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* README for `Pymacs'				allout -*- outline -*-

.. Presentation.

. : What is Pymacs?

    Pymacs is a powerful tool which, once started from Emacs, allows both-way
    communication between Emacs Lisp and Python.  Yet, Pymacs aims Python as
    an extension language for Emacs rather than the other way around; this
    assymetry is reflected in some design choices.  Within Emacs Lisp code,
    one may load and use Python modules.  Python functions may themselves use
    Emacs services, and handle Emacs Lisp objects kept in Emacs Lisp space.

    The goals are to write "naturally" in both languages, debug with ease,
    fall back gracefully on errors, and allow full cross-recursivity.

    It is very easy to install Pymacs, as neither Emacs nor Python need to
    be compiled nor relinked.  Emacs merely starts Python as a subprocess,
    and Pymacs implements a communication protocol between both processes.

. : Warning to Pymacs users.

    I expect average Pymacs users to have a deeper knowledge of Python than
    Emacs Lisp.  So, the few examples at the end of this file are meant for
    Python users having a limited experience with Emacs.  Currently, there
    are only contains two examples, one which is too small, another which
    is too big :-).  It would be useful to have more: my intent is to wait
    until such examples pop up naturally.  As there is no dedicated mailing
    list nor discussion group for Pymacs, let's use `'
    for asking questions or discussing Pymacs related matters.

    This is beta status software: specifications are slightly frozen, yet
    changes may still happen that would require small adaptations in your
    code.  Report problems to François Pinard at `'.
    For discussing specifications or making suggestions, please also copy
    the `' mailing list, to help brain-storming! :-)

. : History and references.

    Pymacs revisits previous Cedric Adjih's works about running Python as a
    process separate from Emacs.  See `',
    or write Cedric at `'.  Cedric presented
    `pyemacs' to me as a proof of concept.  As I simplified that concept
    a bit, I dropped the `e' in `pyemacs' :-).  Cedric also told me that
    there exist some older patches for linking Python right into XEmacs.

    Brian McErlean independently and simultaneously wrote a tool similar
    to this one, we decided to join our projects.  Amusing coincidence, he
    even chose `pymacs' as a name.  Brian paid good attention to complex
    details that escaped my courage, so his help and collaboration have
    been beneficial.  You may reach Brian at `'.

    One other reference of interest is Doug Bagley shoot out project,
    which compares the relative speed of many popular languages.
    See `' for more information.

.. Installation.

. : Install the Pymacs proper.

    We then invite you to follow the instructions below.  Currently,
    there are two installation scripts, and both should be run.  If you
    prefer, you may use `make install lispdir=LISPDIR', where LISPDIR is
    some directory along the list kept in your Emacs `load-path'.

    The first installation script installs the Python package, including the
    Pymacs examples, using the Python standard Distutils tool.  Merely `cd'
    into the Pymacs distribution, then execute `python install'.
    To get an option reminder, do `python install --help'.  Check
    the Distutils documentation if you need more information about this.

    The second installation script installs the Emacs Lisp part only.
    (It used to do everything, but is now doomed to disappear completely.)
    Merely `cd' into the Pymacs distribution, then run `python setup -ie'.
    This will invite you to interactively confirm the Lisp installation
    directory.  Without `-ie', the Lisp part of Pymacs will be installed
    in some automatically guessed place.  Use `-n' to known about the guess
    without proceeding to the actual installation.  `./setup -E xemacs ...'
    may be useful to XEmacs lovers.  See `./setup -H' for all options.

    About Win32 systems, Syver Enstad says: "For Pymacs to operate correctly,
    one should create a batch file with `pymacs-services.bat' as a name,
    which runs the `pymacs-services' script.  The `.bat' file could be
    placed along with `pymacs-services', wherever that maybe.".

    To check that `pymacs.el' is properly installed, start Emacs and give
    it the command `M-x load-library RET pymacs': you should not receive
    any error.  To check that `' is properly installed, start
    an interactive Python session and type `from Pymacs import lisp':
    you should not receive any error.  To check that `pymacs-services'
    is properly installed, type `pymacs-services </dev/null' in a shell;
    you should then get a line ending with "(pymacs-version VERSION)",
    and another saying: "Protocol error: `>' expected.".

    Currently, there is only one installed Pymacs example, which come
    in two parts: a batch script `rebox' and a `Pymacs.rebox' module.
    To check that both are properly installed, type `rebox </dev/null'
    in a shell; you should not receive any output nor see any error.

. : Prepare your `.emacs' file.

    The ".emacs" file is not given in the distribution, you likely have
    one already in your home directory.  You need to add these lines:

      (autoload 'pymacs-load "pymacs" nil t)
      (autoload 'pymacs-eval "pymacs" nil t)
      (autoload 'pymacs-apply "pymacs")
      ;;(eval-after-load "pymacs"
      ;;  (add-to-list 'pymacs-load-path "YOUR-PYMACS-DIRECTORY"))

    If you plan to use a special directory to hold your own Pymacs code in
    Python, which should be searched prior to the usual Python import search
    path, then uncomment the last two lines (by removing the semi-colons)
    and replace YOUR-PYMACS-DIRECTORY by the name of your special directory.
    If the file "$HOME/.emacs" does not exist, merely create it with the
    above lines.  You are now all set to use Pymacs.

    To check this, start a fresh Emacs session, and type `M-x pymacs-eval'.
    Emacs should prompt you for a Python expression.  Try "`2L**111`" (type
    the backquotes, but not the external double-quotes).  The minibuffer
    should display `2596148429267413814265248164610048L'. `M-x pymacs-load'
    should prompt you for a Python module name.  Reply `os'.  After Emacs
    prompts you for a prefix, merely hit Enter to accept the default prefix.
    This should have the effect of importing the Python "os" module within
    Emacs.  Typing `M-: (os-getcwd)' should echo the current directory in
    the message buffer, as returned by the `os.getcwd' Python function.

. : Porting Pymacs.

    Pymacs has been developped on Linux and Emacs (20 and 21), it is expected
    to work out of the box on most other Unices, and also with XEmacs.

    Syver Enstad reports that Pymacs could be made to work on Windows-2000
    (win2k), he suspects it should equally work with NT and XP.  However,
    little shell stunts may be required, I hope to later document them here.

. : Caveats.

    Some later versions of Emacs 20 silently ignore the request for
    creating weak hash tables, they create an ordinary table instead.
    Older Emacses just do not have hash tables.  Pymacs should run on
    all, yet for these, memory will leak on the Python side whenever
    complex objects get transmitted to Emacs, as these objects will not
    be reclaimed on the Python side once Emacs is finished with them.
    It should not be a practical problem in most simple cases.

.. Emacs Lisp structures and Python objects.

. : Conversions.

    Whenever Emacs Lisp calls Python functions giving them arguments,
    these arguments are Emacs Lisp structures that should be converted
    into Python objects in some way.  Conversely, whenever Python calls
    Emacs Lisp functions, the arguments are Python objects that should
    be received as Emacs Lisp structures.  We need some conventions for
    doing such conversions.

    Conversions generally transmit mutable Emacs Lisp structures as mutable
    objects on the Python side, in such a way that transforming the object in
    Python will effectively transform the structure on the Emacs Lisp side
    (strings are handled a bit specially however, see below).  The other
    way around, Python objects transmitted to Emacs Lisp often loose their
    mutability, so transforming the Emacs Lisp structure is not reflected
    on the Python side.

. : Simple objects.

    Emacs Lisp `nil' and the equivalent Emacs Lisp `()' yield Python `None'.
    Python `None' and the Python empty list `[]' are returned as `nil'
    in Emacs Lisp.

    Emacs Lisp numbers, either integer or floating, are converted in
    equivalent Python numbers.  Emacs Lisp characters are really numbers
    and yield Python numbers.  In the other direction, Python numbers are
    converted into Emacs Lisp numbers, with the exception of long Python
    integers and complex numbers.

    Emacs Lisp strings are usually converted into equivalent Python narrow
    strings.  As Python strings do not have text properties, these are not
    reflected.  This may be changed by setting the `pymacs-mutable-strings'
    option: if this variable is not `nil', Emacs Lisp strings are then
    transmitted opaquely.  Python strings, except Unicode, are always
    converted into Emacs Lisp strings.

    Emacs Lisp symbols yield the special `lisp.SYMBOL' or `lisp[STRING]'
    notations on the Python side.  The first notation is used when the
    Emacs Lisp symbol starts with a letter, and contains only letters,
    digits and hyphens, in which case Emacs Lisp hyphens get replaced
    by Python underscores.  This convention is welcome, as Emacs Lisp
    programmers commonly prefer using dashes, where Python programmers
    use underlines.  Otherwise, the second notation is used.  Conversely,
    `lisp.SYMBOL' on the Python side yields an Emacs Lisp symbol with
    underscores replaced with hyphens, while `lisp[STRING]' corresponds
    to an Emacs Lisp symbol printed with that STRING which, of course,
    should then be a valid Emacs Lisp symbol name.

. : Sequences.

    The case of strings has been discussed in the previous section.

    Proper Emacs Lisp lists, those for which the `cdr' of last
    cell is `nil', are normally transmitted opaquely to Python.
    If `pymacs-forget-mutability' is set, or if Python later asks for these
    to be expanded, proper Emacs Lisp lists get converted into Python lists,
    if we except the empty list, which is always converted as Python `None'.
    In the other direction, Python lists are always converted into proper
    Emacs Lisp lists.

    Emacs Lisp vectors are normally transmitted opaquely to Python.  However,
    if `pymacs-forget-mutability' is set, or if Python later asks for these
    to be expanded, Emacs Lisp vectors get converted into Python tuples.
    In the other direction, Python tuples are always converted into Emacs
    Lisp vectors.

    Remember the rule: "Round parentheses correspond to square brackets!".
    It works for lists, vectors, tuples, seen from either Emacs Lisp or Python.

    The above choices were debatable.  Since Emacs Lisp proper lists and
    Python lists are the bread-and-butter of algorithms modifying structures,
    at least in my experience, I guess they are more naturally mapped into
    one another, this spares many casts in practice.  While in Python,
    the most usual idiom for growing lists is appending to their end,
    the most usual idiom in Emacs Lisp to grow a list is by cons'ing new
    items at its beginning:

      (setq accumulator (cons 'new-item accumulator))

    or more simply:

      (push accumulator 'new-item)

    So, in case speed is especially important and many modifications happen
    in a row on the same side, while order of elements ought to be preserved,
    some (nreverse ...) on the Emacs Lisp side or .reverse() on the Python
    side side might be needed.  Surely, proper lists in Emacs Lisp and lists
    in Python are the normal structure for which length is easily modified.

    We cannot so easily change the size of a vector, the same as it is a
    bit more of a stunt to "modify" a tuple.  The shape of these objects is
    fixed.  Mapping vectors to tuples, which is admittedly strange, will only
    be done if the Python side requests an expanded copy, otherwise an opaque
    Emacs Lisp object is seen in Python.  In the other direction, whenever
    an Emacs Lisp vector is needed, one has to write `tuple(python_list)'
    while transmitting the object.  Such transmissions are most probably
    to be unusual, as people are not going to blindly transmit whole big
    structures back and forth between Emacs and Python, they would rather do
    it once in a while only, and do only local modifications afterwards.
    The infrequent casting to `tuple' for getting an Emacs Lisp vector
    seems to suggest that we did a reasonable compromise.

    In Python, both tuples and lists have O(1) access, so there is no real
    speed consideration there.  Emacs Lisp is different: vectors have O(1)
    access while lists have O(N) access.  The rigidity of Emacs Lisp
    vectors is such that people do not resort to vectors unless there
    is a speed issue, so in real Emacs Lisp practice, vectors are used
    rather parsimoniously.  So much, in fact, that Emacs Lisp vectors are
    overloaded for what they are not meant: for example, very small vectors
    are used to represent X events in key-maps, programmers only want
    to test vectors for their type, or users just like bracketed syntax.
    The speed of access is hardly an issue then.

. : Opaque objects.

.  , Emacs Lisp handles.

     When a Python function is called from Emacs Lisp, the function arguments
     have already been converted to Python types from Emacs Lisp types
     and the function result is going to be converted back to Emacs Lisp.

     Several Emacs Lisp objects do not have Python equivalents, like for
     Emacs windows, buffers, markers, overlays, etc.  It is nevertheless
     useful to pass them to Python functions, hoping that these Python
     functions will "operate" on these Emacs Lisp objects.  Of course,
     the Python side may not itself modify such objects, it has to call
     for Emacs services to do so.  Emacs Lisp handles are a mean to ease
     this communication.

     Whenever an Emacs Lisp object may not be converted to a Python object, an
     Emacs Lisp handle is created and used instead.  Whenever that Emacs Lisp
     handle is returned into Emacs Lisp from a Python function, or is used
     as an argument to an Emacs Lisp function from Python, the original Emacs
     Lisp object behind the Emacs Lisp handle is automatically retrieved.

     Emacs Lisp handles are either instances of the internal `Lisp' class,
     or of one of its subclasses.  If `object' is an Emacs Lisp handle, and
     if the underlying Emacs Lisp object is an Emacs Lisp sequence, then
     whenever `object[index]', `object[index] = value' and `len(object)'
     are meaningful, these may be used to fetch or alter an element of the
     sequence directly in Emacs Lisp space.  Also, if `object' corresponds
     to an Emacs Lisp function, `object(ARGUMENTS)' may be used to apply
     the Emacs Lisp function over the given arguments.  Since arguments
     have been evaluated the Python way on the Python side, it would be
     conceptual overkill evaluating them again the Emacs Lisp way on the
     Emacs Lisp side, so Pymacs manage to quotes arguments for defeating
     Emacs Lisp evaluation.  The same logic applies the other way around.

     Emacs Lisp handles have a `value()' method, which merely returns self.
     They also have a `copy()' method, which tries to "open the box"
     if possible.  Emacs Lisp proper lists are turned into Python lists,
     Emacs Lisp vectors are turned into Python tuples.  Then, modifying
     the structure of the copy on the Python side has no effect on the
     Emacs Lisp side.

     For Emacs Lisp handles, `str()' returns an Emacs Lisp representation
     of the handle which should be `eq' to the original object if read back
     and evaluated in Emacs Lisp.  `repr()' returns a Python representation
     of the expanded Emacs Lisp object.  If that Emacs Lisp object has
     an Emacs Lisp representation which Emacs Lisp could read back, then
     `repr()' value is such that it could be read back and evaluated in
     Python as well, this would result in another object which is `equal'
     to the original, but not neccessarily `eq'.

.  , Python handles.

     The same as Emacs Lisp handles are useful to handle Emacs Lisp objects
     on the Python side, Python handles are useful to handle Python objects
     on the Emacs Lisp side.

     Many Python objects do not have direct Emacs Lisp equivalents,
     including long integers, complex numbers, Unicode strings, modules,
     classes, instances and surely a lot of others.  When such are being
     transmitted to the Emacs Lisp side, Pymacs use Python handles.
     These are automatically recovered into the original Python objects
     whenever transmitted back to Python, either as arguments to a Python
     function, as the Python function itself, or as the return value of
     an Emacs Lisp function called from Python.

     The objects represented by these Python handles may be inspected or
     modified using the basic library of Python functions.  For example, in:

       (setq matcher (pymacs-eval "re.compile('PATTERN').match"))
       (pymacs-apply matcher (list ARGUMENT))

     the initial `setq' above could be decomposed into:

       (setq compiled (pymacs-eval "re.compile('PATTERN')")
             matcher (pymacs-apply "getattr" (list compiled "match")))

     This example shows that one may use `pymacs-apply' with "getattr"
     as the function, to get a wanted attribute for a Python object.

.. Usage on the Emacs Lisp side.

. : `pymacs-eval'.

    Function `(pymacs-eval TEXT)' gets TEXT evaluated as a Python expression,
    and returns the value of that expression converted back to Emacs Lisp.

. : `pymacs-apply'.

    Function `(pymacs-apply FUNCTION ARGUMENTS)' will get Python to apply the
    given FUNCTION over the given ARGUMENTS.  ARGUMENTS is a list containing
    all arguments, or `nil' if there is none.  FUNCTION is either a Python
    string holding an expression yielding a Python function, or else, a
    Python handle previously received from Python, and hopefully holding
    a callable Python object.  Each argument gets separately converted
    to Python before the function is called.  `pymacs-apply' returns the
    resulting value of the function call, converted back to Emacs Lisp.

. : `pymacs-load'.

    Function `(pymacs-load MODULE PREFIX)' imports the Python MODULE into
    Emacs Lisp space.  MODULE is the name of the file containing the module,
    without any `.py' or `.pyc' extension.  If the directory part is omitted
    in MODULE, the module will be looked into the current Python search
    path.  Dot notation may be used when the module is part of a package.
    Each top-level function in the module produces a trampoline function in
    Emacs Lisp having the same name, except that underlines in Python names
    are turned into dashes in Emacs Lisp, and that PREFIX is uniformly
    added before the Emacs Lisp name (as a way to avoid name clashes).
    PREFIX may be omitted, in which case it defaults to base name of MODULE
    with underlines turned into dashes, and followed by a dash.

    Whenever `pymacs_load_hook' is defined in the loaded Python module,
    `pymacs-load' calls it without arguments, but before creating the Emacs
    view for that module.  So, the `pymacs_load_hook' function may create
    new definitions or even add `interaction' attributes to functions.

    The return value of a successful `pymacs-load' is the module object.
    An optional third argument, NOERROR, when given and not `nil', will
    have `pymacs-load' to return `nil' instead of raising an error, if
    the Python module could not be found.

    When later calling one of these functions, all provided arguments are
    converted to Python and transmitted, and the function return value is
    later converted back to Emacs Lisp.  It is left to the Python side to
    check for argument consistency.  However, for an interactive function,
    the interaction specification drives some checking on the Emacs Lisp
    side.  There is no provision to collect keyword arguments in Emacs Lisp.

. : Expected usage.

    We do not expect that `pymacs-eval' or `pymacs-apply' will be much used,
    if ever.  In practice, the Emacs Lisp side of a Pymacs application might
    call `pymacs-load' a few times for linking into the Python modules,
    with the indirect effect of defining trampoline functions for these
    modules on the Emacs Lisp side, than can be called like usual Emacs
    Lisp functions.

    These imported functions are really those which are of interest for
    the user, and the preferred way to call Python services with Pymacs.

. : Special Emacs Lisp variables.

    Users could alter the inner working of Pymacs through a few variables,
    which are documented here.  Except for `pymacs-load-path', which should
    be set before calling any Pymacs function, the value of these variables
    can be changed at any time.

.  , pymacs-load-path

     Users might want to use special directories for holding their Python
     modules, when these modules are meant to be used from Emacs.  Best is
     to preset `pymacs-load-path, `nil' by default, to a list of these
     directory names.  (Tilde expansions and such occur automatically.)

.  , pymacs-trace-transit

     The `*Pymacs*' buffer, within Emacs, holds a trace of transactions
     between Emacs and Python.  When `pymacs-trace-transit' is `nil',
     and this is the default setting, the buffer only holds the last
     bi-directional transaction (a request and a reply).  If that variable
     is not `nil', all transactions are kept.  This could be useful for
     debugging, but the drawback is that this buffer could grow big over
     time, to the point of diminishing Emacs performance.

.  , pymacs-forget-mutability

     The default behaviour of Pymacs is to transmit Emacs Lisp objects to
     Python in such a way thay they are fully modifiable from the Python
     side, would it mean triggering Emacs Lisp functions to act on them.
     When `pymacs-forget-mutability' is not `nil', the behaviour is changed,
     and the flexibility is lost.  Pymacs then tries to expand proper lists
     and vectors as full copies when transmitting them on the Python side.
     This variable, seen as a user setting, is best left to `nil'.  It may
     be temporarily overriden within some functions, when deemed useful.

     There is no corresponding variable from objects transmitted to Emacs
     from Python.  Pymacs automatically expands what gets transmitted.
     Mutability is preserved only as a side-effect of not having a natural
     Emacs Lisp representation for the Python object.  This assymetry is on
     purpose, yet debatable.  Maybe Pymacs could have a variable telling
     that mutability _is_ important for Python objects?  That would give
     Pymacs users the capability of restoring the symmetry somewhat,
     yet so far, in our experience, this has never been needed.

.  , pymacs-mutable-strings

     Strictly speaking, Emacs Lisp strings are mutable. Yet, it does not
     come naturally to a Python programmer to modify a string "in-place",
     as Python strings are never mutable.  When `pymacs-mutable-strings' is
     `nil', which is the default setting, Emacs Lisp strings are transmitted
     to Python as Python strings, and so, loose their mutability.
     Moreover, text properties are not reflected on the Python side.
     But if that variable is not `nil', Emacs Lisp strings are rather
     passed as Emacs Lisp handles.  This variable is ignored whenever
     `pymacs-forget-mutability' is set.

.  , pymacs-timeout-at-start
.  , pymacs-timeout-at-reply
.  , pymacs-timeout-at-line

     Emacs needs to protect itself a bit, in case the Pymacs service program,
     which handles the Python side of requests, would not start correctly,
     or maybe later die unexpectedly.  So, whenever Emacs reads data coming
     from that program, it sets a time limit, and take some action whenever
     that time limit expires.  All times are expressed in seconds.

     The timeout at start defaults to 30 seconds, this time should only
     be increased if a given machine is so heavily loaded that the Pymacs
     service program has not enough of 30 seconds to start, in which case
     Pymacs refuses to work, with an appropriate message in the minibuffer.

     The two other variables almost never need to be changed in practice.
     When Emacs is expecting a reply from Python, it might repeatedly
     check the status of the Pymacs service program when that reply is not
     received fast enough, just to make sure that this program did not die.
     The timeout at reply, which defaults to 5, says how many seconds
     to wait without checking, while expecting the first line of a reply.
     The timeout at line, which defaults to 2, says how many seconds to wait
     without checking, while expecting a line of the reply after the first.

.. Usage on the Python side.

. : Python setup.

    Pymacs requires little or no setup in the Python modules which are
    meant to be used from Emacs, for the simple situations where these
    modules receive nothing but Emacs nil, numbers or strings, or return
    nothing but Python None, numbers or strings.

    Otherwise, use `from Pymacs import lisp'.  If you need more Pymacs
    features, like the `Let' class, write `from Pymacs import lisp, Let'.

. : Response mode.

    When Python receives a request from Emacs in the context of Pymacs,
    and until it returns the reply, Emacs keeps listening to serve Python
    requests.  Emacs is not listening otherwise.  Other Python threads,
    if any, may not call Emacs without _very_ careful synchronisation.

. : Emacs Lisp symbols.

    `lisp' is a special object which has useful built-in magic.
    Its attributes do nothing but represent Emacs Lisp symbols, created
    on the fly as needed (symbols also have their built-in magic).

    Except for `lisp.nil' or `lisp["nil"]', which are the same as `None',
    both `lisp.SYMBOL' and `lisp[STRING]' yield objects of the internal
    `Symbol' type.  These are genuine Python objects, that could be referred
    to by simple Python variables.  One may write `quote = lisp.quote',
    for example, and use `quote' afterwards to mean that Emacs Lisp symbol.
    If a Python function received an Emacs Lisp symbol as an argument, it
    can check with `==' if that argument is `lisp.never' or `lisp.ask', say.
    A Python function may well choose to return `lisp.t'.

    In Python, writing `lisp.SYMBOL = VALUE' or `lisp[STRING] = VALUE'
    does assign VALUE to the corresponding symbol in Emacs Lisp space.
    Beware that in such cases, the `lisp.' prefix may not be spared.
    After `result = lisp.result', one cannot hope that a later `result = 3'
    will have any effect in the Emacs Lisp space: this would merely change
    the Python variable `result', which was a reference to a `Symbol'
    instance, so it is now a reference to the number 3.

    The `Symbol' class has `value()' and `copy()' methods.  One can use
    either `lisp.SYMBOL.value()' or `lisp.SYMBOL.copy()' to access the
    Emacs Lisp value of a symbol, after conversion to some Python object,
    of course.  However, if `value()' would have given an Emacs Lisp handle,
    `lisp.SYMBOL.copy()' has the same effect of `lisp.SYMBOL.value().copy()',
    that is, it returns the value of the symbol as opened as possible.

    A symbol may also be as if it was a Python function, in which case
    it really names an Emacs Lisp function that should be applied over the
    following function arguments.  The result of the Emacs Lisp function
    becomes the value of the call, with all due conversions of course.

. : Dynamic bindings.

    As Emacs Lisp uses dynamic bindings, it is common that Emacs Lisp
    programs use `let' for temporarily setting new values for some Emacs
    Lisp variables having global scope.  These variables recover their
    previous value automatically when the `let' gets completed, even if
    an error occurs which interrupts the normal flow of execution.

    Pymacs has a `Let' class to represent such temporary settings.  Suppose
    for example that you want to recover the value of `lisp.mark()' when the
    transient mark mode is active on the Emacs Lisp side.  One could surely
    use `lisp.mark(lisp.t)' to "force" reading the mark in such cases,
    but for the sake of illustration, let's ignore that, and temporarily
    deactivate transient mark mode instead.  This could be done this way:

            let = Let()
            ... USER CODE ...

    `let.push()' accepts any number of keywords arguments.  Each keyword
    name is interpreted as an Emacs Lisp symbol written the Pymacs way,
    with underlines.  The value of that Emacs Lisp symbol is saved on the
    Python side, and the value of the keyword becomes the new temporary value
    for this Emacs Lisp symbol.  A later `let.pop()' restores the previous
    value for all symbols which were saved together at the time of the
    corresponding `let.push()'.  There may be more than one `let.push()'
    call for a single `Let' instance, they stack within that instance.
    Each `let.pop()' will undo one and only one `let.push()' from the stack,
    in the reverse order or the pushes.

    When the `Let' instance disappears, either because the programmer does
    `del let' or `let = None', or just because the Python `let' variable
    goes out of scope, all remaining `let.pop()' get automatically executed,
    so the `try'/'finally' statement may be omitted in practice.  For this
    omission to work flawlessly, the programmer should be careful at not
    keeping extra references to the `Let' instance.

    The constructor call `let = Let()' also has an implied initial `.push()'
    over all given arguments, so the explicit `let.push()' may be omitted
    as well.  In practice, this sums up and the above code could be reduced
    to a mere:

        let = Let(transient_mark_mode=None)
        ... USER CODE ...

    Be careful at assigning the result of the constructor to some Python
    variable.  Otherwise, the instance would disappear immediately after
    having been created, restoring the Emacs Lisp variable much too soon.

    Any variable to be bound with `Let' should have been bound in advance
    on the Emacs Lisp side.  This restriction usually does no kind of harm.
    Yet, it will likely be lifted in some later version of Pymacs.

    The `Let' class has other methods meant for some macros which are
    common in Emacs Lisp programming, in the spirit of `let' bindings.
    These method names look like `push_*' or `pop_*', where Emacs
    Lisp macros are `save-*'.  One has to use the matching `pop_*' for
    undoing the effect of a given `push_*' rather than a mere `.pop()':
    the Python code is clearer, this also ensures that things are undone
    in the proper order.  The same `Let' instance may use many `push_*'
    methods, their effects nest.

    `push_excursion()' and `pop_excursion()' save and restore the current
    buffer, point and mark.  `push_match_data()' and `pop_match_data()'
    save and restore the state of the last regular expression match.
    `push_restriction()' and `pop_restriction()' save and restore
    the current narrowing limits.  `push_selected_window()' and
    `pop_selected_window()' save and restore the fact that a window holds
    the cursor.  `push_window_excursion()' and `pop_window_excursion()'
    save and restore the current window configuration in the Emacs display.

    As a convenience, `let.push()' and all other `push_*' methods return
    the `Let' instance.  This helps chaining various `push_*' right after
    the instance generation.  For example, one may write:

        let = Let().push_excursion()
        if 1:
            ... USER CODE ...
        del let

    The `if 1:' (some people might prefer writing `if let:'), has the only
    goal of indenting USER CODE, so the scope of the `let' variable is made
    very explicit.  This is purely stylistic, and not at all necessary.
    The last `del let' might be omitted in a few circumstances, for example
    if the excursion lasts until the end of the Python function.

. : Raw Emacs Lisp expressions.

    Pymacs offers a device for evaluating a raw Emacs Lisp expression
    expressed as a string.  One merely uses `lisp' as a function, like this:


    The Emacs Lisp value of the expression becomes the value of the `lisp'
    call, after conversion back to Python.

. : User interaction.

    Emacs functions have the concept of user interaction for completing
    the specification of their arguments while being called.  This happens
    only when a function is interactively called by the user, it does
    not happen when a function is programmatically called by another.
    As Python does not have a corresponding facility, a bit of trickery
    was needed to retrofit that facility on the Python side.

    Whenever a Python module contains an `interactions' global variable
    which is a dictionary, Pymacs uses that dictionary at load time to find
    out the interaction specification of each imported function from this
    module.  If the dictionary has no entry for a given function, or if the
    dictionary value is None, that function is not going to be interactive
    on the Emacs side.  Otherwise, the function is interactive, and the
    dictionary value should either be a string or another Python function.

    If the interaction specification is a string, that string is used
    verbatim as the argument to the `(interactive ...)' function on the
    Emacs side.  Within Emacs, you may execute `C-h f interactive' to get a
    short reminder about how this string is interpreted on the Emacs side.
    For an interactive function having no arguments, use the empty string:

        from Pymacs import lisp
        interactions = {}

        def hello_world():
            "`Hello world' from Python."
            lisp.insert("Hello from Python!")
        interactions[hello_world] = ''

    If the interaction specification is not a string, but another Python
    function, that function is called without arguments and should return
    a list of the actual arguments to be used for the original function.

    Python 2.x users may install an `interaction' attribute to a given
    function, in which case it will be used instead of the `interactions'
    dictionary.  So the last line in the above example could be written as:

        hello_world.interaction = ''

    However, such writings are not portable to any Python released before
    PEP 232 was integrated, and should probably be discouraged for a while.

    One might wonder why we did not use `lisp.interactive(...)' from Python.
    The requirement that "(interactive ...)" be first in an Emacs Lisp
    `defun' suggests that there is some magic in the Emacs Lisp interpreter
    itself, which looks for that call _before_ the function is actually
    entered.  Looking for `lisp.interactive' would be the difficult part,
    as this Python code is already in compiled form when it gets executed.
    Pymacs could try to scan the generated code, but given the computation
    of `lisp.interactive' arguments could get arbitrarily complex, it would
    be rather difficult to un-compile that evaluation into Emacs Lisp code.

. : Keybindings.

    An interactive function may be bound to a key sequence.

    To translate bindings like `C-x w', say, one might have to know a bit
    more how Emacs Lisp processes string escapes like `\C-x' or `\M-\C-x' in
    Emacs Lisp, and emulate it within Python strings, since Python does not
    have such escapes.  `\C-L', where L is an upper case letter, produces a
    character which ordinal is the result of subtracting 0x40 from ordinal
    of `L'.  `\M-' has the ordinal one gets by adding 0x80 to the ordinal
    of following described character.  So people can use self-inserting
    non-ASCII characters, `\M-' is given another representation, which is
    to replace the addition of 0x80 by prefixing with `ESC', that is 0x1b.

    So "\C-x" in Emacs is '\x18' in Python.  This is easily found, using an
    interactive Python session, by givin it: chr(ord('X') - ord('A') + 1).
    An easier way would be using the `kbd' function on the Emacs Lisp side,
    like with lisp.kbd('C-x w') or lisp.kbd('M-<f2>').

    To bind the F1 key to the `helper' function in some `module':

        lisp.global_set_key((lisp.f1,), lisp.module_helper)

    (item,) is a Python tuple yielding an Emacs Lisp vector.  `lisp.f1'
    translates to the Emacs Lisp symbol `f1'.  So, Python `(lisp.f1,)' is
    Emacs Lisp `[f1]'.  Keys like `[M-f2]' might require some more ingenuity,
    one may write either (lisp['M-f2'],) or (lisp.M_f2,) on the Python side.

.. Debugging.

. : The `*Pymacs*' buffer.

    Emacs and Python are two separate processes (well, each may use more
    than one process).  Pymacs implements a simple communication protocol
    between both, and does whatever needed so the programmers do not have to
    worry about details.  The main debugging tool is the communication buffer
    between Emacs and Python, which is named `*Pymacs*'.  To make good use
    of it, first set `pymacs-trace-transit' to `t', so all exchanges are
    accumulated in that buffer.  It helps understanding the communication
    protocol, so it is shortly explained here.  Consider:

    (pymacs-eval "lisp('(pymacs-eval \"`2L**111`\")')")

    Here, Emacs asks Python to ask Emacs to ask Python for a simple bignum
    computation.  Note that Emacs does not natively know how to handle big
    integers, nor has an internal representation for them.  This is why I
    use backticks, so Python returns a string representation of the result,
    instead of the result itself.  Here is a trace for this example.  The `<'
    character flags a message going from Python to Emacs and is followed
    by an expression written in Emacs Lisp.  The '>' character flags a
    message going from Emacs to Python and is followed by a expression
    written in Python.  The number gives the length of the message.

    <22   (pymacs-version "0.3")
    >49   eval("lisp('(pymacs-eval \"`2L**111`\")')")
    <25   (pymacs-eval "`2L**111`")
    >18   eval("`2L**111`")
    <47   (pymacs-reply "2596148429267413814265248164610048L")
    >45   reply("2596148429267413814265248164610048L")
    <47   (pymacs-reply "2596148429267413814265248164610048L")

    Python evaluation is done in the context of the `Pymacs.pymacs' module,
    so for example a mere `reply' really means `Pymacs.pymacs.reply'.  On the
    Emacs Lisp side, there is no concept of module namespaces, so we use
    the `pymacs-' prefix as an attempt to stay clean.  Users should ideally
    refrain from naming their Emacs Lisp objects with a `pymacs-' prefix.

    `reply' and `pymacs-reply' are special functions meant to indicate that
    an expected result is finally transmitted.  `error' and `pymacs-error'
    are special functions that introduce a string which explains an exception
    which recently occurred.  `pymacs-expand' is a special function
    implementing the `copy()' methods of Emacs Lisp handles or symbols.
    In all other cases, the expression is a request for the other side,
    that request stacks until a corresponding reply is received.

    Part of the protocol manages memory, and this management generates
    some extra-noise in the `*Pymacs*' buffer.  Whenever Emacs passes a
    structure to Python, an extra pointer is generated on the Emacs side to
    inhibit garbage collection by Emacs.  Python garbage collector detects
    when the received structure is no longer needed on the Python side,
    at which time the next communication will tell Emacs to remove the
    extra pointer.  It works symmetrically as well, that is, whenever Python
    passes a structure to Emacs, an extra Python reference is generated to
    inhibit garbage collection on the Python side.  Emacs garbage collector
    detects when the received structure is no longer needed on the Emacs
    side, after which Python will be told to remove the extra reference.
    For efficiency, those allocation-related messages are delayed, merged and
    batched together within the next communication having another purpose.

. : Emacs usual debugging.

    If cross-calls between Emacs Lisp and Python nest deeply, an error will
    raise successive exceptions alternatively on both sides as requests
    unstack, and the diagnostic gets transmitted back and forth, slightly
    growing as we go.  So, errors will eventually be reported by Emacs.
    I made no kind of effort to transmit the Emacs Lisp backtrace on the
    Python side, as I do not see a purpose for it: all debugging is done
    within Emacs windows anyway.

    On recent Emacses, the Python backtrace gets displayed in the
    mini-buffer, and the Emacs Lisp backtrace is simultaneously shown in the
    `*Backtrace*' window.  One useful thing is to allow to mini-buffer to
    grow big, so it has more chance to fully contain the Python backtrace,
    the last lines of which are often especially useful.  Here, I use:

      (setq resize-mini-windows t
            max-mini-window-height .85)

    in my `.emacs' file, so the mini-buffer may use 85% of the screen,
    and quickly shrinks when fewer lines are needed.  The mini-buffer
    contents disappear at the next keystroke, but you can recover the
    Python backtrace by looking at the end of the `*Messages*' buffer.
    In which case the `ffap' package in Emacs may be yet another friend!
    From the `*Messages*' buffer, once `ffap' activated, merely put the
    cursor on the file name of a Python module from the backtrace, and
    `C-x C-f RET' will quickly open that source for you.

. : Auto-reloading on save.

    I found useful to automatically `pymacs-load' some Python files whenever
    they get saved from Emacs.  Here is how I do it.  The code below assumes
    that Python files meant for Pymacs are kept in `~/share/emacs/python'.

      (defun fp-maybe-pymacs-reload ()
        (let ((pymacsdir (expand-file-name "~/share/emacs/python/")))
          (when (and (string-equal (file-name-directory buffer-file-name)
                     (string-match "\\.py\\'" buffer-file-name))
            (pymacs-load (substring buffer-file-name 0 -3)))))
      (add-hook 'after-save-hook 'fp-maybe-pymacs-reload)

.. Exemples.

. : Paul Winkler's.

.  , The problem.

     Let's say I have a a module, call it `', containing this
     simple python function:

         def break_on_whitespace(some_string):
             words = some_string.split()
             return '\n'.join(words)

     The goal is telling Emacs about this function so that I can call it on
     a region of text and replace the region with the result of the call.
     And bind this action to a key, of course, let's say `[f7]'.

     The Emacs buffer ought to be handled in some way.  If this is not on the
     Emacs Lisp side, it has to be on the Python side, but we cannot escape
     handling the buffer.  So, there is an equilibrium in the work to do for
     the user, that could be displaced towards Emacs Lisp or towards Python.

.  , Python side.

     Here is a first draft for the Python side of the problem:

         from Pymacs import lisp

         def break_on_whitespace():
             start = lisp.point()
             end = lisp.mark(lisp.t)
             if start > end:
                 start, end = end, start
             text = lisp.buffer_substring(start, end)
             words = text.split()
             replacement = '\n'.join(words)
             lisp.delete_region(start, end)

         interactions = {break_on_whitespace: ''}

     For various stylistic reasons, this could be rewritten into:

         from Pymacs import lisp
         interactions = {}

         def break_on_whitespace():
             start, end = lisp.point(), lisp.mark(lisp.t)
             words = lisp.buffer_substring(start, end).split()
             lisp.delete_region(start, end)

         interactions[break_on_whitespace] = ''

     The above relies, in particular, on the fact that for those Emacs
     Lisp functions used here, `start' and `end' may be given in any order.

.  , Emacs side.

     On the Emacs side, one would do:

       (pymacs-load "manglers")
       (global-set-key [f7] 'manglers-break-on-whitespace)

. : The `rebox' tool.

.  , The problem.

     For comments held within boxes, it is painful to fill paragraphs, while
     stretching or shrinking the surrounding box "by hand", as needed.
     This piece of Python code eases my life on this.  It may be used
     interactively from within Emacs through the Pymacs interface, or in
     batch as a script which filters a single region to be reformatted.
     Pymacs features are used exclusively within the `Emacs_Rebox' class.

     In batch, the reboxing is driven by command options and arguments
     and expects a complete, self-contained boxed comment from a file.
     Emacs function `rebox-region' also presumes that the region encloses
     a single boxed comment.  Emacs `rebox-comment' is different, as it
     has to chase itself the extent of the surrounding boxed comment.

.  , Python side.

     The Python code is too big to be inserted in this documentation: see
     file `Pymacs/' in the Pymacs distribution.  This section is
     meant to discuss some of the design choices, in Pymacs context.

     In batch mode, as well as with `rebox-region', the text to handle is
     turned over to Python, and fully processed in Python, with practically
     no Pymacs interaction while the work gets done.  On the other hand,
     `rebox-comment' is rather Pymacs intensive: the comment boundaries
     are chased right from the Emacs buffer, as directed by the function
     `Emacs_Rebox.find_comment'.  Once the boundaries are found, the
     remainder of the work is essentially done on the Python side.

     Once the boxed comment has been reformatted in Python, the old comment
     is removed in a single delete operation, the new comment is inserted in
     a second operation, this occurs in `Emacs_Rebox.process_emacs_region'.
     But by doing so, if point was within the boxed comment before the
     reformatting, its precise position is lost.  To well preserve point,
     Python might have driven all reformatting details directly in the
     Emacs buffer.  We really preferred doing it all on the Python side:
     as we gain legibility by expressing the algorithms in pure Python,
     the same Python code may be used in batch or interactively, and we
     avoid the slowdown that would result from heavy use of Emacs services.

     To avoid completely loosing point, I kludged a `Marker' class,
     which goal is to estimate the new value of point from the old.
     Reformatting may change the amount of white space, and either delete
     or insert an arbitrary number characters meant to draw the box.
     The idea is to initially count the number of characters between the
     beginning of the region and point, while ignoring any problematic
     character.  Once the comment has been reboxed, point is advanced from
     the beginning of the region until we get the same count of characters,
     skipping all problematic characters.  This `Marker' class works fully
     on the Python side, it does not involve Pymacs at all, but it does
     solve a problem that resulted from my choice of keeping the data on
     the Python side instead of handling it directly in the Emacs buffer.

     We want a comment reformatting to appear as a single operation, in
     the context of Emacs Undo.  The method `Emacs_Rebox.clean_undo_after'
     handles the general case for this.  Not that we do so much in practice:
     a reformatting implies one `delete-region' and one `insert', and maybe
     some other little adjustements at `Emacs_Rebox.find_comment' time.
     Even if this method scans and mofifies an Emacs Lisp list directly in
     the Emacs memory, the code doing this stays neat and legible.  However,
     I found out that the undo list may grow quickly when the Emacs buffer
     use markers, with the consequence of making this routine so Pymacs
     intensive that most of the CPU is spent there.  I rewrote that
     routine in Emacs Lisp so it executes in a single Pymacs interaction.

     Function `Emacs_Rebox.remainder_of_line' could have been written in
     Python, but it was probably not worth going away from this one-liner in
     Emacs Lisp.  Also, given this routine is often called by `find_comment',
     a few Pymacs protocol interactions are spared this way.  This function
     is useful when there is a need to apply a regexp already compiled on
     the Python side, it is probably better fetching the line from Emacs
     and do the pattern match on the Python side, than transmitting the
     source of the regexp to Emacs for it to compile and apply it.

     For refilling, I could have either used the refill algorithm built
     within in Emacs, programmed a new one in Python, or relied on Ross
     Paterson's `fmt', distributed by GNU and available on most Linuxes.
     In fact, `refill_lines' prefers the latter.  My own Emacs setup is
     such that the built-in refill algorithm is _already_ overridden by GNU
     `fmt', and it really does a much better job.  Experience taught me
     that calling an external program is fast enough to be very bearable,
     even interactively.  If Python called Emacs to do the refilling, Emacs
     would itself call GNU `fmt' in my case, I preferred that Python calls
     GNU `fmt' directly.  I could have reprogrammed GNU `fmt' in Python.
     Despite interesting, this is an uneasy project: `fmt' implements
     the Knuth refilling algorithm, which depends on dynamic programming
     techniques; Ross fine tuned them, and took care of many details.
     If GNU `fmt' fails, for not being available, say, `refill_lines'
     falls back on a dumb refilling algorithm, which is better than none.

.  , Emacs side.

     The Emacs recipe appears under the `Emacs usage' section, near the
     beginning of `Pymacs/', so I do not repeat it here.

                                        François Pinard,
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