Auxiliary Data Download

Sometimes Satpy components need some extra data files to get their work done properly. These include files like Look Up Tables (LUTs), coefficients, or Earth model data (ex. elevations). This includes any file that would be too large to be included in the Satpy python package; anything bigger than a small text file. To help with this, Satpy includes utilities for downloading and caching these files only when your component is used. This saves the user from wasting time and disk space downloading files they may never use. This functionality is made possible thanks to the Pooch library.

Downloaded files are stored in the directory configured by Data Directory.

Adding download functionality

The utility functions for data downloading include a two step process:

  1. Registering: Tell Satpy what files might need to be downloaded and used later.

  2. Retrieving: Ask Satpy to download and store the files locally.

Registering

Registering a file for downloading tells Satpy the remote URL for the file, and an optional hash. The hash is used to verify a successful download. Registering can also include a filename to tell Satpy what to name the file when it is downloaded. If not provided it will be determined from the URL. Once registered, Satpy can be told to retrieve the file (see below) by using a “cache key”. Cache keys follow the general scheme of <component_type>/<filename> (ex. readers/README.rst).

Satpy includes a low-level function and a high-level Mixin class for registering files. The higher level class is recommended for any Satpy component like readers, writers, and compositors. The lower-level register_file() function can be used for any other use case.

The DataMixIn class is automatically included in the FileYAMLReader and Writer base classes. For any other component (like a compositor) you should include it as another parent class:

from satpy.aux_download import DataDownloadMixin
from satpy.composites import GenericCompositor

class MyCompositor(GenericCompositor, DataDownloadMixin):
    """Compositor that uses downloaded files."""

    def __init__(self, name, url=None, known_hash=None, **kwargs):
        super().__init__(name, **kwargs)
        data_files = [{'url': url, 'known_hash': known_hash}]
        self.register_data_files(data_files)

However your code registers files, to be consistent it must do it during initialization so that the find_registerable_files(). If your component isn’t a reader, writer, or compositor then this function will need to be updated to find and load your registered files. See Offline Downloads below for more information.

As mentioned, the mixin class is included in the base reader and writer class. To register files in these cases, include a data_files section in your YAML configuration file. For readers this would go under the reader section and for writers the writer section. This parameter is a list of dictionaries including a url, known_hash, and optional filename. For example:

reader:
    name: abi_l1b
    short_name: ABI L1b
    long_name: GOES-R ABI Level 1b
    ... other metadata ...
    data_files:
      - url: "https://example.com/my_data_file.dat"
      - url: "https://raw.githubusercontent.com/pytroll/satpy/master/README.rst"
        known_hash: "sha256:5891286b63e7745de08c4b0ac204ad44cfdb9ab770309debaba90308305fa759"
      - url: "https://raw.githubusercontent.com/pytroll/satpy/master/RELEASING.md"
        filename: "satpy_releasing.md"
        known_hash: null

See the DataDownloadMixin for more information.

Retrieving

Files that have been registered (see above) can be retrieved by calling the retrieve() function. This function expects a single argument: the cache key. Cache keys are returned by registering functions, but can also be pre-determined by following the scheme <component_type>/<filename> (ex. readers/README.rst). Retrieving a file will download it to local disk if needed and then return the local pathname. Data is stored locally in the Data Directory. It is up to the caller to then open the file.

Offline Downloads

To assist with operational environments, Satpy includes a retrieve_all() function that will try to find all files that Satpy components may need to download in the future and download them to the current directory specified by Data Directory. This function allows you to specify a list of readers, writers, or composite_sensors to limit what components are checked for files to download.

The retrieve_all function is also available through a command line script called satpy_retrieve_all_aux_data. Run the following for usage information.

satpy_retrieve_all_aux_data --help

To make sure that no additional files are downloaded when running Satpy see Download Auxiliary Data.