Document gotchas when interoperation with python

docs/make.jl

@@ -24,6 +24,7 @@ makedocs(;
     pages=[
         "Home" => "index.md",
         "Low-level library bindings" => "api_bindings.md",
+        "Python interoperability" => "h5py.md"
     ],
 )
 

docs/src/h5py.md

@@ -0,0 +1,105 @@
+# Python interoperability
+
+When loading python created hdf5 files from Julia the dimensions of arrays are reversed.
+The reason is that in python C-memory layout is the default, while Julia uses Fortran layout.
+Here is an example:
+```python
+import h5py
+import numpy as np
+path = "created_by_h5py.h5"
+file = h5py.File(path, "w")
+arr1d = np.array([1,2,3])
+arr2d = np.array([[1,2,3], [4,5,6]])
+arr3d = np.array([[[1,2,3], [4,5,6]]])
+assert arr1d.shape == (3,)
+assert arr2d.shape == (2,3)
+assert arr3d.shape == (1,2,3)
+file["1d"] = arr1d
+file["2d"] = arr2d
+file["3d"] = arr3d
+file.close()
+```
+When we try to load it from julia, dimensions are reversed:
+```julia
+using HDF5
+path = "created_by_h5py.h5"
+h5open(path, "r") do file
+    arr1d = read(file["1d"])
+    arr2d = read(file["2d"])
+    arr3d = read(file["3d"])
+    @assert size(arr1d) == (3,)
+    @assert size(arr2d) == (3,2)
+    @assert size(arr3d) == (3,2,1)
+end
+```
+To fix this, we can simply reverse the dimensions again:
+
+```julia
+using HDF5
+function reversedims(arr)
+    dims = ntuple(identity, Val(ndims(arr)))
+    return permutedims(arr, reverse(dims))
+end
+
+path = "created_by_h5py.h5"
+h5open(path, "r") do file
+    arr1d = reversedims(read(file["1d"]))
+    arr2d = reversedims(read(file["2d"]))
+    arr3d = reversedims(read(file["3d"]))
+    @assert arr1d == [1,2,3]
+    @assert arr2d == [1 2 3; 4 5 6]
+    @assert arr3d == reshape(arr2d, (1,2,3))
+end
+```
+Similarly `reversedims` can be used before saving arrays intended for use from python.
+If copying of data is undesirable, other options are:
+* using Fortran memory layout on the python side
+* using C-memory layout on the Julia side (e.g. replace `permutedims` by `PermutedDimsArray` above)
+
+The whole example as a Julia executable script:
+```julia
+using PyCall
+py"""
+import h5py
+import numpy as np
+path = "created_by_h5py.h5"
+file = h5py.File(path, "w")
+arr1d = np.array([1,2,3])
+arr2d = np.array([[1,2,3], [4,5,6]])
+arr3d = np.array([[[1,2,3], [4,5,6]]])
+assert arr1d.shape == (3,)
+assert arr2d.shape == (2,3)
+assert arr3d.shape == (1,2,3)
+file["1d"] = arr1d
+file["2d"] = arr2d
+file["3d"] = arr3d
+file.close()
+"""
+
+using HDF5
+path = "created_by_h5py.h5"
+h5open(path, "r") do file
+    arr1d = read(file["1d"])
+    arr2d = read(file["2d"])
+    arr3d = read(file["3d"])
+    @assert size(arr1d) == (3,)
+    @assert size(arr2d) == (3,2)
+    @assert size(arr3d) == (3,2,1)
+end
+
+using HDF5
+function reversedims(arr)
+    dims = ntuple(identity, Val(ndims(arr)))
+    return permutedims(arr, reverse(dims))
+end
+
+path = "created_by_h5py.h5"
+h5open(path, "r") do file
+    arr1d = reversedims(read(file["1d"]))
+    arr2d = reversedims(read(file["2d"]))
+    arr3d = reversedims(read(file["3d"]))
+    @assert arr1d == [1,2,3]
+    @assert arr2d == [1 2 3; 4 5 6]
+    @assert arr3d == reshape(arr2d, (1,2,3))
+end
+```