[Python-Dev] PEP 350: Codetags

Micah Elliott mde at micah.elliott.name
Tue Sep 27 00:35:21 CEST 2005

Please read/comment/vote.  This circulated as a pre-PEP proposal
submitted to c.l.py on August 10, but has changed quite a bit since
then.  I'm reposting this since it is now "Open (under consideration)"
at <http://www.python.org/peps/pep-0350.html>.


Micah Elliott <mde at tracos.org>

PEP: 350
Title: Codetags
Version: $Revision: 1.2 $
Last-Modified: $Date: 2005/09/26 19:56:53 $
Author: Micah Elliott <mde at tracos.org>
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 27-Jun-2005
Post-History: 10-Aug-2005, 26-Sep-2005


This informational PEP aims to provide guidelines for consistent use
of *codetags*, which would enable the construction of standard
utilities to take advantage of the codetag information, as well as
making Python code more uniform across projects.  Codetags also
represent a very lightweight programming micro-paradigm and become
useful for project management, documentation, change tracking, and
project health monitoring.  This is submitted as a PEP because its
ideas are thought to be Pythonic, although the concepts are not unique
to Python programming.  Herein are the definition of codetags, the
philosophy behind them, a motivation for standardized conventions,
some examples, a specification, a toolset description, and possible
objections to the Codetag project/paradigm.

This PEP is also living as a wiki_ for people to add comments.

What Are Codetags?

Programmers widely use ad-hoc code comment markup conventions to serve
as reminders of sections of code that need closer inspection or
review.  Examples of markup include ``FIXME``, ``TODO``, ``XXX``,
``BUG``, but there many more in wide use in existing software.  Such
markup will henceforth be referred to as *codetags*.  These codetags
may show up in application code, unit tests, scripts, general
documentation, or wherever suitable.

Codetags have been under discussion and in use (hundreds of codetags
in the Python 2.4 sources) in many places (e.g., c2_) for many years.
See References_ for further historic and current information.


If you subscribe to most of these values, then codetags will likely be
useful for you.

1. As much information as possible should be contained **inside the
   source code** (application code or unit tests).  This along with
   use of codetags impedes duplication.  Most documentation can be
   generated from that source code; e.g., by using help2man, man2html,
   docutils, epydoc/pydoc, ctdoc, etc.

2. Information should be almost **never duplicated** -- it should be
   recorded in a single original format and all other locations should
   be automatically generated from the original, or simply be
   referenced.  This is famously known as the Single Point Of
   Truth (SPOT) or Don't Repeat Yourself (DRY) rule.

3. Documentation that gets into customers' hands should be
   **auto-generated** from single sources into all other output
   formats.  People want documentation in many forms.  It is thus
   important to have a documentation system that can generate all of

4. The **developers are the documentation team**.  They write the code
   and should know the code the best.  There should not be a
   dedicated, disjoint documentation team for any non-huge project.

5. **Plain text** (with non-invasive markup) is the best format for
   writing anything.  All other formats are to be generated from the
   plain text.

Codetag design was influenced by the following goals:

A. Comments should be short whenever possible.

B. Codetag fields should be optional and of minimal length.  Default
   values and custom fields can be set by individual code shops.

C. Codetags should be minimalistic.  The quicker it is to jot
   something down, the more likely it is to get jotted.

D. The most common use of codetags will only have zero to two fields
   specified, and these should be the easiest to type and read.


* **Various productivity tools can be built around codetags.**

  See Tools_.

* **Encourages consistency.**

  Historically, a subset of these codetags has been used informally in
  the majority of source code in existence, whether in Python or in
  other languages.  Tags have been used in an inconsistent manner with
  different spellings, semantics, format, and placement.  For example,
  some programmers might include datestamps and/or user identifiers,
  limit to a single line or not, spell the codetag differently than
  others, etc.

* **Encourages adherence to SPOT/DRY principle.**

  E.g., generating a roadmap dynamically from codetags instead of
  keeping TODOs in sync with separate roadmap document.

* **Easy to remember.**

  All codetags must be concise, intuitive, and semantically
  non-overlapping with others.  The format must also be simple.

* **Use not required/imposed.**

  If you don't use codetags already, there's no obligation to start,
  and no risk of affecting code (but see Objections_).  A small subset
  can be adopted and the Tools_ will still be useful (a few codetags
  have probably already been adopted on an ad-hoc basis anyway).  Also
  it is very easy to identify and remove (and possibly record) a
  codetag that is no longer deemed useful.

* **Gives a global view of code.**

  Tools can be used to generate documentation and reports.

* **A logical location for capturing CRCs/Stories/Requirements.**

  The XP community often does not electronically capture Stories, but
  codetags seem like a good place to locate them.

* **Extremely lightweight process.**

  Creating tickets in a tracking system for every thought degrades
  development velocity.  Even if a ticketing system is employed,
  codetags are useful for simply containing links to those tickets.


This shows a simple codetag as commonly found in sources everywhere
(with the addition of a trailing ``<>``)::

    # FIXME: Seems like this loop should be finite. <>
    while True: ...

The following contrived example demonstrates a typical use of
codetags.  It uses some of the available fields to specify the
assignees (a pair of programmers with initials *MDE* and *CLE*), the
Date of expected completion (*Week 14*), and the Priority of the item

    # FIXME: Seems like this loop should be finite. <MDE,CLE d:14w p:2>
    while True: ...

This codetag shows a bug with fields describing author, discovery
(origination) date, due date, and priority::

    # BUG: Crashes if run on Sundays.
    # <MDE 2005-09-04 d:14w p:2>
    if day == 'Sunday': ...

Here is a demonstration of how not to use codetags.  This has many
problems: 1) Codetags cannot share a line with code; 2) Missing colon
after mnemonic; 3) A codetag referring to codetags is usually useless,
and worse, it is not completable; 4) No need to have a bunch of fields
for a trivial codetag; 5) Fields with unknown values (``t:XXX``)
should not be used::

    i = i + 1   # TODO Add some more codetags.
    # <JRNewbie 2005-04-03 d:2005-09-03 t:XXX d:14w p:0 s:inprogress>


This describes the format: syntax, mnemonic names, fields, and
semantics, and also the separate DONE File.

General Syntax

Each codetag should be inside a comment, and can be any number of
lines.  It should not share a line with code.  It should match the
indentation of surrounding code.  The end of the codetag is marked by
a pair of angle brackets ``<>`` containing optional fields, which must
not be split onto multiple lines.  It is preferred to have a codetag
in ``#`` comments instead of string comments.  There can be multiple
fields per codetag, all of which are optional.

.. NOTE: It may be reasonable to allow fields to fit on multiple
   lines, but it complicates parsing and defeats minimalism if you
   use this many fields.

In short, a codetag consists of a mnemonic, a colon, commentary text,
an opening angle bracket, an optional list of fields, and a closing
angle bracket.  E.g., ::

    # MNEMONIC: Some (maybe multi-line) commentary. <field field ...>


The codetags of interest are listed below, using the following format:

| ``recommended mnemonic (& synonym list)``
|     *canonical name*: semantics

   *To do*: Informal tasks/features that are pending completion.

   *Fix me*: Areas of problematic or ugly code needing refactoring or

   *Bugs*: Reported defects tracked in bug database.

   *Will Not Be Fixed*: Problems that are well-known but will never be
   addressed due to design problems or domain limitations.

   *Requirements*: Satisfactions of specific, formal requirements.

   *Requests For Enhancement*: Roadmap items not yet implemented.

   *Ideas*: Possible RFE candidates, but less formal than RFE.

   *Questions*: Misunderstood details.

``!!! (ALERT)``
   *Alerts*: In need of immediate attention.

   *Hacks*: Temporary code to force inflexible functionality, or
   simply a test change, or workaround a known problem.

   *Portability*: Workarounds specific to OS, Python version, etc.

   *Caveats*: Implementation details/gotchas that stand out as

   *Notes*: Sections where a code reviewer found something that needs
   discussion or further investigation.

   *Frequently Asked Questions*: Interesting areas that require
   external explanation.

   *Glossary*: Definitions for project glossary.

   *See*: Pointers to other code, web link, etc.

   *Needs Documentation*: Areas of code that still need to be

   *Credits*: Accreditations for external provision of enlightenment.

   *Status*: File-level statistical indicator of maturity of this

   *Reviewed*: File-level indicator that review was conducted.

File-level codetags might be better suited as properties in the
revision control system, but might still be appropriately specified in
a codetag.

Some of these are temporary (e.g., ``FIXME``) while others are
persistent (e.g., ``REQ``).  A mnemonic was chosen over a synonym
using three criteria: descriptiveness, length (shorter is better),
commonly used.

Choosing between ``FIXME`` and ``XXX`` is difficult.  ``XXX`` seems to
be more common, but much less descriptive.  Furthermore, ``XXX`` is a
useful placeholder in a piece of code having a value that is unknown.
Thus ``FIXME`` is the preferred spelling.  `Sun says`__ that ``XXX``
and ``FIXME`` are slightly different, giving ``XXX`` higher severity.
However, with decades of chaos on this topic, and too many millions of
developers who won't be influenced by Sun, it is easy to rightly call
them synonyms.

__ http://java.sun.com/docs/codeconv/html/CodeConventions.doc9.html#395

``DONE`` is always a completed ``TODO`` item, but this should probably
be indicated through the revision control system and/or a completion
recording mechanism (see `DONE File`_).

It may be a useful metric to count ``NOTE`` tags: a high count may
indicate a design (or other) problem.  But of course the majority of
codetags indicate areas of code needing some attention.

An ``FAQ`` is probably more appropriately documented in a wiki where
users can more easily view and contribute.


All fields are optional.  The proposed standard fields are described
in this section.  Note that upper case field characters are intended
to be replaced.

The *Originator/Assignee* and *Origination Date/Week* fields are the
most common and don't usually require a prefix.

.. NOTE: the colon after the prefix is a new addition that became
   necessary when it was pointed out that a "codename" field (with no
   digits) such as "cTiger" would be indistinguishable from a username.
   <MDE 2005-9-24>

.. NOTE: This section started out with just assignee and due week.  It
   has grown into a lot of fields by request.  It is still probably
   best to use a tracking system for any items that deserve it, and
   not duplicate everything in a codetag (field). <MDE>

This lengthy list of fields is liable to scare people (the intended
minimalists) away from adopting codetags, but keep in mind that these
only exist to support programmers who either 1) like to keep ``BUG``
or ``RFE`` codetags in a complete form, or 2) are using codetags as
their complete and only tracking system.  In other words, many of
these fields will be used very rarely.  They are gathered largely from
industry-wide conventions, and example sources include `GCC
Bugzilla`__ and `Python's SourceForge`__ tracking systems.

.. ???: Maybe codetags inside packages (__init__.py files) could have
   special global significance. <MDE>

__ http://gcc.gnu.org/bugzilla/
__ http://sourceforge.net/tracker/?group_id=5470

    List of *Originator* or *Assignee* initials (the context
    determines which unless both should exist).  It is also okay to
    use usernames such as ``MicahE`` instead of initials.  Initials
    (in upper case) are the preferred form.

    List of *Assignee* initials.  This is necessary only in (rare)
    cases where a codetag has both an assignee and an originator, and
    they are different.  Otherwise the ``a:`` prefix is omitted, and
    context determines the intent.  E.g., ``FIXME`` usually has an
    *Assignee*, and ``NOTE`` usually has an *Originator*, but if a
    ``FIXME`` was originated (and initialed) by a reviewer, then the
    assignee's initials would need a ``a:`` prefix.

``YYYY[-MM[-DD]]`` or ``WW[.D]w``
    The *Origination Date* indicating when the comment was added, in
    `ISO 8601`_ format (digits and hyphens only).  Or *Origination
    Week*, an alternative form for specifying an *Origination Date*.
    A day of the week can be optionally specified.  The ``w`` suffix
    is necessary for distinguishing from a date.

``d:YYYY[-MM[-DD]]`` or ``d:WW[.D]w``
    *Due Date (d)* target completion (estimate).  Or *Due Week (d)*,
    an alternative to specifying a *Due Date*.

    *Priority (p)* level.  Range (N) is from 0..3 with 3 being the
    highest.  0..3 are analogous to low, medium, high, and
    showstopper/critical.  The *Severity* field could be factored into
    this single number, and doing so is recommended since having both
    is subject to varying interpretation.  The range and order should
    be customizable.  The existence of this field is important for any
    tool that itemizes codetags.  Thus a (customizable) default value
    should be supported.

    *Tracker (t)* number corresponding to associated Ticket ID in
    separate tracking system.

The following fields are also available but expected to be less

    *Category (c)* indicating some specific area affected by this

    *Status (s)* indicating state of item.  Examples are "unexplored",
    "understood", "inprogress", "fixed", "done", "closed".  Note that
    when an item is completed it is probably better to remove the
    codetag and record it in a `DONE File`_.

    Development cycle *Iteration (i)*.  Useful for grouping codetags into
    completion target groups.

    Development cycle *Release (r)*.  Useful for grouping codetags into
    completion target groups.

    .. NOTE: SourceForge does not recognize a severity and I think
       that *Priority* (along with separate RFE codetags) should
       encompass and obviate *Severity*. <MDE>

    .. NOTE: The tools will need an ability to sort codetags in order
       of targeted completion.  I feel that *Priority* should be a
       unique, lone indicator of that addressability order.  Other
       categories such as *Severity*, *Customer Importance*, etc. are
       related to business logic and should not be recognized by the
       codetag tools.  If some groups want to have such logic, then it
       is best factored (externally) into a single value (priority)
       that can determine an ordering of actionable items. <MDE>

To summarize, the non-prefixed fields are initials and origination
date, and the prefixed fields are: assignee (a), due (d), priority
(p),tracker (t), category (c), status (s), iteration (i), and release

It should be possible for groups to define or add their own fields,
and these should have upper case prefixes to distinguish them from the
standard set.  Examples of custom fields are *Operating System (O)*,
*Severity (S)*, *Affected Version (A)*, *Customer (C)*, etc.


Some codetags have an ability to be *completed* (e.g., ``FIXME``,
``TODO``, ``BUG``).  It is often important to retain completed items
by recording them with a completion date stamp.  Such completed items
are best stored in a single location, global to a project (or maybe a
package).  The proposed format is most easily described by an example,
say ``~/src/fooproj/DONE``::

    # TODO: Recurse into subdirs only on blue
    # moons. <MDE 2003-09-26>
    [2005-09-26 Oops, I underestimated this one a bit.  Should have
    used Warsaw's First Law!]

    # FIXME: ...

You can see that the codetag is copied verbatim from the original
source file.  The date stamp is then entered on the following line
with an optional post-mortem commentary.  The entry is terminated by a
blank line (``\n\n``).

It may sound burdensome to have to delete codetag lines every time one
gets completed.  But in practice it is quite easy to setup a Vim or
Emacs mapping to auto-record a codetag deletion in this format (sans
the commentary).


Currently, programmers (and sometimes analysts) typically use *grep*
to generate a list of items corresponding to a single codetag.
However, various hypothetical productivity tools could take advantage
of a consistent codetag format.  Some example tools follow.

.. NOTE: Codetag tools are mostly unimplemented (but I'm getting
   started!) <MDE>

Document Generator
    Possible docs: glossary, roadmap, manpages

Codetag History
    Track (with revision control system interface) when a ``BUG`` tag
    (or any codetag) originated/resolved in a code section

Code Statistics
    A project Health-O-Meter

Codetag Lint
    Notify of invalid use of codetags, and aid in porting to codetags

Story Manager/Browser
    An electronic means to replace XP notecards.  In MVC terms, the
    codetag is the Model, and the Story Manager could be a graphical
    Viewer/Controller to do visual rearrangement, prioritization, and
    assignment, milestone management.

Any Text Editor
    Used for changing, removing, adding, rearranging, recording

There are some tools already in existence that take advantage of a
smaller set of pseudo-codetags (see References_).  There is also an
example codetags implementation under way, known as the `Codetag

__ http://tracos.org/codetag


:Objection: Extreme Programming argues that such codetags should not
    ever exist in code since the code is the documentation.

:Defense: Maybe you should put the codetags in the unit test files
    instead.  Besides, it's tough to generate documentation from
    uncommented source code.


:Objection: Too much existing code has not followed proposed

:Defense: [Simple] utilities (*ctlint*) could convert existing codes.


:Objection: Causes duplication with tracking system.

:Defense: Not really, unless fields are abused.  If an item exists in
    the tracker, a simple ticket number in the codetag tracker field
    is sufficient.  Maybe a duplicated title would be acceptable.
    Furthermore, it's too burdensome to have a ticket filed for every
    item that pops into a developer's mind on-the-go.  Additionally,
    the tracking system could possibly be obviated for simple or small
    projects that can reasonably fit the relevant data into a codetag.


:Objection: Codetags are ugly and clutter code.

:Defense: That is a good point.  But I'd still rather have such info
    in a single place (the source code) than various other documents,
    likely getting duplicated or forgotten about.  The completed
    codetags can be sent off to the `DONE File`_, or to the bit


:Objection: Codetags (and allcomments) get out of date.

:Defense: Not so much if other sources (externally visible
    documentation) depend on their being accurate.


:Objection: Codetags tend to only rarely have estimated completion
    dates of any sort.  OK, the fields are optional, but you want to
    suggest fields that actually will be widely used.

:Defense: If an item is inestimable don't bother with specifying a
    date field.  Using tools to display items with order and/or color
    by due date and/or priority, it is easier to make estimates.
    Having your roadmap be a dynamic reflection of your codetags makes
    you much more likely to keep the codetags accurate.


:Objection: Named variables for the field parameters in the ``<>``
    should be used instead of cryptic one-character prefixes.  I.e.,
    <MDE p:3> should rather be <author=MDE, priority=3>.

:Defense: It is just too much typing/verbosity to spell out fields.  I
    argue that ``p:3 i:2`` is as readable as ``priority=3,
    iteration=2`` and is much more likely to by typed and remembered
    (see bullet C in Philosophy_).  In this case practicality beats
    purity.  There are not many fields to keep track of so one letter
    prefixes are suitable.


:Objection: Synonyms should be deprecated since it is better to have a
    single way to spell something.

:Defense: Many programmers prefer short mnemonic names, especially in
    comments.  This is why short mnemonics were chosen as the primary
    names.  However, others feel that an explicit spelling is less
    confusing and less prone to error.  There will always be two camps
    on this subject.  Thus synonyms (and complete, full spellings)
    should remain supported.


:Objection: It is cruel to use [for mnemonics] opaque acronyms and
    abbreviations which drop vowels; it's hard to figure these things
    out.  On that basis I hate: MLSTN RFCTR RFE FEETCH, NYI, FR, FTRQ,

:Defense: Mnemonics are preferred since they are pretty easy to
    remember and take up less space.  If programmers didn't like
    dropping vowels we would be able to fit very little code on a
    line.  The space is important for those who write comments that
    often fit on a single line.  But when using a canon everywhere it
    is much less likely to get something to fit on a line.


:Objection: It takes too long to type the fields.

:Defense: Then don't use (most or any of) them, especially if you're
    the only programmer.  Terminating a codetag with ``<>`` is a small
    chore, and in doing so you enable the use of the proposed tools.
    Editor auto-completion of codetags is also useful:  You can
    program your editor to stamp a template (e.g. ``# FIXME . <MDE
    {date}>``) with just a keystroke or two.


:Objection: *WorkWeek* is an obscure and uncommon time unit.

:Defense: That's true but it is a highly suitable unit of granularity
    for estimation/targeting purposes, and it is very compact.  The
    `ISO 8601`_ is widely understood but allows you to only specify
    either a specific day (restrictive) or month (broad).


:Objection: I aesthetically dislike for the comment to be terminated
    with <> in the empty field case.

:Defense: It is necessary to have a terminator since codetags may be
    followed by non-codetag comments.  Or codetags could be limited to
    a single line, but that's prohibitive.  I can't think of any
    single-character terminator that is appropriate and significantly
    better than <>.  Maybe ``@`` could be a terminator, but then most
    codetags will have an unnecessary @.


:Objection: I can't use codetags when writing HTML, or less
    specifically, XML.  Maybe ``@fields@`` would be a better than
    ``<fields>`` as the delimiters.

:Defense: Maybe you're right, but ``<>`` looks nicer whenever
    applicable.  XML/SGML could use ``@`` while more common
    programming languages stick to ``<>``.


Some other tools have approached defining/exploiting codetags.
See http://tracos.org/codetag/wiki/Links.

.. _wiki: http://tracos.org/codetag/wiki/Pep
.. _ISO 8601: http://en.wikipedia.org/wiki/ISO_8601
.. _c2: http://c2.com/cgi/wiki?FixmeComment

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