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Loading Data in Starlark

You can load static data from within BUCK or .bzl files, as long as that static data is stored in a JSON or TOML file in a package.

load("//foo:bar.toml", "value")

some_rule(
    name = "my_rule",
    data = value,
)

This is useful for example if the data being loaded is generated outside of the build process, but is necessary for the build itself - like URLs and checksums being passed into http_archive.

The file name must end in .toml or .json, and load() can only import the name value from these files. For TOML, value is always going to be a dictionary with str keys (i.e. a TOML table), and for JSON it maps to whatever value is in the file.

If a name other than value is needed, load() allows aliasing:

load("//foo:bar.toml", my_more_specific_name = "value")

print(my_more_specific_name)

TOML types map unambiguously to JSON, and JSON types map unambiguously to Starlark like you might expect:

  • null maps to None, and true/false maps to True/False
  • numbers map to int or float
  • strings map to str
  • arrays map to list
  • objects map to dict
  • tables (in TOML) map to str-keyed dict