P31. Step 5. Code error(s)?

jc. · August 2nd, 2015, 04:00 PM

I think Step 5 should start with:

>>> c2 = tc.Circle2(42)

Rather than:

>>> c2 = Circle2(42)

Otherwise you get:

>>> c2 = Circle2(42)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'Circle2' is not defined

Or did I miss something?

The downloaded code also generated this error:
>>> c2.area()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'float' object is not callable

Thanks.

Alan G · August 2nd, 2015, 05:22 PM

You are quite right. My fault entirely, the error is in the original manuscript.
Thanks for pointing it out. The first line should indeed read

>>> c2 = tc.Circle2(42)

My apologies,

Alan G.

Alan G · August 2nd, 2015, 05:36 PM

The downloaded code issue is slightly different. If you look closely at the
text you will see that the correct usage is

>>> c2.area

No parens after area.

That's the point of using the @property.
It means you can access area as if it were a data attribute even though,
in reality, its calling a method.

Alan G.

jc. · August 3rd, 2015, 03:41 AM

Thanks for both replies Alan, and for being so quick. Oh yes, I see what you mean now about c2.area. My mistake!

Paul_Friesen · February 29th, 2016, 12:08 PM

When I do step 5, I don't get the expected result, even with the correction. The line c2.radius = 12 seems to have no effect. c2.area prints out 5541.76476 just as before. c2.radius = -4 does not generate the error message, and the radius is apparently unchanged.

I thought this might be the little ARM board I am trying this on, so I tried it on my laptop, with the code downloaded from the web to make sure I had no typo. Same result.

Alan G · February 29th, 2016, 09:24 PM

> When I do step 5, I don't get the expected result, even with the correction.
> The line c2.radius = 12 seems to have no effect. c2.area prints out
> 5541.76476 just as before. c2.radius = -4 does not generate the
> error message, and the radius is apparently unchanged.

That's odd, I don't know what's happening there. Can you cut 'n paste
the transcript of your interpreter session to me so I can see exactly
what you get? Either post it here, or send by email to

[email protected]

Paul_Friesen · March 1st, 2016, 06:17 PM

I have realized that I have python 2.7 on both computers.

Here is the interpreter sesioun:

Code:

odroid@odroid:~/Documents/tests/programming/python/test01$ python
Python 2.7.6 (default, Mar 22 2014, 23:30:12) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import testcircle as tc
>>> c2 = Circle2(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'Circle2' is not defined
>>> c2 = tc.Circle2(42)
>>> c2.area
5541.76476
>>> print(c2.radius)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: unreadable attribute
>>> c2.radius = 12
>>> c2.area
5541.76476
>>> c2.radius = -4
>>> c2.area
5541.76476
>>>

Alan G · March 1st, 2016, 08:21 PM

Quote:

Originally Posted by Paul_Friesen

I have realized that I have python 2.7 on both computers.

Here is the interpreter sesioun:

Code:

odroid@odroid:~/Documents/tests/programming/python/test01$ python
Python 2.7.6 (default, Mar 22 2014, 23:30:12) 
[GCC 4.8.2] on linux2

Yep, that's the problem. The book is written for Python v3.
The problems you are hitting are due to Python v2.

Properties only work on "new style classes" in v2 (in v3 all
classes are "new style"). To fix it you need to explicitly
inherit from 'object' in the class definition:

class Circle2(object):
... as before

That should work.

But I strongly recommend grabbing a copy of v3 for the rest of the
book because there are a lot of subtle changes like that as you
go through it.

Regards,

Alan G.