Fill matrix with transposed version

I have a pairwise matrix: ``` >>> m a b c d a 1.0 NaN NaN NaN b 0.5 1.0 NaN NaN c 0.6 0.0 1.0 NaN d 0.5 0.4 0.3 1.0 ``` I want to replace the NaN in the the top right with the same values as in the bottom left: ``` >>> m2 a b c d a 1.0 0.5 0.6 0.5 b 0.5 1.0 0.0 0.4 c 0.6 0.0 1.0 0.3 d 0.5 0.4 0.3 1.0 ``` I can do it by swapping columns and indexes: ``` cols = m.columns idxs = m.index for c in cols: for i in idxs: m[i][c] = m[c][i] ``` But that's slow with my actual data, and I'm sure there's a way to do it in one step. I know I can generate the upper right version with "m.T" but I don't know how to replace NaN with non-NaN values to get the complete matrix. There's probably a single-step way to do this in numpy, but I don't know from matrix algebra.