ntLog — a NestedText logfile aggregation utility
================================================

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:Author: Ken Kundert
:Version: 0.4
:Released: 2024-01-29


Command-Line Application
------------------------

*ntLog* is a simple command line utility used to append discretely generated log 
files into a running log formulated as a NestedText_ file.  You can specify 
limits on how many log entries there are or how old they should be.  Any entries 
that do not satisfy the limits are purged.

A discrete log file is a log file generated by program that runs at regular 
intervals or after particular events.  This contrasts from log files generated 
by continuously running programs such as daemons.  With discretely running 
programs a existing log file will be overwritten on the next run.  By using 
*ntLog* you can add the most recently generated log file to a running log that 
will not be overwritten.

::

    Usage:

        ntlog [options] <logfile>

    Options:

        -k, --keep-for [days]     drop entries older than this [default: 7]
        -n, --max-entries [N]     maximum number of log entries to keep
        -N, --min-entries [N]     minimum number of log entries to keep [default: 1]
        -d, --delete              delete given logfile after incorporating it
        -h, --help                print this usage message
        -Y, --year <fmt>          add year headers
        -M, --month <fmt>         add month headers
        -D, --day <fmt>           add day headers
        -H, --hour <fmt>          add hour headers
        -E, --entry <fmt>         add entry headers
        -d, --description <text>  add entry headers
        --fold-marker <mapping>   map fold markers contained in logfile

When run, *ntLog* copies the contents of ``<logfile>`` into ``<logfile>.nt``.

Log entries older than ``--keep-for`` days are deleted.  If units are not 
specified, ``--keep-for`` is given in days.  However you can directly specify 
the units in terms of seconds (s, sec, second, seconds), minutes (m, min, 
minute, minutes), hours (h, hr, hour, hours), days (d, day, days), weeks (w, W, 
week, weeks), months (M, month, months), and years (y, Y, year, years).

If the number of entries exceeds ``--max-entries``, the oldest entries are 
deleted even if they are younger than the ``--keep-for`` limit.

If ``--delete`` is specified, the given log file is deleted after its contents 
are incorporated into the running log file.

The key used when filing log entries into the *NestedText* document is the 
timestamp of the modification time of the given log file.

The given log file is always kept, even if it is older than the ``--keep-for`` 
limit.

Log entries are sorted from most recent to oldest, with the most recent at the 
top of the *NestedText* file.  The one exception to this rule is that the given 
log file is always listed first, even if its modification time is older than 
existing log entries.


Headers
"""""""

Any headers you specify are added as comments above the appropriate entries.
When a year header is specified, it is added before the first entry found from 
new year.  The same is true for month, day, and hour headers.  The entry header 
it given before each entry.  An entry is treated as an Arrow_ format.  It 
consists of tokens that are to be replaced by components from the entry date 
stamp.  For example, if you specify::

    --entry 'D MMMM YYYY, h:mm A'

Then you will get a entry header command that looks like this::

    # 31 December 2023, 6:00 PM
    2023-12-31T18:00:25.680009-08:00:
        > ...

If you specify a description, it will be prepended to the datestamp in the key 
for the new log entry and will be prepended to the entry header.  For example, 
if ``--description create`` were specified, the result might look like::

    # create — 31 December 2023, 6:00 PM
    create — 2023-12-31T18:00:25.680009-08:00:
        > ...


It is attractive to include editor fold markers in the headers.  In doing so you 
can collapse large log entries into a single line folds until they are needed, 
at which point you can easily open the fold and examine the contents of the log 
file.  Here is an example set of header specifications that include Vim fold 
markers::

    --year 'YYYY  {{{1'
    --month 'MMMM YYYY  {{{2'
    --day 'D MMMM YYYY  {{{3'
    --entry 'D MMMM YYYY, h:mm A  {{{4'

If your logfiles tend to include fold markers, they can confuse the folds.  You 
can prevent this by mapping the marker into a different string.   Use 
``--fold-marker``::

    --fold-marker '{{{ {<{'


NTlog API
---------

*ntLog* provides the *NTlog* class, whose instances can be used as an output 
file stream in Python programs.  Instead of writing to stand-alone files their 
output is incorporated directly into a NestedText composite logfile.

*NTlog* provides the normal methods for output file streams, *write*, *flush* 
and *close*, and can act as a context manager.  Only *write* takes an argument, 
the text to be written to the logfile.

*NTlog* Arguments:
    running_log_file: (str, os.PathLike):
        The path to the composite log file.  Normally this uses .log.nt as
        the suffix.  If not given, then the name of the temp_log_file with
        an added .nt suffix is used.
    temp_log_file: (str, os.PathLike):
        The path to the temporary log file.  Normally this uses .log as the 
        suffix.  This is optional.
    keep_for (float, str):
        Any entries older than keep_for (in seconds) are dropped.
        If keep_for is a string, it is converted to seconds.  In this case
        it assumed to be a number followed by a unit.  For example, '1w',
        '6M', etc.
    max_entries (int):
        Maximum number of log entries to keep.
    min_entries (int):
        Minimum number of log entries to keep.
    retain_temp (bool):
        Do not delete the temporary log file after writing composite log
        file.
    ctime (datetime):
        Used as the creation time of the log entry.
        If not specified, the current time is used.
    year_header (str):
        When specified, this header is added above the first entry from a new year.
    month_header (str):
        When specified, this header is added above the first entry from a new month.
    day_header (str):
        When specified, this header is added above the first entry from a new day.
    hour_header (str):
        When specified, this header is added above the first entry from a new hour.
    entry_header (str):
        When specified, this header is added above every entry.
    description (str):
        This string is prepended to the datestamp to create the key for the new 
        log entry.  It is also prepended to the entry header, if it is 
        requested.
    fold_marker_mapping ([str, str]):
        When specified, any instances of the first string in a log file are
        replaced by the second string when incorporating that log into the
        output NestedText file.

Raises:
    OSError, NTlogError

    *NTlogError* is a clone of the Error_ exception from Inform_.

The use of the *temp_log_file* is optional.  It is helpful with long running 
processes as it provides a way of monitoring the progress of the process, 
especially if the logfile is routinely flushed.

**Example** (with error reporting)::

    from ntlog import NTlog, NTlogError
    from inform import Inform, fatal, os_error

    try:
        with NTlog('appname.log.nt', keep_for='7d', max_entries=20):
            ntlog.write('a log message')
            ntlog.write('another log message')
            ...
    except OSError as e:
        fatal(os_error(e))
    except NTlogError as e:
        e.terminate()

**Example** (with temp log)::

    with NTlog('appname.log.nt', 'appname.log', keep_for='7d', retain_temp=True):
        ntlog.write('log message')
        ntlog.flush()
        ...

*NTlog* can be specified as the logfile to Inform_.

**Example** (with inform)::

    from ntlog import NTlog
    from inform import Inform, display, error, log

    with (
        NTlog('appname.log.nt', keep_for='7d') as ntlog,
        Inform(logfile=ntlog) as inform,
    ):
        display('status message')
        log('log message')
        if there_is_a_problem:
            error('error message')
        ...

**Example** (with temp log and inform)::

    with (
        NTlog('appname.log.nt', 'appname.log', keep_for='7d') as ntlog,
        Inform(logfile=ntlog, flush=True) as inform,
    ):
        display('status message')
        log('log message')
        if there_is_a_problem:
            error('error message')
        ...


Installation
------------

Install with::

    pip install ntlog

Releases
--------

Latest Development Version
""""""""""""""""""""""""""
| Version: 0.4
| Released: 2024-01-29

0.4 (2024-01-29)
""""""""""""""""
- Add support for headers and fold markers.

0.3 (2023-05-01)
""""""""""""""""
- Add Python API.

0.2 (2023-04-10)
""""""""""""""""
- Improve error handling

0.1 (2023-04-08)
""""""""""""""""
- Add support for time units on ``--keep-for``.

.. _NestedText: https://nestedtext.org
.. _Inform: https://inform.readthedocs.io
.. _Error: https://inform.readthedocs.io/en/stable/api.html#inform.Error
.. _Arrow: https://arrow.readthedocs.io/en/latest/guide.html#supported-tokens