Command line interface

Specification scope and status

This incomplete specification describes the current implementation.

The command line interface (CLI) implementation is located at datalad.cli. It provides a console entry point that automatically constructs an argparse-based command line parser, which is used to make adequately parameterized calls to the targeted command implementations. It also performs error handling. The CLI automatically supports all commands, regardless of whether they are provided by the core package, or by extensions. It only requires them to be discoverable via the respective extension entry points, and to implement the standard datalad.interface.base.Interface.

Basic workflow of a command line based command execution

The functionality of the main command line entrypoint described here is implemented in datalad.cli.main.

  1. Construct an argparse parser.

    • this is happening with inspection of the actual command line arguments in order to avoid needless processing

    • when insufficient arguments or other errors are detected, the CLI will fail informatively already at this stage

  2. Detect argument completions events, and utilize the parser in a optimized fashion for this purpose.

  3. Determine the to-be-executed command from the given command line arguments.

  4. Read any configuration overrides from the command line arguments.

  5. Change the process working directory, if requested.

  6. Execute the target command in one of two modes:

    1. With a basic exception handler

    2. With an exception hook setup that enables dropping into a debugger for any exception that reaches the command line main() routine.

  7. Unless a debugger is utilized, five error categories are distinguished (in the order given below):

    1. Insufficient arguments (exit code 2)

      A command was called with inadequate or incomplete parameters.

    2. Incomplete results (exit code 1)

      While processing an error occurred.

    3. A specific internal shell command execution failed (exit code relayed from underlying command)

      The error is reported, as if the command would have been executed directly in the command line. Its output is written to the stdout, stderr streams, and the exit code of the DataLad process matches the exit code of the underlying command.

    4. Keyboard interrupt (exit code 3)

      The process was interrupted by the equivalent of a user hitting Ctrl+C.

    5. Any other error/exception.

Command parser construction by Interface inspection

The parser setup described here is implemented in datalad.cli.parser.

A dedicated sub-parser for any relevant DataLad command is constructed. For normal execution use cases, only a single subparser for the target command will be constructed for speed reasons. However, when the command line help system is requested (--help) subparsers for all commands (including extensions) are constructed. This can take a considerable amount of time that grows with the number of installed extensions.

The information necessary to configure a subparser for a DataLad command is determined by inspecting the respective Interface class for that command, and reusing individual components for the parser. This includes:

  • the class docstring

  • a _params_ member with a dict of parameter definitions

  • a _examples_ member, with a list of example definitions

All docstrings used for the parser setup will be processed by applying a set of rules to make them more suitable for the command line environment. This includes the processing of CMD markup macros, and stripping their PYTHON counter parts. Parameter constraint definition descriptions are also altered to exclude Python-specific idioms that have no relevance on the command line (e.g., the specification of None as a default).

CLI-based execution of Interface command

The execution handler described here is implemented in datalad.cli.exec.

Once the main command line entry point determine that a command shall be executed, it triggers a handler function that was assigned and parameterized with the underlying command Interface during parser construction. At the time of execution, this handler is given the result of argparse-based command line argument parsing (i.e., a Namespace instance).

From this parser result, the handler constructs positional and keyword arguments for the respective Interface.__call__() execution. It does not only process command-specific arguments, but also generic arguments, such as those for result filtering and rendering, which influence the central processing of result recorded yielded by a command.

If an underlying command returns a Python generator it is unwound to trigger the respective underlying processing. The handler performs no error handling. This is left to the main command line entry point.