P304, Appendix A, Answer 7

jc. · August 4th, 2015, 04:40 PM

I believe

Code:

class RotatingCounter:
   def __init__(self, start = 0)

should read:

Code:

class RotatingCounter:
   def __init__(self, start = 0):

And should:

Code:

if 0 < value < 9:

read?

Code:

if 0 < current_value < 9:

Alan G · August 4th, 2015, 06:00 PM

Quote:

Originally Posted by jc.

I believe

Code:

class RotatingCounter:
   def __init__(self, start = 0)

should read:

Code:

class RotatingCounter:
   def __init__(self, start = 0):

Quite right. We should have had you as a proof reader!

Quote:

Originally Posted by jc.

And should:

Code:

if 0 < value < 9:

read?

Code:

if 0 < current_value < 9:

But not for this one, although there is a slight error in the text. Where it says "the latter returns the current count and then sets the count back to zero" it should say sets the count to value if value is valid and otherwise leaves it as-is. (the default behaviour is of course to set it to zero.) So the code is what I intended but the description is inadequate.

Thanks again for highlighting it. I'm trying to get these things onto the book's errata page.

Alan G.

jc. · August 5th, 2015, 03:48 AM

Cheers Alan.

[warning: I suspect I'm way off base here]

So calling reset() with a 'value' is optional, right? If it's not provided, zero is used, yes?

A value is provided, but not valid

I don't see how the default value is then used to reset self.counter, as we just (in the else block) raise an exception and return the current value of self.counter, don't we?

No value is provided, and zero is used

Doesn't the if block fail, as zero is not less than zero, so self.counter is not set to zero?

Value is provided, and is valid

Assuming the code does work as intended (likely given my understanding), why return the old value of self.counter (held in current_value), why not just return self.counter in both situations (and dispense with using current_value)?

Thanks for your patience!

Alan G · August 5th, 2015, 05:33 AM

Groan! I obviously had a bad day at the office with this one.

Yes value is optional, if not given the default zero should be used.

You are also right that the test for value should be

if 0<= value <=9:

Finally the reason to return the old value is mainly an arbitrary
choice, but its a practical one. Since reset() now allows you to
set the counter at any value you choose(within the range) in
practice you may want to remember the old value, change the
counter temporarily, then later go back to how it was. You could
do that in multiple steps but having reset give you the old value
for free saves a step. If you didn't need the old value you just
ignore it!As i say it was an arbitrary decision intended to make
the users life slightly easier And you should always try to design
a class (or a program for that matter) from the users perspective..

One thing to remember with the Appendix answers is that they
are *example* solutions. Any answer that meets the spec as given
is correct. In this case i tried to be a bit (too?) clever and provide
some added value (the original spec didn't say anything about setting
the value, it just said reset to zero...)

And another thing that will be obvious by now is that the sample
answers didn't get tested as thoroughly as the code in the main
text. That was probably a mistake....!

jc. · August 5th, 2015, 06:13 AM

Ahh. Thank you.

re: user perspective / examples only. Good advice / yes, of course.

Much appreciated.