Step I/O Design

API Summary

Step command-line options

  • --output_dir: Directory where all output will go.

  • --output_file: File name upon which output files will be based.

Step configuration options

  • output_dir: Directory where all output will go.

  • output_file: File name upon which output files will be based.

  • suffix: Suffix defining the output of this step.

  • save_results: True to create output files. [more]

  • search_output_file: True to retrieve the output_file from a parent Step or Pipeline. [more]

  • output_use_model: True to always base output file names on the DataModel.meta.filename of the DataModel being saved.

  • input_dir: Generally defined by the location of the primary input file unless otherwise specified.

Classes, Methods, Functions

  • Step.open_model: Open a DataModel

  • Step.load_as_level2_asn(): Open a list or file as Level2 association.

  • Step.load_as_level3_asn(): Open a list or file as Level3 association.

  • Step.make_input_path: Create a file name to be used as input

  • Step.save_model: Save a DataModel immediately

  • Step.make_output_path: Create a file name to be used as output


The Step architecture is designed such that a Step’s intended sole responsibility is to perform the calculation required. Any input/output operations are handled by the surrounding Step architecture. This is to help facilitate the use of Step’s from both a command-line environment, and from an interactive Python environment, such as Jupyter notebooks or ipython.

For command-line usage, all inputs and outputs are designed to come from and save to files.

For interactive Python use, inputs and outputs are expected to be Python objects, negating the need to save and reload data after every Step call. This allows users to write Python scripts without having to worry about doing I/O at every step, unless, of course, if the user wants to do so.

The high-level overview of the input/output design is given in Writing a step. The following discusses the I/O API and best practices.

To facilitate this design, a basic Step is suggested to have the following structure:

class MyStep(jwst.stpipe.step.Step):

    spec = ''  # Desired configuration parameters

    def process(self, input):

        with as input_model:

            # Do awesome processing with final result
            # in `result`
            result = final_calculation(input_model)

        return (result)

When run from the command line:

strun MyStep input_data.fits

the result will be saved in a file called:


Similarly, the same code can be used in a Python script or interactive environment as follows:

>>> import jwst
>>> input ='input_data.fits')
>>> result =
    # `result` contains the resulting data
    # which can then be used by further `Steps`'s or
    # other functions.
    # when done, the data can be saved with the ``
    # method

Input and JWST Conventions

A Step gets its input from two sources:

  • Configuration parameters

  • Arguments to the Step.process method

The definition and use of parameters is documented in Writing a step.

When using the Step.process arguments, the code must at least expect strings. When invoked from the command line using strun, how many arguments to expect are the same number of arguments defined by Step.process. Similarly, the arguments themselves are passed to Step.process as strings.

However, to facilitate code development and interactive usage, code is expected to accept other object types as well.

A Step’s primary argument is expected to be either a string containing the file path to a data file, or a JWST DataModel object. The method open_model() handles either type of input, returning a DataModel from the specified file or a shallow copy of the DataModel that was originally passed to it. A typical pattern for handling input arguments is:

class MyStep(jwst.stpipe.step.Step):

    def process(self, input_argument):

        input_model = self.open_model(input_argument)


input_argument can either be a string containing a path to a data file, such as FITS file, or a DataModel directly.

open_model() handles Step-specific issues, such ensuring consistency of input directory handling.

If some other file type is to be opened, the lower level method make_input_path() can be used to specify the input directory location.

Input and Associations

Many of the JWST calibration steps and pipelines expect an Association file as input. When opened with open_model(), a ModelContainer is returned. ModelContainer is, among other features, a list-like object where each element is the DataModel of each member of the association. The meta.asn_table is populated with the association data structure, allowing direct access to the association itself.

To read in a list of files, or an association file, as an association, use the load_as_level2_asn or load_as_level3_asn methods.

Input Source

In general, all input, except for references files provided by CRDS, are expected to be co-resident in the same directory. That directory is determined by the directory in which the primary input file resides. For programmatic use, this directory is available in the Step.input_dir attribute.


When Files are Created

Whether a Step produces an output file or not is ultimately determined by the built-in parameter option save_results. If True, output files will be created. save_results is set under a number of conditions:

  • Explicitly through a parameter file or as a command-line option.

  • Implicitly when a step is called by strun.

Output File Naming

File names are constructed based on three components: basename, suffix, and extension:


The extension will often be the same as the primary input file. This will not be the case if the data format of the output needs to be something different, such as a text table with ecsv extension.

Similarly, the basename will usually be derived from the primary input file. However, there are some caveats discussed below.

Ultimately, the suffix is what Step’s use to identify their output. The most common suffixes are listed in the Pipeline/Step Suffix Definitions.

A Step’s suffix is defined in a couple of different ways:

  • By the attribute. This is the default.

  • By the suffix parameter.

  • Explicitly in the code. Often this is done in Pipelines where a single pipeline creates numerous different output files.

Basename Determination

Most often, the output file basename is determined through any of the following, given from higher precedence to lower:

  • The --output_file command-line option.

  • The output_file parameter option.

  • Primary input file name.

  • If the output is a DataModel, from the DataModel.meta.filename.

In all cases, if the originating file name has a known suffix on it, that suffix is removed and replaced by the Step’s own suffix.

In very rare cases, when there is no other source for the basename, a basename of step_<step_name> is used. This can happen when a Step is being programmatically used and only the save_results parameter option is given.

Sub-Steps and Output

Normally, the value of a parameter option is completely local to the Step: A Step, called from another Step or Pipeline, can only access its own parameters. Hence, options such as save_results do not affect a called Step.

The exceptions to this are the parameters output_file and output_dir. If either of these parameters are queried by a Step, but are not defined for that Step, values will be retrieved up through the parent. The reason is to provide consistency in output from Step and Pipelines. All file names will have the same basename and will all appear in the same directory.

As expected, if either parameter is specified for the Step in question, the local value will override the parent value.

Also, for output_file, there is another option, search_output_file, that can also control this behavior. If set to False, a Step will never query its parent for its value.

Basenames, Associations, and Stage 3 Pipelines

Stage 3 pipelines, such as calwebb_image3 or calwebb_spec3, take associations as their primary input. In general, the association defines what the output basename should be. A typical pattern used to handle associations is:

class MyStep(jwst.stpipe.step.Step):

    spec = ''  # Desired configuration parameters

    def process(self, input):

        with as input_model:

            # If not already specified, retrieve the output
            # file name from the association.
            if self.save_results and self.output_file is None:
                   self.output_file = input_model.meta.asn_table.products[0].name

                except AttributeError:

            # Do awesome processing with final result
            # in `result`
            result = final_calculation(input_model)

        return (result)

Some pipelines, such as calwebb_spec3, call steps which are supposed to save their results, but whose basenames should not be based on the association product name. An example is the OutlierDetectionStep step. For such steps, one can prevent using the Pipeline.output_file specification by setting the parameter search_output_file=False. When such steps then save their output, they will go through the standard basename search. If nothing else is specified, the basename will be based on DataModel.meta.filename that step’s result, creating appropriate names for that step.

Output API: When More Control Is Needed

In summary, the standard output API, as described so far, is basically “set a few parameters, and let the Step framework handle the rest”. However, there are always the exceptions that require finer control, such as saving intermediate files or multiple files of different formats. This section discusses the method API and conventions to use in these situations.

Save That Model: Step.save_model

If a Step needs to save a DataModel before the step completes, use of Step.save_model is the recommended over directly calling Step.save_model uses the Step framework and hence will honor the following:

  • If Step.save_results is False, nothing will happen.

  • Will ensure that Step.output_dir is used.

  • Will use Step.suffix if not otherwise specified.

  • Will determine the output basename through the Step framework, if not otherwise specified.

The basic usage, in which nothing is overridden, is:

class MyStep(Step):

    def process(self, input):
        result = some_DataModel

The most common use case, however, is for saving some intermediate results that would have a different suffix:

self.save_model(intermediate_result_datamodel, suffix='intermediate')

See jwst.stpipe.step.Step.save_model() for further information.

Make That Filename: Step.make_output_path

For the situations when a filename is needed to be constructed before saving, either to know what the filename will be or for data that is not a DataModel, use Step.make_output_path. By default, calling make_output_path without any arguments will return what the default output file name will be:

output_path = self.make_output_path()

This method encapsulates the following Step framework functions:

  • Will ensure that Step.output_dir is used.

  • Will use Step.suffix if not otherwise specified.

  • Will determine the output basename through the Step framework, if not otherwise specified.

A typical use case is when a Step needs to save data that is not a DataModel. The current Step architecture does not know how to handle these, so saving needs to be done explicitly. The pattern of usage would be:

# A table need be saved and needs a different
# suffix than what the Step defines.
table = some_astropy_table_data
if self.save_results:
    table_path = self.make_output_path(suffix='cat', ext='ecsv'), format='ascii.ecsv', overwrite=True)