BUG: Using a length-only coord causes it to be volatile

[JasonTam]

Describe the issue:

When a variable is defined with a coord, it's considered volatile. Then sample_posterior_predictive does not transfer the node over. I believe the volatility condition that's met is https://github.com/pymc-devs/pymc/blob/9eaf92ab40ff95cf7f5cb2763377e06bb0d7be0f/pymc/sampling/forward.py#L208-L212

The coord is intended to be immutable, but after https://github.com/pymc-devs/pymc/pull/7047, coords always mutable

Somewhat related:

• https://github.com/pymc-devs/pymc/issues/7069
• https://github.com/pymc-devs/pymc/issues/6876
• https://github.com/pymc-devs/pymc/issues/7183 (lmk if this issue is considered a duplicate of anything)

Reproduceable code example:

import pymc as pm

y_obs = [0] * 3
with pm.Model() as m:
    m.add_coord("coord0", length=1)
    x = pm.Data("x", [0] * 3)
    b = pm.Normal("b", dims="coord0")  # Causes `b` to be volatile
    # b = pm.Normal("b", shape=1)      # Works as expected (`b` is transferred)
    y = pm.Normal("y", b[0] * x, observed=y_obs)
    idata = pm.sample()
    
    pm.sample_posterior_predictive(trace=idata, var_names='y')

Error message:

Sampling: [b, y]

PyMC version information:

5.15.1

Context for the issue:

Cannot use sample_posterior_predictive as intended

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[ricardoV94]

The volatility logic is supposed to check if the values of mutable data actually changed (it's not enough to be mutable) so this is quite surprising

[JasonTam]

Ah I think I understand what the issue is. The condition check for constant_coords is: https://github.com/pymc-devs/pymc/blob/9eaf92ab40ff95cf7f5cb2763377e06bb0d7be0f/pymc/sampling/forward.py#L797-L803

But if you add a coord without any values (just the length)

m.add_coord("coord0", length=1)

then the coord, will be None. We will only have dim_lengths information. https://github.com/pymc-devs/pymc/blob/9eaf92ab40ff95cf7f5cb2763377e06bb0d7be0f/pymc/model/core.py#L1030-L1031

I guess my question now: is this the intended?

[ricardoV94]

I see, we always always get tricked by coords without values (as in we tend to forget they exist all the time). In that case the volatility check should be whether the length changed or not?

What do you think? And do you want to open a PR to fix it?

[JasonTam]

For sure it threw me in for a loop. So I think it should either be considered constant if the length stays the same, or a log message should be fired.

I've opened a PR https://github.com/pymc-devs/pymc/pull/7381 and we can continue any discussion there