Progress reporting

Specification scope and status

This specification describes the current implementation.

Progress reporting is implemented via the logging system. A dedicated function datalad.log.log_progress() represents the main API for progress reporting. For some standard use cases, the utilities datalad.log.with_progress() and datalad.log.with_result_progress() can simplify result reporting further.

Design and implementation

This basic idea is to use an instance of datalad’s loggers to emit log messages with particular attributes that are picked up by datalad.log.ProgressHandler (derived from logging.Handler), and are acted on differently, depending on configuration and conditions of a session (e.g., interactive terminal sessions vs. non-interactive usage in scripts). This variable behavior is implemented via the use of logging standard library log filters and handlers. Roughly speaking, datalad.log.ProgressHandler will only be used for interactive sessions. In non-interactive cases, progress log messages are inspected by datalad.log.filter_noninteractive_progress(), and are either discarded or treated like any other log message (see datalad.log.LoggerHelper.get_initialized_logger() for details on the handler and filter setup).

datalad.log.ProgressHandler inspects incoming log records for attributes with names starting with dlm_progress. It will only process such records and pass others on to the underlying original log handler otherwise.

datalad.log.ProgressHandler takes care of creating, updating and destroying any number of simultaneously running progress bars. Progress reports must identify the respective process via an arbitrary string ID. It is the caller’s responsibility to ensure that this ID is unique to the target process/activity.

Reporting progress with log_progress()

Typical progress reporting via datalad.log.log_progress() involves three types of calls.

1. Start reporting progress about a process

A typical call to start of progress reporting looks like this

log_progress(
    # the callable used to emit log messages
    lgr.info,
    # a unique identifiers of the activity progress is reported for
    identifier,
    # main message
    'Unlocking files',
    # optional unit string for a progress bar
    unit=' Files',
    # optional label to be displayed in a progress bar
    label='Unlocking',
    # maximum value for a progress bar
    total=nfiles,
)

A new progress bar will be created automatically for any report with a previously unseen activity identifier. It can be configured via the specification of a number of arguments, most notably a target total for the progress bar. See datalad.log.log_progress() for a complete overview.

Starting a progress report must be done with a dedicated call. It cannot be combined with a progress update.

2. Update progress information about a process

Any subsequent call to datalad.log.log_progress() with an activity identifier that has already been seen either updates, or finishes the progress reporting for an activity. Updates must contain an update key which either specifies a new value (if increment=False, the default) or an increment to previously known value (if increment=True):

log_progress(
    lgr.info,
    # must match the identier used to start the progress reporting
    identifier,
    # arbitrary message content, string expansion supported just like
    # regular log messages
    "Files to unlock %i", nfiles,
    # critical key for report updates
    update=1,
    # ``update`` could be an absolute value or an increment
    increment=True
)

Updating a progress report can only be done after a progress reporting was initialized (see above).

3. Report completion of a process

A progress bar will remain active until it is explicitly taken down, even if an initially declared total value may have been reached. Finishing a progress report requires a final log message with the corresponding identifiers which, like the first initializing message, does NOT contain an update key.

log_progress(
    lgr.info,
    identifier,
    # closing log message
    "Completed unlocking files",
)

Progress reporting in non-interactive sessions

datalad.log.log_progress() takes a noninteractive_level argument that can be used to specify a log level at which progress is logged when no progress bars can be used, but actual log messages are produced.

import logging

log_progress(
    lgr.info,
    identifier,
    "Completed unlocking files",
    noninteractive_level=logging.INFO
)

Each call to log_progress() can be given a different log level, in order to control the verbosity of the reporting in such a scenario. For example, it is possible to log the start or end of an activity at a higher level than intermediate updates. It is also possible to single out particular intermediate events, and report them at a higher level.

If no noninteractive_level is specified, the progress update is unconditionally logged at the level implied by the given logger callable.

Reporting progress with with_(result_)progress()

For cases were a list of items needs to be processes sequentially, and progress shall be communicated, two additional helpers could be used: the decorators datalad.log.with_progress() and datalad.log.with_result_progress(). They require a callable that takes a list (or more generally a sequence) of items to be processed as the first positional argument. They both set up and perform all necessary calls to log_progress().

The difference between these helpers is that datalad.log.with_result_progress() expects a callable to produce DataLad result records, and supports customs filters to decide which particular result records to consider for progress reporting (e.g., only records for a particular action and type).

Output non-progress information without interfering with progress bars

log_progress() can also be useful when not reporting progress, but ensuring that no other output is interfering with progress bars, and vice versa. The argument maint can be used in this case, with no particular activity identifier (it always impacts all active progress bars):

log_progress(
    lgr.info,
    None,
    'Clear progress bars',
    maint='clear',
)

This call will trigger a temporary discontinuation of any progress bar display. Progress bars can either be re-enabled all at once, by an analog message with maint='refresh', or will re-show themselves automatically when the next update is received.