Writing a step

Writing a new step involves writing a class that has a process method to perform work and a spec member to define its configuration parameters. (Optionally, the spec member may be defined in a separate spec file).

Inputs and outputs

A Step provides a full framework for handling I/O. Below is a short description. A more detailed discussion can be found in Step I/O Design.

Steps get their inputs from two sources:

  • Configuration parameters come from the parameter file or commandline and are set as member variables on the Step object by the stpipe framework.

  • Arguments are passed to the Step’s process function as regular function arguments.

Parameters should be used to specify things that must be determined outside of the code by a user using the class. Arguments should be used to pass data that needs to go from one step to another as part of a larger pipeline. Another way to think about this is: if the user would want to examine or change the value, use a parameter.

The parameters are defined by the Step.spec member.

Input Files, Associations, and Directories

It is presumed that all input files are co-resident in the same directory. This directory is whichever directory the first input file is found in. This is particularly important for associations. It is assumed that all files referenced by an association are in the same directory as the association file itself.

Output Files and Directories

The step will generally return its output as a data model. Every step has implicitly created parameters output_dir and output_file which the user can use to specify the directory and file to save this model to. Since the stpipe architecture generally creates output file names, in general, it is expected that output_file be rarely specified, and that different sets of outputs be separated using output_dir.

Output Suffix

There are three ways a step’s results can be written to a file:

  1. Implicitly when a step is run from the command line or with Step.from_cmdline

  2. Explicity by specifying the parameter save_results

  3. Explicity by specifying a file name with the paremeter output_file

In all cases, the file, or files, is/are created with an added suffix at the end of the base file name. By default this suffix is the class name of the step that produced the results. Use the suffix parameter to explicitly change the suffix.

For pipelines, this can be done either in a parameter file, or within the code itself. See calwebb_dark for an example of specifying in the parameter file.

For an example where the suffix can only be determined at runtime, see calwebb_detector1. For an example of a pipeline that returns many results, see calwebb_spec2.

The Python class

At a minimum, the Python Step class should inherit from stpipe.Step, implement a process method to do the actual work of the step and have a spec member to describe its parameters.

  1. Objects from other Steps in a pipeline are passed as arguments to the process method.

  2. The parameters described in Configuring a Step are available as member variables on self.

  3. To support the caching suspend/resume feature of pipelines, images must be passed between steps as model objects. To ensure you’re always getting a model object, call the model constructor on the parameter passed in. It is good idea to use a with statement here to ensure that if the input is a file path that the file will be appropriately closed.

  4. Use get_reference_file_model method to load any CRDS reference files used by the Step. This will make a cached network request to CRDS. If the user of the step has specified an override for the reference file in either the parameter file or at the command line, the override file will be used instead. (See Interfacing with CRDS).

  5. Objects to pass to other Steps in the pipeline are simply returned from the function. To return multiple objects, return a tuple.

  6. The parameters for the step are described in the spec member in the configspec format.

  7. Declare any CRDS reference files used by the Step. (See Interfacing with CRDS).

from jwst.stpipe import Step

from jwst.datamodels import ImageModel
from my_awesome_astronomy_library import combine

class ExampleStep(Step):
    Every step should include a docstring.  This docstring will be
    displayed by the `strun --help`.

    # 1.
    def process(self, image1, image2):"Inside ExampleStep")

        # 2.
        threshold = self.threshold

        # 3.
        with ImageModel(image1) as image1, ImageModel(image2) as image2:
            # 4.
            with self.get_reference_file_model(image1, "flat_field") as flat:
                new_image = combine(image1, image2, flat, threshold)

        # 5.
        return new_image

   # 6.
   spec = """
   # This is the configspec file for ExampleStep

   threshold = float(default=1.0)  # maximum flux

   # 7.
   reference_file_types = ['flat_field']

The Python Step subclass may be installed anywhere that your Python installation can find it. It does not need to be installed in the stpipe package.

The spec member

The spec member variable is a string containing information about the parameters. It is in the configspec format defined in the ConfigObj library that stpipe uses.

The configspec format defines the types of the parameters, as well as allowing an optional tree structure.

The types of parameters are declared like this:

n_iterations = integer(1, 100)  # The number of iterations to run
factor = float()                # A multiplication factor
author = string()               # The author of the file

Note that each parameter may have a comment. This comment is extracted and displayed in help messages and docstrings etc.

Parameters can be grouped into categories using ini-file-like syntax:

offset = float()
scale = float()

offset = float()
scale = float()

offset = float()
scale = float()

Default values may be specified on any parameter using the default keyword argument:

name = string(default="John Doe")

While the most commonly useful parts of the configspec format are discussed here, greater detail can be found in the configspec documentation.

Configspec types

The following is a list of the commonly useful configspec types.

integer: matches integer values. Takes optional min and max arguments:

integer(3, 9)  # any value from 3 to 9
integer(min=0) # any positive value

float: matches float values Has the same parameters as the integer check.

boolean: matches boolean values: True or False.

string: matches any string. Takes optional keyword args min and max to specify min and max length of string.

list: matches any list. Takes optional keyword args min, and max to specify min and max sizes of the list. The list checks always return a list.

force_list: matches any list, but if a single value is passed in will coerce it into a list containing that value.

int_list: Matches a list of integers. Takes the same arguments as list.

float_list: Matches a list of floats. Takes the same arguments as list.

bool_list: Matches a list of boolean values. Takes the same arguments as list.

string_list: Matches a list of strings. Takes the same arguments as list.

option: matches any from a list of options. You specify this test with:

option('option 1', 'option 2', 'option 3')

Normally, steps will receive input files as parameters and return output files from their process methods. However, in cases where paths to files should be specified in the parameter file, there are some extra parameter types that stpipe provides that aren’t part of the core configobj library.

input_file: Specifies an input file. Relative paths are resolved against the location of the parameter file. The file must also exist.

output_file: Specifies an output file. Identical to input_file, except the file doesn’t have to already exist.

Interfacing with CRDS

If a Step uses CRDS to retrieve reference files, there are two things to do:

  1. Within the process method, call self.get_reference_file or self.get_reference_file_model to get a reference file from CRDS. These methods take as input a) a model for the input file, whose metadata is used to do a CRDS bestref lookup, and b) a reference file type, which is just a string to identify the kind of reference file.

  2. Declare the reference file types used by the Step in the reference_file_types member. This information is used by the stpipe framework for two purposes: a) to pre-cache the reference files needed by a Pipeline before any of the pipeline processing actually runs, and b) to add override parameters to the Step’s configspec.

For each reference file type that the Step declares, an override_* parameter is added to the Step’s configspec. For example, if a step declares the following:

reference_file_types = ['flat_field']

then the user can override the flat field reference file using the parameter file:

override_flat_field = /path/to/my_reference_file.fits

or at the command line:


Making a simple commandline script for a step

Any step can be run from the commandline using Running a Step from the commandline. However, to make a step even easier to run from the commandline, a custom script can be created. stpipe provides a function stpipe.cmdline.step_script to make those scripts easier to write.

For example, to make a script for the step mypackage.ExampleStep:

# ExampleStep

# Import the custom step
from mypackage import ExampleStep

# Import stpipe.cmdline
from jwst.stpipe import cmdline

if __name__ == '__main__':
    # Pass the step class to cmdline.step_script

Running this script is similar to invoking the step with Running a Step from the commandline, with one difference. Since the Step class is known (it is hard-coded in the script), it does not need to be specified on the commandline. To specify a config file on the commandline, use the --config-file option.

For example:

> ExampleStep

> ExampleStep --config-file=example_step.asdf

> ExampleStep --parameter1=42.0 input_file.fits