[Numpy-discussion] Memory mapping and NPZ files

Sebastian Berg sebastian at sipsolutions.net
Thu Dec 10 09:35:38 EST 2015

On Mi, 2015-12-09 at 15:51 +0100, Mathieu Dubois wrote:
> Dear all,
> If I am correct, using mmap_mode with Npz files has no effect i.e.:
> f = np.load("data.npz", mmap_mode="r")
> X = f['X']
> will load all the data in memory.

My take on it is, that no, I do not want implicit extraction/copy of the
However, npz files are not necessarily compressed, and I expect that in
the non-compressed version, memory-mapping is possible on the
uncompressed version.
If that is possible, it would ideally work for uncompressed npz files
and could raise an error which suggests to manually uncompress the file
when mmap_mode is given.

- Sebastian

> Can somebody confirm that?
> If I'm correct, the mmap_mode argument could be passed to the NpzFile
> class which could in turn perform the correct operation. One way to
> handle that would be to use the ZipFile.extract method to write the
> Npy file on disk and then load it with numpy.load with the mmap_mode
> argument. Note that the user will have to remove the file to reclaim
> disk space (I guess that's OK).
> One problem that could arise is that the extracted Npy file can be
> large (it's the purpose of using memory mapping) and therefore it may
> be useful to offer some control on where this file is extracted (for
> instance /tmp can be too small to extract the file here). numpy.load
> could offer a new option for that (passed to ZipFile.extract).
> Does it make sense?
> Thanks in advance,
> Mathieu
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