ENH: Add compresslevel input parameter in savez_compressed() function

Proposed new feature or change:

I'm using bunch of large 2D arrays (about 10000 x 10000 ) and I need to compress and save data quickly.
The problem is that the savez_compressed() function uses only default compress level of zipfile.ZipFile class.
(The default compress level of zipfile.ZIP_DEFLATED is '6')

If I could use compress level 1 for savez_compressed() function

• w/ Compress level 1: Compression ratio: 66.8%, exec. time: about 3.2 seconds 
• w/ Compress level 6: Compression ratio: 69.5%, exec. time: about 24.7 seconds
  --> The compress level 1 is the most suitable for my project.
  Therefore I want to configure the compression level in savez_compressed() function.

Here is my solution. I could solve it with adding just few lines.

my modification code snippet of _savez() function definition of the numpy/lib/npyio.py file.

def _savez(file, args, kwds, compress, allow_pickle=True, pickle_kwargs=None):
    # Import is postponed to here since zipfile depends on gzip, an optional
    # component of the so-called standard library.
    import zipfile

    if not hasattr(file, 'write'):
        file = os_fspath(file)
        if not file.endswith('.npz'):
            file = file + '.npz'

    namedict = kwds
    for i, val in enumerate(args):
        key = 'arr_%d' % i
        if key in namedict.keys():
            raise ValueError(
                "Cannot use un-named variables and keyword %s" % key)
        namedict[key] = val

    if compress:
        compression = zipfile.ZIP_DEFLATED
    else:
        compression = zipfile.ZIP_STORED

    if 'compresslevel' in namedict:
        compresslevel = namedict['compresslevel']
        if not isinstance(compresslevel, int) or compresslevel < 1 or compresslevel > 9:
            compresslevel = None
    else:
        compresslevel = None

    zipf = zipfile_factory(file, mode="w", compression=compression, compresslevel=compresslevel)

    for key, val in namedict.items():
        fname = key + '.npy'
        val = np.asanyarray(val)
        # always force zip64, gh-10776
        with zipf.open(fname, 'w', force_zip64=True) as fid:
            format.write_array(fid, val,
                               allow_pickle=allow_pickle,
                               pickle_kwargs=pickle_kwargs)

    zipf.close()

It works!!

I haven't understand the github process from my idea to the merge in this kind of big open source repository cause I have no experience of github upstream activity yet. So... just let me share my idea through an issue.

thanks.

Start with numpy/lib/npyio.py, especially the _savez() implementation and the savez_compressed() entry point, then inspect how zipfile_factory receives compression options. Compare the existing behavior with the requested configurable compression level and determine where the public API should expose it. Done means callers can select a compression level without changing the saved array contents.