The constants module contains a number of predefined tags, sources and other structures either useable as arguments in MPI functions or returned from MPI functions. It is intended to be read-only (except for DEFAULT_LOGDIR) and changing anything voids the warranty.
MPI_TAG_ANY is a special tag matching any other tag - a wildcard if you will. This is also the default tag used if you do not specify a particular tag. You can use this if you don’t want to filter the incoming message on a specific tag.
MPI_SOURCE_ANY is a special rank matching any rank - a wildcard like the any-tag. This is also the default rank used if you do not specify a particular rank. You can use this if you don’t want to filter the incoming message on sender rank.
MPI_COMM_NULL represents the empty communicator. You shouldn’t normally see this unless you tried to create a communicator with conditions resulting in it being empty.
MPI_GROUP_EMPTY represents the empty group. You shouldn’t normally see this unless you tried to create a group with conditions resulting in it being empty.
MPI_IDENT signals that two groups or communicators are identical (when compared with the requisite compare function, ie. group_compare or comm_compare). For groups, this means that having the same members in the same order. For communicators, it must be the same communicator instance (context in regular MPI).
MPI_CONGRUENT signals that two communicators have the same members in the same order, but it is not the same communicator instance (context in regular MPI).
MPI_SIMILAR signals that two groups or communicators have the same members, but not in the same order.
MPI_UNEQUAL signals that two groups or communicators are not identical, and do not have the same members.
Used for method parameters that is undefined, such as for Communicator.comm_split() or when the result of a method call is undefined in some context, such as for Group.translate_ranks().