
mess up. This method is called Stir.format.
It's not a special method, but it is pretty powerful.
We're going to go take a look again at this output that we have in our back
test, where we have CashRegister and a colon and so on.
And, it would be nice to be able to base our return value, our string, on the
format that we have here. And in fact, we can.
Let's copy this and paste it. In order to keep the line length short,
we're going to deal with the cash register part separately from the
numbers. We are also want to replace these numbers
with place holders that we're going to replace later with whatever values we
want. These place holders are integers
indicating the order in which we're going to substitute values.
4:24
This first place holder should be replaced by the reuslt of self dot get
total. This is the number of loonies, then
toonies, fives, tens, and twenties. I hope you can see how much easier this
is to read as well as to write We can comment out what we had and run it.
Then we see that we have exactly the output that we used to have, but in a
much cleaner fashion. Special method str is called whenever we
print an object, call function str on an object, or any other time that we need a
human readable version of an object.
Jennifer CampbellAssociate Professor, Teaching Stream
Paul GriesAssociate Professor, Teaching Stream
