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Here's a grammar that roughly describes the subset that NumPy supports.
As for implementing this in the struct module: There is a new data
description language on the horizon:
 http://datashape.readthedocs.org/en/latest/
It does not have all the low-level capabilities (e.g changing alignment
on the fly), but it is far more readable. Example:
PEP-3118: "(2,3)10f0fZdT{10B:x:(2,3)d:y:Q:z:}B"
Datashape: "2 * 3 * (10 * float32, 0 * float32, complex128, {x: 10 * uint8, y: 2 * 3 * float64, z: int64}, uint8)"
There are a lot of open questions still. Should "10f" be viewed as an
array[10] of float, i.e. equivalent to (10)f?
In the context of PEP-3118, I think so.