Import Policy Guard

Activated by repeated --deny-import NAME (CLI) or deny_imports=[...] (API). Implemented in hermetic/guards/imports_guard.py.

What it patches

Hermetic checks Python's import hook, importlib.import_module, PathFinder.find_spec, and the standard source, bytecode, and zip loaders. It blocks any import whose resolved, fully qualified name matches a denied module or package prefix.

Examples:

  • --deny-import pickle blocks pickle
  • --deny-import xml.etree blocks xml.etree and xml.etree.ElementTree

The check is dot-boundary aware, so denying pickle does not block an unrelated module whose name merely starts with the same characters.

Composition with native blocking

--deny-import composes with --block-native; both use the same import-hook layer. The generic deny-list is user-supplied, while --block-native adds Hermetic's built-in deny-lists for FFI and native extension loading.

What it does not catch

  • Captured module references obtained before hermetic installed.
  • Already-imported modules that user code still holds references to.
  • Arbitrary user-defined loaders that execute code without using Python's standard loader implementations.

Examples

hermetic --deny-import pickle --deny-import marshal -- python tool.py

from hermetic import hermetic_blocker

with hermetic_blocker(deny_imports=["pickle", "xml.etree"]):
    ...