Enumeration idioms: Values from different enumerations

[Ben Finney]

Mike Meyer <mwm at mired.org> writes:
> Peter Hansen <peter at engcorp.com> writes:
>> That is, [perhaps] trying to compare enumerations that should not
>> be compared *is* an error (raising an exception) *because* the
>> whole point of enumerations is to avoid errors in such cases.
>
> Except it might not be an error. For instance, if I've got a list of
> enum objects taken from various types (say I've got one set of enums
> for days of the week, another for months of the year, and so on, and
> which I use depends on whether the user wants to select days of the
> week, months of the year, etc), it makes perfect sense to want to know
> if a specific enum value is in the list, and the obvious way to check
> it is with "my_value in enum_list". That won't work if you raise an
> exception - it takes a relatively convoluted bit of code to make this
> test.

The 'enum' package in Cheeseshop doesn't do that. Enumerations are
sequences (of unique arbitrary values), that can be iterated and tested
for membership.

What's being discussed here is what happens when comparing the *values*
from the enumeration.

> Python generally uses '==' to mean "is the same value".  To do that,
> a simple true/false return is enough. In raising an exception,
> you're making '==' carry an extra meaning (I'm not sure *what* that
> is, though).

The problem with "is the same value" as an explanation for '==' is
that it doesn't help in cases such as::

    >>> ShirtSize = Enum('small', 'medium', 'large')
    >>> AppleSize = Enum('small', 'large')

What should be the result of this comparison::

    >>> ShirtSize.small == AppleSize.small

Are they "the same value"? They're both "small" (and they both coerce
to the same string value, and in this case the same integer value).

If not, is 'False' the right way to indicate that?

Or is it an error to even try comparing them?

> Do any python builtins behave that way? How about anything in the
> python standard library?

No to both; I believe this may be a defining property of
enumerations. Am I wrong?

-- 
 \       "Man cannot be uplifted; he must be seduced into virtue."  -- |
  `\                                       Donald Robert Perry Marquis |
_o__)                                                                  |
Ben Finney