In numpy, why is it ok to do matrix.mean(), but not ok to do matrix.median()?

[C W]

It's interesting how mean() can be implemented, but median() will break
other packages.

So, the default way in numpy is to use functions, not methods?

When I first learned Python, I was told to create an object and to play
around, there are methods for that object. List has list methods, tuple has
tuple methods, etc.

Now, the default way in numpy is to use function instead of methods? I'm
confused. What happened to object-oriented programming?

Thanks,

-M




On Tue, May 1, 2018 at 3:38 PM, Thomas Jollans <tjol at tjol.eu> wrote:

> On 01/05/18 19:57, C W wrote:
> > matrix.median()        # throws error message
>
> READ error messages. At the very least, quote error messages when asking
> questions somewhere like here. There I was, wondering why the numpy docs
> didn't mention ndarray.median when you were clearly using it...
>
> Anyway,
>
> > Hello everyone,
> >
> > In numpy, why is it ok to do matrix.mean(), but not ok to do
> > matrix.median()? To me, they are two of many summary statistics. So, why
> > median() is different?
>
> First, this is how it's different: the method ndarray.median simply does
> not exist.
>
> Now, of course, most numpy functions don't have method versions, so
> there's nothing special about median here. However, as you quite rightly
> point out, median would be "a good fit".
>
> As with most things that are "a bit odd" about numpy this probably boils
> to "historical reasons".
>
> numpy mostly goes back to an earlier package called "Numeric". Numeric's
> array did not have methods like mean(). Early (anno 2005) numpy also
> incorporated features from a package called "numarray"; one of these
> features were array methods like ".mean". numarray did not have a
> .median method, though it *did* have a function numarray.mlab.median. So
> far, so good. ndarray.mean *must* exist for compatibility reasons,
> ndarray.median need not.
>
> So why not add it later? Other methods have been added, after all. Well,
> for starters, "who cares?". Most of numpy is functions; the methods are
> nice, but not that important. The other obstacle is that numpy.median is
> implemented in Python, not in C. For historical reasons™, it has to stay
> that way. They tried to change it in 2014, but that broke some other
> packages...
>
>
> >
> > Here's an example code,
> >
> > import numpy as np
> > matrix = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
> >
> > # find the mean
> > np.mean(matrix)      # ok
> > matrix.mean()          # ok
> >
> > # find the median
> > np.median(matrixA)  # ok
>
> >
> >
> > Also, why have two of the same thing: np.mean(matrix) and matrix.mean()?
> > When to use which one?
> >
> > The documentation below looks almost identical! What am I missing here?
> > [1] Median documentation:
> > https://docs.scipy.org/doc/numpy-1.14.0/reference/
> generated/numpy.median.html
> > [2] Mean documentation:
> > https://docs.scipy.org/doc/numpy-1.14.0/reference/
> generated/numpy.mean.html
> >
> >
> > Thank you so much,
> >
> > M
> >
>
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