how can I copy the bytes from numpy.save to stdout?-CodePudding

I have some python that needs to send a pickled numpy array to stdout. If I try to

numpy.save(sys.stdout.buffer, data, allow_pickle=False)

I get the following error

Traceback (most recent call last):
  File "/home/thoth/work/TandbergLabs/2022/iot-rendezvous/picture-server/src/main/resources/com/ericsson/duluthMadScience/pictureServer/histogram.py", line 29, in <module>
    numpy.save(sys.stdout.buffer, data, allow_pickle=False)
  File "<__array_function__ internals>", line 180, in save
  File "/usr/lib/python3.10/site-packages/numpy/lib/npyio.py", line 502, in save
    format.write_array(fid, arr, allow_pickle=allow_pickle,
  File "/usr/lib/python3.10/site-packages/numpy/lib/format.py", line 689, in write_array
    array.tofile(fp)
OSError: obtaining file position failed

What writable file-like object can I pass to numpy.save that I can later fetch bytes from?

CodePudding user response：

You can pass any file-like object if the file has been opened in binary mode it seems.

You can see this in examples such as this one from the numpy.save documentation:

import numpy as np

with open('test.npy', 'wb') as f:
    np.save(f, np.array([1, 2]))
    np.save(f, np.array([1, 3]))
with open('test.npy', 'rb') as f:
    a = np.load(f)
    b = np.load(f)
print(a, b)

Notices that both when opening and reading the file, it passes the character 'b' to second function parameter:

with open('test.npy', 'wb') as f:
....
with open('test.npy', 'rb') as f:

This signals to the open function to open the file in binary mode, you can see more details about this in the documentation

Hope that was helpful!

CodePudding user response：

you can write to io.BytesIO buffer then write it to stdout or even transfer it over network.

import numpy as np
import io
buffer = io.BytesIO()
arr = np.array([1, 2, 3])
np.save(buffer, arr, allow_pickle=False)
bytes_value = buffer.getvalue()
print(bytes_value)
sys.stdout.buffer.write(bytes_value)  # prints it in non-human readable binary