Preferred (or most common) file extension for a Python pickle

At times, I've seen .pickle, .pck, .pcl, and .db for files that contain Python pickles, but I am unsure what is the most common or best practice. I know that the latter three extensions are also used for other things.

The related question is: What MIME type is preferred for sending pickles between systems using a REST API?

Python 2

From the Python 2 documentation, while serializing (i.e. writing to a pickle file), use:

output = open('data.pkl', 'wb')

I would choose .pkl as the extension when using Python 2.

Python 3

The example in the Python 3 documentation now uses .pickle as the file extension for serialization:

with open('data.pickle', 'wb') as f:
    pickle.dump(...)

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The MIME type preferred for sending pickles from martineau's comment below:

> application/octet-stream

See https://stackoverflow.com/questions/13223855/what-is-the-http-content-type-to-use-for-a-blob-of-bytes