[Python-ideas] Fwd: [RFC] draft PEP: Dedicated infix operators for matrix multiplication and matrix power

[Ronan Lamy]

I have a few remarks, in random order:

* The PEP is not just about the multiplication of matrices. The 
matrix-vector and vector-vector semantics are required in order to get 
nice expressions. This should probably be mentioned more prominently.

* The complicated semantics for higher-dimensional arrays are 
effectively incompatible with the matrix-vector and vector-vector cases: 
for instance, writing the product of a list of matrices by a list of 
vectors requires something like "mats @ vecs[..., np.newaxis]". In other 
words, A @ B is AFAICT nearly equivalent to numpy.dot(A, B), therefore 
people who cannot use numpy.dot() today will not be able to use "@" either.

*  A big part of the problem with np.matrix is that it subclasses 
np.ndarray but has a completely different __mul__(). IMHO, the problems 
with it are a good argument for the importance of the Liskov 
substitution principle, but say rather little about the viability of a 
correctly implemented matrix type.