[Tutor] Why do I not get an error when I mistakenly type "humdrum.sigh_strenght" instead of the correct "humdrum.sigh_strength"?

[Cameron Simpson]

On 22Jan2016 22:14, boB Stepp <robertvstepp at gmail.com> wrote:
>On Thu, Jan 21, 2016 at 4:57 AM, Steven D'Aprano <steve at pearwood.info> wrote:
>> Danny is correct. And it is a useful feature too. For instance, we can
>> add attributes to functions:
>>
>> def spam(x, y):
>>     ...
>>
>> spam.extra_info = "whatever"
>
>A new thing that I did not suspect I could do.  This bothers me for two reasons:
>
>    1)  It does not seem right adding attributes to functions outside
>of its definition.

Have you tried to do it from inside? Anyway, a function is just another object. 
What do you think it should be particularly special?

>    2)  spam.extra_info appears to be global:
>
>>>> spam.extra_info = "whatever"
>>>> z = 'WOW!'
>>>> def print_global_stuff():
>        print('z =', z)
>        print('spam.extra_info =', spam.extra_info)

No, "spam" is global. So you can name it anywhere in that module; therefore you 
can name anything inside it. Again, like any other object.

>>>> print_global_stuff()
>z = WOW!
>spam.extra_info = whatever
>
>And I imagine I am being dense about something that is quite obvious
>to you:  How is this a useful feature to have?  What does it give me
>that is more useful than just saying something like:
>
>just_another_global variable = "whatever"

spam.extra_info = "blah"
snot = spam
print(snot.extra_info)

Consider if you passed "snot" (or "spam") to a function:

  def print_extra_info(obj):
    print(obj.extra_info)

This is not something you could do with just_another_global_variable.

Consider: this is information you want assicated with a specific object.  
Therefore it really _is_ an arribute of the object so that it can follow it 
around.

>And what bothered me about my original example that started this
>thread is that when my typo
>
>humdrum.sigh_strenght = 'high'
>
>was accepted and did not generate an error, it seemed to mean to me
>that I was violating my object's data encapsulation.  It just seems to
>me that I should not be able to arbitrarily add new attributes from
>outside the class definition that created my object.  That seems
>similar to having a class Dog, to which from outside the class'
>definition, I decide to add a new Dog attribute that all dogs in this
>class can now have a tail sticking out of their noses.  I have no
>qualms about subclassing Dog with a MutantDog subclass, but adding new
>attributes from outside the class definition?  That just does not seem
>right to at this point of my understanding.  As in your function
>example, I am sure that I am missing something quite obvious, but I am
>just not seeing it now.

All you're missing is realising that setting an attribute is not a special 
operation.

To take an anecdote from elsewhere:

  UNIX was not designed to stop you from doing stupid things, because that 
  would also stop you from doing clever things.   - Doug Gwyn

There's no need to prevent you setting "sigh_strenght". There probably is 
benefit in linting tools reporting to you about attributes which are set but 
apparently never used.

Cheers,
Cameron Simpson <cs at zip.com.au>