On Fri, Sep 27, 2019 at 09:34:05AM +0700, Sébastien Eskenaz wrote:
> I have a class `dataset` which can be built as follows: `mydata1 =
> dataset('path/to/dataset1')`
> Then I can create a second dataset `mydata2 = dataset('path/to/dataset1')`
> (no typo about `path/to/dataset1`, it is a 1).
> Thus testing `mydata1==mydata2` will return false while they actually
> contain the same information and data.
Do they? I have to believe you, because it is your class and you should
know, but we can't make that assumption in general. It depends on what
the dataset class does.
For instance, think of:
myfile1 = open('path/to/file')
# some time later...
myfile2 = open('path/to/file')
Are they the same file? No. They point to the same *pathname*, but they
are independent file objects. On Linux systems, they might not even end
up reading from the same file on disk, for example:
* path/to/file is a hard link to spam.txt
* open('path/to/file') --> spam.txt
* another process deletes path/to/file
* then creates a new file at path/to/file.
(Untested, but I'm pretty sure that will work.)
Without knowing the internal details of your mydata1 and mydata2, there
is no way that I can predict whether or not the two objects should
compare as "the same".
Which brings us to another point. What does "the same" mean? In Python,
we have two well-defined operators for sameness:
- the ``is`` operator, which tests for object identity
(the two operands are the same object)
- the ``==`` operator, which tests for value equality
(the two operands have equal value)
Javascript defines at least four different versions of "sameness"
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness
none of which are the same as either of Python's standard comparisons.
What does your compare function do? Obviously it doesn't test for object
identity, but it doesn't test for equality either since
``compare_objs(1, 1.0)`` will return False.
I've now taken the time to read the whole function
https://gist.github.com/SebastienEske/5a9c04e718becd93b7928514e80f0211
but the main thing that is missing is an explanation of *what it does*.
If I call compare_objs(spam, eggs) and it returns True, what does that
tell me about spam and eggs?
Without having a good, solid description of the *meaning* of this
comparison, it is difficult to tell whether it is useful in general or
not, or whether other people would find it helpful as well as you.
> Regarding parent class comparison, I would assume that there is a good
> reason to have made that class `MyStr` hence either some of its methods or
> attributes will end up being different from `str`.
Perhaps. But the *value* of the strings are still equal: MyStr("hello")
and built-in string "hello" have the same sequence of characters and
should be considered equal. But we've established that your comparison
isn't an equality comparison.
> So testing directly for
> the class is only a faster way to find the difference. I don't really see
> the problem with saying that objects of two different classes will always
> be different (unless the object implements its own `__eq__` function in
> which case the user can use the `==` operator). It's like comparing apples
> to oranges. They're bound to be different.
That depends on what you mean by "different".
If the subclass is designed to obey the Liskov Substitution Principle,
you ought to be able to substitute the subclass and the behaviour will
remain the same. In other words, anything you can do with the string
"hello", you should be able to do with MyStr("hello") -- they are
identical in the limited sense of the Liskov Substitution Principle.
So not so much like apples and oranges, but more like apples from one
tree, and apples from another tree. Possibly from the same variety, or
possibly not.
--
Steven
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