Databases Reference
In-Depth Information
Visiting Items
Given the name supplied to your callback, you could retrieve the object by simply using
getitem
on the group you're iterating over:
>>>
def
printobj
(
name
):
...
print
grp
[
name
]
But that's a pain; since the
name
argument supplied by
visit
is a
relative
path, your
function has to know in advance what group it'll be applied to. The previous example
will work properly only when applied to
grp
.
Thankfully, HDF5 provides a more general way to deal with this. The method
visit
items
supplies both the relative name and an instance of each object:
>>>
def
printobj2
(
name
,
obj
):
...
print
name
,
obj
>>>
grp
.
visititems
(
printobj2
)
hardlink_to_dataset <HDF5 dataset "hardlink_to_dataset": shape (), type "<f8">
subgroup_1 <HDF5 group "/top_group_1/subgroup_1" (1 members)>
Since each object has to be opened, there is some overhead involved. You're better off
using
visititems
only in the case where you really need access to each object; for
example, if you need to inspect attributes.
One way to make
visit
a little more generic is by using the built-in Python widget
functools.partial
. For example, here's a trivial function that prints the absolute path
of each object in the group:
>>>
import
posixpath
>>>
from
functools
import
partial
>>>
def
print_abspath
(
somegroup
,
name
):
...
""" Print *name* as an absolute path
...
somegroup: HDF5 base group (*name* is relative to this)
...
name: Object name relative to *somegroup*
...
"""
...
print
posixpath
.
join
(
somegroup
.
name
,
name
)
>>>
grp
.
visit
(
partial
(
print_abspath
,
grp
))
/top_group_1/hardlink_to_dataset
/top_group_1/subgroup_1
Using this technique, you can avoid “embedding” the group you intend to iterate over
in the function itself.
Canceling Iteration: A Simple Search Mechanism
There's a simple way to “bail out” when visiting items. You might notice that our
print
name
function has no explicit return value; in Python that means that the function re‐
