Migrating from PyWPS 3.x to 4.x

The basic concept of PyWPS 3.x and 4.x remains the same: You deploy PyWPS once and can have many instances with set of processes. It’s good practice to store processes in single files, although it’s not required.

Note

Unluckily, there is not automatic tool for conversion of processes nor compatibility module. If you would like to sponsor development of such module, please contact Project Steering Committee via PyWPS mailing list or members of PSC directly.

Configuration file

Configuration file format remains the same (it’s the one used by configparser module). The sections are shift a bit, so they are more alike another GeoPython project - pycsw.

See section Configuration.

Single process definition

The main principle remains the same between 3.x and 4.x branches: You have to define process class class and it’s __init__ method with inputs and outputs.

The former execute() method can now be any function and is assigned as handler attribute.

handler function get’s two arguments: request and response. In requests, all input data are stored, response will have output data assinged.

The main difference between 3.x and 4.x is, every input is list of inputs. The reason for such behaviour is, that you, as user of PyWPS define input defined by type and identifier. When PyWPS process is turned to running job, there can be usually more then one input with same identifier defined. Therefore instead of calling:

def execute(self):

        ...

        # 3.x inputs
        input = self.my_input.getValue()

you shall use first index of an input list:

def handler(request, response):

        ...

        # 4.X inputs
        input = request.inputs['my_input'][0].file

Inputs and outputs data manipulation

Btw, PyWPS Inputs do now have file, data and stream attributes. They are transparently converting one data-representation-type to another. You can read input data from file-like object using stream or get directly the data into variable with input.data. You can also save output data directly using output.data = { ..... }.

See more in Processes

Deployment

While PyWPS 3.x was usually deployed as CGI application, PyWPS 4.x is configured as WSGI application. PyWPS 4.x is distributed without any processes or sample deploy script. We provide such example in our pywps-flask project.

Note

PYWPS_PROCESSES environment variable is gone, you have to assing processes to deploy script manually (or semi-automatically).

For deployment script, standard WSGI application as used by flask microframework has to be defined, which get’s two parameters: list of processes and configuration files:

from pywps.app.Service import Service
from processes.buffer import Buffer

processes = [Buffer()]

application = Service(processes, ['wps.cfg'])

Those 4 lines of code do deploy PyWPS with Buffer process. This gives you more flexible way, how to define processes, since you can pass new variables and config values to each process class instance during it’s definition.

Sample processes

For sample processes, please refer to pywps-flask project on GITHub.

Needed steps summarization

  1. Fix configuration file
  2. Every processes needs new class and inputs and outputs definition
  3. In execute method, you just need to review inputs and outputs data assignment, but the core of the method should remain the same.
  4. Replace shell or python-based CGI script with Flask-based WSGI script