Why does numpy.savez() output non reproducible files?

Question

The function numpy.savez() allows to store numpy objects in a file. Storing the same same object in two files results in two different files:

import numpy as np
some_array = np.arange(42)
np.savez('/tmp/file1', some_array=some_array)
np.savez('/tmp/file2', some_array=some_array)

The two files differ:

$ diff /tmp/file1.npz /tmp/file2.npz 
Binary files /tmp/file1.npz and /tmp/file2.npz differ

Why aren't the files identical? Is there some random behavior, filename or time stamp included? Can this be workaround or fixed? (Is it a bug?)

Note that this is not the case for np.save(). Files produced by np.save() are identical for identical inputs. So I guess it is related to zipping of the data.

AFAICS there are only two bits different:

$ xxd /tmp/file1.npz > /tmp/file1.hex
$ xxd /tmp/file2.npz > /tmp/file2.hex
$ diff -u0 /tmp/file1.hex /tmp/file2.hex    
--- /tmp/file1.hex      2018-03-13 13:39:12.235897095 +0100
+++ /tmp/file2.hex      2018-03-13 13:39:08.743927081 +0100
@@ -1 +1 @@
-0000000: 504b 0304 1400 0000 0000 ce6c 6d4c 9c9d  PK.........lmL..
+0000000: 504b 0304 1400 0000 0000 cf6c 6d4c 9c9d  PK.........lmL..
@@ -30 +30 @@
-00001d0: 1403 1400 0000 0000 ce6c 6d4c 9c9d 6ad9  .........lmL..j.
+00001d0: 1403 1400 0000 0000 cf6c 6d4c 9c9d 6ad9  .........lmL..j.

I can't find any good hint in the implementation of the function, but I haven't checked the zipping code yet (also Python 3.6 might make a difference).

Note: Tested with Python 2.7 and numpy 1.9.2.

Answer 1

There is an github-issue about this here:

savez() is not deterministic #9439

which seems to boil down to Zip files attaching timestamps to files (as you guessed) in combination with the usage of temporary-files.

Workarounds are discussed too, but it seems this issue is still open (although Python>=3.6.0 might be unaffected now (which again seems to be observed by you)).