TypeError: cannot pickle 'starry._c_ops.Ops' object

The eclipsing binary example notebook with MCMC is failing at the NUTS sampling step. It has something to do with the new version of PyMC3 (3.9) because it works fine with PyMC3 3.7.

The following cell fails with the error TypeError: cannot pickle 'starry._c_ops.Ops' object:

with model:
    trace = pm.sample(
        tune=500,
        draws=500,
        start=map_soln,
        chains=4,
        step=xo.get_dense_nuts_step(target_accept=0.9),
    )

It works if you set chains=1.

Starry version: 1.0.1.dev161+g9c462a2c exoplanet version: 0.3.3 PyMC3 version: 3.9.3 macOS Catalina install from source

[rodluger]

Looking at @bmorris3’s comment, the workaround is

import multiprocessing as mp

pm.sample(
    ..., 
    mp_ctx=mp.get_context("fork")
)

[dfm]

It’s all the same issue and it’s really a Python issue, not pymc3 really. I’ve found that using fork instead of forkserver works fine in general.