Skip to content

Components Using External Tools

It often makes sense to write a component which makes use of third-party tools in one way or another. If these tools are distributed in Python modules, the module can simply be added to the requirements in the OFRAK package's file. But for dependencies on tools which cannot be simply installed with pip, OFRAK has another mechanism for the making the dependency clear to users and easy to install.


ComponentExternalTool is a class that encapsulates the information that OFRAK tracks about an external tool which a component depends on. Each component that uses an external tool should include a ComponentExternalTool object for that tool in its external_dependencies field (empty by default, this field does not need to be provided for components which do not use an external tool). The ZipUnpacker is an example:

  1. At the top of the file, the ComponentExternalTool is declared:
UNZIP_TOOL = ComponentExternalTool(
  1. In the declaration of the component itself, the tool is listed in the component's external_dependencies:
class ZipUnpacker(Unpacker[None]):
    Unpack (decompress) a zip archive.

    targets = (ZipArchive,)
    children = (File, Folder, SpecialFileType)
    external_dependencies = (UNZIP_TOOL,)  # HERE

    async def unpack(self, resource: Resource, config=None):

And that's it! This allows OFRAK to do a couple things: 1. When a user requests a list of the non-Python OFRAK dependencies they need to install, OFRAK can see that ZipUnpacker depends on unzip and reports this:

python3 -m ofrak deps
[✓] unzip
    [ApkIdentifier, ZipUnpacker]
  1. OFRAK can catch certain errors when the component runs which obviously arise from the dependency not being found, and re-raise them in a way that makes it more clear to the user they are missing a dependency with some hints on how to install it:
ofrak.component.abstract.ComponentMissingDependencyError: Missing unzip tool needed for ZipUnpacker!
    apt installation: apt install unzip
    brew installation: brew install unzip
    See for more info and installation help.

See the ComponentExternalTool docs for a breakdown of the fields of that class.

Edge Cases

One of the functions of ComponentExternalTool is to provide a way for OFRAK to check if each dependency is installed. By default, this is done by running a command formed from fields of the ComponentExternalTool:

retcode =[self.tool, self.install_check_arg], ...)

This works for most cases (the install_check_arg provides a lot of flexibility), but does not cover certain edge cases. For example, SquashfsUnpacker requires specifically versions of unsquashfs with the -no-exit-code flag. A user may already have unsquashfs installed, but an unsuitable version, so simply checking for unsquashfs can give a false negative result when a user is checking for missing dependencies.

In such cases, ComponentExternalTool should be subclassed for that edge case dependency and its is_tool_installed method be overwritten. For the unsquashfs example:

class _UnsquashfsV45Tool(ComponentExternalTool):
    def __init__(self):
        super().__init__("unsquashfs", "", "")

    def is_tool_installed(self) -> bool:
            result =
                ["unsquashfs", "-help"],
        except FileNotFoundError:
            return False

        if 0 != result.returncode:
            return False

        if b"-no-exit" not in result.stdout:
            # Version 4.5+ has the required -no-exit option
            return False

        return True

Then the dependency on unsquashfs can be included in the unpacker as usual:

UNSQUASHFS = _UnsquashfsV45Tool()


class SquashfsUnpacker(Unpacker[None]):
    """Unpack a SquashFS filesystem."""

    targets = (SquashfsFilesystem,)
    children = (File, Folder, SpecialFileType)
    external_dependencies = (UNSQUASHFS,)

    async def unpack(self, resource: Resource, config=None):