Getting `divide by zero` warning when not storing chain in memory

General information:

• emcee version: 3.0.0
• platform: Ubutnu 18.04
• installation method (pip/conda/source/other?): conda (conda-forge)

Problem description:

Expected behavior:

I would expect emcee to be able to return the acceptance rate even when the chain is not stored in memory.

Actual behavior:

When the chain is not stored in memory (I am using sampler.sample([other args], store=False) and saving to file manually) there is a warning about a division by zero and sampler.acceptance_rate always returns NaN.

What have you tried so far?:

I tried to determine why this is happening. As far as I understood, with save=False the accepted and iteration attributes of the backend are initialized to an array of zeroes and zero respectively, but then never updated, because it only happens if backend.save_state is called. For this reason, any call to sampler.acceptance_rate will trigger a warning about the division by zero.

Minimal example:

Start by reproducing the warning with sampler.sample(..., store=False) and inspect how the backend's accepted and iteration state is updated. Trace the acceptance_rate calculation and confirm that a run without storing the chain reports a valid rate without a division-by-zero warning.