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2.1.1. Example NeXus C programs using native HDF5 commands

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h5py example writing the simplest NeXus data file

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2.1.2. Python Examples using h5py

One way to gain a quick familiarity with NeXus is to start working with some data. For at least the first few examples in this section, we have a simple two-column set of 1-D data, collected as part of a series of alignment scans by the APS USAXS instrument during the time it was stationed at beam line 32ID. We will show how to write this data using the Python language and the h5py package [1] (using h5py calls directly rather than using the NeXus NAPI). The actual data to be written was extracted (elsewhere) from a spec [2] data file and read as a text block from a file by the Python source code. Our examples will start with the simplest case and add only mild complexity with each new case since these examples are meant for those who are unfamiliar with NeXus.

[1]h5py: http://code.google.com/p/h5py
[2]SPEC: http://certif.com/spec.html

The data shown plotted in the next figure will be written to the NeXus HDF5 file using the only two required NeXus objects NXentry and NXdata in the first example and then minor variations on this structure in the next two examples. The data model is identical to the one in the Introduction chapter except that the names will be different, as shown below:

simple data structure

data structure, (from Introduction)

our h5py example

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/entry:NXentry
    /mr_scan:NXdata
       /mr : float64[31]
       /I00 : int32[31]
Example-H5py-Plot

plot of our mr_scan

two-column data for our mr_scan

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17.92608    1037
17.92591    1318
17.92575    1704
17.92558    2857
17.92541    4516
17.92525    9998
17.92508    23819
17.92491    31662
17.92475    40458
17.92458    49087
17.92441    56514
17.92425    63499
17.92408    66802
17.92391    66863
17.92375    66599
17.92358    66206
17.92341    65747
17.92325    65250
17.92308    64129
17.92291    63044
17.92275    60796
17.92258    56795
17.92241    51550
17.92225    43710
17.92208    29315
17.92191    19782
17.92175    12992
17.92158    6622
17.92141    4198
17.92125    2248
17.92108    1321

2.1.2.1. Writing the simplest data using h5py

These two examples show how to write the simplest data (above). One example writes the data directly to the NXdata group while the other example writes the data to NXinstrument/NXdetector/data and then creates a soft link to that data in NXdata.

2.1.2.2. Complete h5py example writing and reading a NeXus data file

Writing the HDF5 file using h5py

In the main code section of BasicWriter.py, a current time stamp is written in the format of ISO 8601 (yyyy-mm-ddTHH:MM:SS). For simplicity of this code example, we use a text string for the time, rather than computing it directly from Python support library calls. It is easier this way to see the exact type of string formatting for the time. When using the Python datetime package, one way to write the time stamp is:

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timestamp = "T".join( str( datetime.datetime.now() ).split() )

The data (mr is similar to “two_theta” and I00 is similar to “counts”) is collated into two Python lists. We use the numpy package to read the file and parse the two-column format.

The new HDF5 file is opened (and created if not already existing) for writing, setting common NeXus attributes in the same command from our support library. Proper HDF5+NeXus groups are created for /entry:NXentry/mr_scan:NXdata. Since we are not using the NAPI, our support library must create and set the NX_class attribute on each group.

Note

We want to create the desired structure of /entry:NXentry/mr_scan:NXdata/.

  1. First, our support library calls f = h5py.File() to create the file and root level NeXus structure.
  2. Then, it calls nxentry = f.create_group("entry") to create the NXentry group called entry at the root level.
  3. Then, it calls nxdata = nxentry.create_group("mr_scan") to create the NXentry group called entry as a child of the NXentry group.

Next, we create a dataset called title to hold a title string that can appear on the default plot.

Next, we create datasets for mr and I00 using our support library. The data type of each, as represented in numpy, will be recognized by h5py and automatically converted to the proper HDF5 type in the file. A Python dictionary of attributes is given, specifying the engineering units and other values needed by NeXus to provide a default plot of this data. By setting signal="I00" as an attribute on the group, NeXus recognizes I00 as the default y axis for the plot. The axes="mr" attribute on the NXdata group connects the dataset to be used as the x axis.

Finally, we must remember to call f.close() or we might corrupt the file when the program quits.

BasicWriter.py: Write a NeXus HDF5 file using Python with h5py

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#!/usr/bin/env python
'''Writes a NeXus HDF5 file using h5py and numpy'''

import h5py    # HDF5 support
import numpy

print "Write a NeXus HDF5 file"
fileName = "prj_test.nexus.hdf5"
timestamp = "2010-10-18T17:17:04-0500"

# load data from two column format
data = numpy.loadtxt('input.dat').T
mr_arr = data[0]
i00_arr = numpy.asarray(data[1],'int32')

# create the HDF5 NeXus file
f = h5py.File(fileName, "w")
# point to the default data to be plotted
f.attrs['default']          = 'entry'
# give the HDF5 root some more attributes
f.attrs['file_name']        = fileName
f.attrs['file_time']        = timestamp
f.attrs['instrument']       = 'APS USAXS at 32ID-B'
f.attrs['creator']          = 'BasicWriter.py'
f.attrs['NeXus_version']    = '4.3.0'
f.attrs['HDF5_Version']     = h5py.version.hdf5_version
f.attrs['h5py_version']     = h5py.version.version

# create the NXentry group
nxentry = f.create_group('entry')
nxentry.attrs['NX_class'] = 'NXentry'
nxentry.attrs['default'] = 'mr_scan'
nxentry.create_dataset('title', data='1-D scan of I00 v. mr')

# create the NXentry group
nxdata = nxentry.create_group('mr_scan')
nxdata.attrs['NX_class'] = 'NXdata'
nxdata.attrs['signal'] = 'I00'      # Y axis of default plot
nxdata.attrs['axes'] = 'mr'         # X axis of default plot
nxdata.attrs['mr_indices'] = [0,]   # use "mr" as the first dimension of I00

# X axis data
ds = nxdata.create_dataset('mr', data=mr_arr)
ds.attrs['units'] = 'degrees'
ds.attrs['long_name'] = 'USAXS mr (degrees)'    # suggested X axis plot label

# Y axis data
ds = nxdata.create_dataset('I00', data=i00_arr)
ds.attrs['units'] = 'counts'
ds.attrs['long_name'] = 'USAXS I00 (counts)'    # suggested Y axis plot label

f.close()   # be CERTAIN to close the file

print "wrote file:", fileName

Reading the HDF5 file using h5py

The file reader, BasicReader.py, is very simple since the bulk of the work is done by h5py. Our code opens the HDF5 we wrote above, prints the HDF5 attributes from the file, reads the two datasets, and then prints them out as columns. As simple as that. Of course, real code might add some error-handling and extracting other useful stuff from the file.

Note

See that we identified each of the two datasets using HDF5 absolute path references (just using the group and dataset names). Also, while coding this example, we were reminded that HDF5 is sensitive to upper or lowercase. That is, I00 is not the same is i00.

BasicReader.py: Read a NeXus HDF5 file using Python with h5py

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#!/usr/bin/env python
'''Reads NeXus HDF5 files using h5py and prints the contents'''

import h5py    # HDF5 support

fileName = "prj_test.nexus.hdf5"
f = h5py.File(fileName,  "r")
for item in f.attrs.keys():
    print item + ":", f.attrs[item]
mr = f['/entry/mr_scan/mr']
i00 = f['/entry/mr_scan/I00']
print "%s\t%s\t%s" % ("#", "mr", "I00")
for i in range(len(mr)):
    print "%d\t%g\t%d" % (i, mr[i], i00[i])
f.close()

Output from BasicReader.py is shown next.

Output from BasicReader.py

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file_name: prj_test.nexus.hdf5
file_time: 2010-10-18T17:17:04-0500
creator: BasicWriter.py
HDF5_Version: 1.8.5
NeXus_version: 4.3.0
h5py_version: 1.2.1
instrument: APS USAXS at 32ID-B
#   mr  I00
0   17.9261 1037
1   17.9259 1318
2   17.9258 1704
3   17.9256 2857
4   17.9254 4516
5   17.9252 9998
6   17.9251 23819
7   17.9249 31662
8   17.9247 40458
9   17.9246 49087
10  17.9244 56514
11  17.9243 63499
12  17.9241 66802
13  17.9239 66863
14  17.9237 66599
15  17.9236 66206
16  17.9234 65747
17  17.9232 65250
18  17.9231 64129
19  17.9229 63044
20  17.9228 60796
21  17.9226 56795
22  17.9224 51550
23  17.9222 43710
24  17.9221 29315
25  17.9219 19782
26  17.9217 12992
27  17.9216 6622
28  17.9214 4198
29  17.9213 2248
30  17.9211 1321

Plotting the HDF5 file

Now that we are certain our file conforms to the NeXus standard, let’s plot it using the NeXpy [3] client tool. To help label the plot, we added the long_name attributes to each of our datasets. We also added metadata to the root level of our HDF5 file similar to that written by the NAPI. It seemed to be a useful addition. Compare this with plot of our mr_scan and note that the horizontal axis of this plot is mirrored from that above. This is because the data is stored in the file in descending mr order and NeXpy has plotted it that way (in order of appearance) by default.

[3]NeXpy: http://nexpy.github.io/nexpy/
fig-Example-H5py-nexpy-plot

plot of our mr_scan using NeXpy