Working with groups

Groups are the fundamental containers in NeXus. Technically NeXus groups are nothing else than ordinary HDF5 groups (pninexus.h5cpp.node.Group) with two additional features

• in order to be NeXus compliant a the name of a groups link must adhere to the NeXus naming convention

• the group can represent a particular base class if it has a string attribute NX_class attached to it and set to the name of the desired base class.

The task of creating a NeXus compliant group or base class can be split into two subtasks

• first check if the name of the new group obeys the NeXus naming rules

• attach a NX_class attribute to the new group and set it to the name of the desired base class.

In principle one could create a group simply by using the constructor of the h5cpp.node.Group class and perform the two takes mentioned above manually. However to make life easier the nexus package provides a utility class pninexus.nexus.BaseClassFactory which provides a single static method create() which takes away the burden to write all the boiler plate code yourself.

from pninexus.nexus import BaseClassFactory

#open a file an obtain the root group

entry = nexus.BaseClassFactory.create(parent = root_group,
                                      name = "scan_00001",
                                      base_class = "NXentry")

In order to customize the group creation this static method can be provided with three additional named arguments

argument	type	description
lcpl	pninexus.h5cpp.property.LinkCreationList	link creation property list
gcpl	pninexus.h5cpp.property.GroupCreationList	group creation property list
gapl	pninexus.h5cpp.property.GroupAccessList	group access property list