JAX Scan: Fix bug when recurring trace matches final requested size #1826
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ricardoV94
commented
Jan 5, 2026
| if trace.shape[0] > buffer_size: | ||
| # Trace is longer than buffer, keep just the last `buffer.shape[0]` entries | ||
| partial_trace = trace[-buffer_size:] | ||
| elif trace.shape[0] == buffer_size: |
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The else branch would end up doing [-0:] slice (which selects everything ... yay Python). I suppose when I wrote this code I found no test failing, and in the cases where I could trigger it it always did the right thing, so I removed it. This is more readable anyway
ricardoV94
commented
Jan 5, 2026
| trace[-buffer_size:] if trace.shape[0] > buffer else trace | ||
| ) | ||
|
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| assert partial_trace.shape[0] == buffer_size |
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This is a case where assert does the right thing. Signals a bug to the developer that a user should never find :)
jessegrabowski
approved these changes
Jan 5, 2026
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Showed up in pymc-devs/pymc-extras#615
I don't actually know how to define such a Scan naturally, but there's no reason why it shouldn't be handled correctly.