OWSLib versus Birdy

This notebook shows a side-by-side comparison of owslib.wps.WebProcessingService and birdy.WPSClient.



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from owslib.wps import WebProcessingService
from birdy import WPSClient

url = "https://bovec.dkrz.de/ows/proxy/emu?Service=WPS&Request=GetCapabilities&Version=1.0.0"

wps = WebProcessingService(url)
cli = WPSClient(url=url)

Displaying available processes

With owslib, wps.processes is the list of processes offered by the server. With birdy, the client is like a module with functions. So you just write cli. and press Tab to display a drop-down menu of processes.



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wps.processes

Documentation about a process

With owslib, the process title and abstract can be obtained simply by looking at these attributes. For the process inputs, we need to iterate on the inputs and access their individual attributes. To facilitate this, owslib.wps provides the printInputOuput function.

With birdy, just type help(cli.hello) and the docstring will show up in your console. With the IPython console or a Jupyter Notebook, cli.hello? would do as well. The docstring follows the NumPy convention.



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from owslib.wps import printInputOutput
p = wps.describeprocess('hello')
print("Title: ", p.title)
print("Abstract: ", p.abstract)

for inpt in p.dataInputs:
    printInputOutput(inpt)



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help(cli.hello)

Launching a process and retrieving literal outputs

With owslib, processes are launched using the execute method. Inputs are an an argument to execute and defined by a list of key-value tuples. These keys are the input names, and the values are string representations. The execute method returns a WPSExecution object, which defines a number of methods and attributes, including isComplete and isSucceeded. The process outputs are stored in the processOutputs list, whose content is stored in the data attribute. Note that this data is a list of strings, so we may have to convert it to a float to use it.



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resp = wps.execute('binaryoperatorfornumbers', inputs=[('inputa', '1.0'), ('inputb', '2.0'), ('operator', 'add')])
if resp.isSucceded:
    output, = resp.processOutputs
    print(output.data)

With birdy, inputs are just typical keyword arguments, and outputs are already converted into python objects. Since some processes may have multiple outputs, processes always return a namedtuple, even in the case where there is only a single output.



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z = cli.binaryoperatorfornumbers(1, 2, operator='add').get()[0]
z



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out = cli.inout().get()
out.date

Retrieving outputs by references

For ComplexData objects, WPS servers often return a reference to the output (an http link) instead of the actual data. This is useful if that output is to serve as an input to another process, so as to avoid passing back and forth large files for nothing.

With owslib, that means that the data attribute of the output is empty, and we instead access the reference attribute. The referenced file can be written to the local disk using the writeToDisk method.

With birdy, the outputs are by default the references themselves, but it's also possible to download these references in the background and convert them into python objects. To trigger this automatic conversion, set convert_objects to True when instantating the client WPSClient(url, convert_objects=True). Ini the example below, the first output is a plain text file, and the second output is a json file. The text file is converted into a string, and the json file into a dictionary.



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resp = wps.execute('multiple_outputs', inputs=[('count', '1')])
output, ref = resp.processOutputs
print(output.reference)
print(ref.reference)
output.writeToDisk('/tmp/output.txt')



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output = cli.multiple_outputs(1).get()[0]
print(output)
# as reference
output = cli.multiple_outputs(1).get(asobj=True)[0]
print(output)