represented by an explicit structure. (There are still too many casts
in the code, but that may be unavoidable.)
Also added code so that with -vv it is very chatty about what it does.
buffer increment, and sometimes the new buffer size. Make it do what
its name says, and fix the one place where this matters to the caller.
Also add a comment explaining why we call lseek() and then ftell().
The MS compiler doesn't call it 'long long', it uses __int64,
so a new #define, LONG_LONG, has been added and all occurrences
of 'long long' are replaced with it.
Previously, this said "unsubscriptable object"; in 1.5.1, the reverse
problem existed, where None[''] would complain about a non-integer
index. This fix does the right thing in all cases (for get, set and
del item).
before calling it. This check was there when the objects were of the
same type *before* coercion, but not if they initially differed but
became the same *after* coercion.
Sparc Solaris 2.6 (fully patched!) that I don't want to dig into, but
which I suspect is a bug in the multithreaded malloc library that only
shows up when run on a multiprocessor. (The program wasn't using
threads, it was just using the multithreaded C library.)
faster (using PyList_GetSlice()). Also added a test for a NULL
argument, as with PySequence_Tuple(). (Hmm... Better names for these
two would be PyList_FromSequence() and PyTuple_FromSequence(). Oh well.)
"indefinite length" sequences. These should still have a length, but
the length is only used as a hint -- the actual length of the sequence
is determined by the item that raises IndexError, which may be either
smaller or larger than what len() returns. (This is a novelty; map(),
filter() and reduce() only allow the actual length to be larger than
what len() returns, not shorter. I'll fix that shortly.)
conversions. Formerly, for example, int('-') would return 0 instead
of raising ValueError, and int(' 0') would raise ValueError
(complaining about a null byte!) instead of 0...
+ Took the "list" argument out of the other functions that no longer need
it. This speeds things up a little more.
+ Small comment changes in accord with that.
+ Exploited the now-safe ability to cache values in the partitioning loop.
Makes no timing difference on my flavor of Pentium, but this machine ran out
of registers 12 iterations ago. It should yield a small speedup on a RISC
machine, and not hurt in any case.
instead of testing whether the list changed size after each
comparison, temporarily set the type of the list to an immutable list
type. This should allow continued use of the list for legitimate
purposes but disallows all operations that can change it in any way.
(Changes to the internals of list items are not caught, of cause;
that's not possible to detect, and it's not necessary to protect the
sort code, either.)
not in restricted mode.
__dict__ can be set to any dictionary; the cl_getattr, cl_setattr and
cl_delattr slots are refreshed.
__name__ can be set to any string.
__bases__ can be set to to a tuple of classes, provided they are not
subclasses of the class whose attribute is being assigned.
__getattr__, __setattr__ and __delattr__ can be set to anything, or
deleted; the appropriate slot (cl_getattr, cl_setattr, cl_delattr) is
refreshed.
(Note: __name__ really doesn't need to be a special attribute, but
that would be more work.)
From: "Tim Peters" <tim_one@email.msn.com>
To: "Guido van Rossum" <guido@CNRI.Reston.VA.US>
Date: Sat, 23 May 1998 21:45:53 -0400
Guido, the overflow checking in PyLong_AsLong is off a little:
1) If the C in use sign-extends right shifts on signed longs, there's a
spurious overflow error when converting the most-negative int:
Python 1.5.1 (#0, Apr 13 1998, 20:22:04) [MSC 32 bit (Intel)] on win32
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> x = -1L << 31
>>> x
-2147483648L
>>> int(x)
Traceback (innermost last):
File "<stdin>", line 1, in ?
OverflowError: long int too long to convert
>>>
2) If C does not sign-extend, some genuine overflows won't be caught.
The attached should repair both, and, because I installed a new disk and a C
compiler today, it's even been compiled this time <wink>.
Python 1.5.1 (#0, May 23 1998, 20:24:58) [MSC 32 bit (Intel)] on win32
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> x = -1L << 31
>>> x
-2147483648L
>>> int(x)
-2147483648
>>> int(-x)
Traceback (innermost last):
File "<stdin>", line 1, in ?
OverflowError: long int too long to convert
>>> int(-x-1)
2147483647
>>> int(x-1)
Traceback (innermost last):
File "<stdin>", line 1, in ?
OverflowError: long int too long to convert
>>>
end-casing-ly y'rs - tim
Make sure that no tp_as_numbers->nb_<whatever> function is called
without checking for a NULL pointer. Marc-Andre Lemburg will love it!
(Except that he's just rewritten all this code for a different
approach to coercions ;-( )
programming style.
Recoded many routines to incorporate better error checking, and/or
better versions of the same function found elsewhere
(e.g. bltinmodule.c or ceval.c). In particular,
Py_Number_{Int,Long,Float}() now convert from strings, just like the
built-in functions int(), long() and float().
Sequences and mappings are now safe to have NULL function pointers
anywhere in their tp_as_sequence or tp_as_mapping fields. (A few
places in other files need to be checked in too.)
Renamed PySequence_In() to PySequence_Contains().
clear_carefully() used to do in import.c. Differences: leave only
__builtins__ alone in the 2nd pass; and don't clear the dictionary (on
the theory that as long as there are references left to the
dictionary, those might be destructors that might expect __builtins__
to be alive when they run; and __builtins__ can't normally be part of
a cycle).
PyNumber_Coerce() except that when the coercion can't be done and no
other exceptions happen, it returns 1 instead of raising an
exception.
Use this function in PyObject_Compare() to avoid raising an exception
simply because two objects with numeric behavior can't be coerced to a
common type; instead, proceed with the non-numeric default comparison.
Note that this is a somewhat questionable practice -- comparisons for
numeric objects shouldn't default to random behavior like this, but it
is required for backward compatibility. (Case in point, it broke
comparison of kjDict objects to integers in Aaron Watters' kjbuckets
extension.) A correct fix (for python 2.0) should involve a different
definiton of comparison altogether.
sys.stdin.readline(), you get a fatal error (no current thread). This
is because there was a call to PyErr_CheckSignals() while there was no
current thread. I wonder how many more of these we find... I bnetter
go hunting for PyErr_CheckSignals() now...
in libmath.a so they are available to mathmodule.so (in case it is
shared). While this still gets triggered on Solaris 2.x, this appears
to be harmless there.
__getitem__(). This method never raises an exception; if the key is
not in the dictionary, the second (optional) argument is returned. If
the second argument is not provided and the key is missing, None is
returned.
mapp_methods: added "get" method.
arbitrary nested parens in a %(...)X style format.
#Also folded two lines and added more detail to the error message for
#unsupported format character.
former lets you give an instance a set of new instance vars. The
latter lets you give it a new class. Both are typechecked and
disallowed in restricted mode.
For classes, the check for read-only special attributes is tightened
so that only assignments to __dict__, __bases__, __name__,
__getattr__, __setattr__, and __delattr__ (these could be made to work
as well, but I don't know if that's useful -- let's see first whether
mucking with instances will help).
from the interned table. There are references in hard-to-find static
variables all over the interpreter, and it's not worth trying to get
rid of all those; but "uninterning" isn't fair either and may cause
subtle failures later -- so we have to keep them in the interned
table.
Also get rid of no-longer-needed insert of None in interned dict.
no valid directory is passed in. This prevents __del__ to fail when
invoked after __builtins__ has already been discarded.
Also add PyFrame_Fini() to discard the cache of frames.
In _Py_PrintReferences(), no longer suppress once-referenced string.
Add Py_Malloc and friends and PyMem_Malloc and friends (malloc
wrappers for third parties).
complexity saved much any more. A simple benchmark (grail) showed
that there were 3 times as many misses as hits, and the same number of
times again the code was bypassed altogether due to the existence of
setattro/getattro.
this many bytes have been read, readlines stops. Because of
buffering, the amount of bytes read is usually at least 8K more than
the hint.
Also changed read() and readline() to use PyArg_ParseTuple().
(Note that the *previous* checkin also fixed error handling and
narrowed the range of thread unblocking for all methods using
fread().)
see if we can guess the #bytes until the end of the file. If we
can't, increment the buffer size increments up to 0.5Meg to avoid
realloc'ing too much.
The table size is now constrained to be a power of two, and we use a
variable increment based on GF(2^n)-{0} (not that I have the faintest
idea what that is :-) which helps avoid the expensive '%' operation.
Some of the entries in the table of polynomials have been modified
according to a post by Tim Peters.
Rather than allocating a list object for the fast locals and another
(extensible one) for the value stack and allocating the block stack
dynamically, allocate the block stack with a fixed size (CO_MAXBLOCKS
from compile.h), and stick the locals and value stack at the end of
the object (this is now possible since the stack size is known
beforehand). Get rid of the owner field and the nvalues argument --
it is available in the code object, like nlocals.
This requires small changes in ceval.c only.
sequence, otherwise
operator.indexOf([4, 3, 2, 1], 9) would raise a SystemError!
Note: it might be wise to double check all these functions. I haven't
done that yet.
object pointers. Should be a bit faster than the C library's qsort(),
and doesn't have the prohibition on recursion that Solaris qsort() has
in the threaded version of their C library.
Thanks to discussions with Tim Peters.
defines that a shorter dictionary is always smaller than a longer one.
For dictionaries of the same size, the smallest differing element
determines the outcome (which yields the same results as before,
without explicit sorting).
be Ellipsis!).
Bumped the API version because a linker-visible symbol is affected.
Old C code will still compile -- there's a b/w compat macro.
Similarly, old Python code will still run, builtin exports both
Ellipses and Ellipsis.
Removed im_doc attribute; __name__ and __doc__ are now handled by
special casing in instancemethodgetattr(). This saves a few bytes and
INCREF/DECREF calls per i.m. object allocation/deallocation.
getcounts() returns a list of counts of allocations and
deallocations for all different object types.
getobjects(n [, type ]) returns a list of recently allocated
and not-yet-freed objects of the given type (all
objects if no type given). Only the n most recent
(all if n==0) objects are returned.
getcounts is only available if compiled with -DCOUNT_ALLOCS,
getobjects is only available if compiled with -DTRACE_REFS. Note that
everything must be compiled with these options!
entirely redone operator overloading. The rules for class
instances are now much more relaxed than for other built-in types
(whose coerce must still return two objects of the same type)
* Objects/floatobject.c: add overflow check when converting float
to int and implement truncation towards zero using ceil/float
* Objects/longobject.c: change ValueError to OverflowError when
converting to int
* Objects/rangeobject.c: modernized
* Objects/stringobject.c: use HAVE_LIMITS instead of __STDC__
* Objects/xxobject.c: changed to use new style (not finished?)
and __setattr__ support to override getattr(x, name) and
setattr(x, name, value) for class instances. This uses a special
hack whereby the class is supposed to be static: the __getattr__
and __setattr__ methods are looked up only once and saved in the
instance structure for speed
* funcobject.c (func_repr): don't call getstringvalue(None) for anonymous
functions.
* bltinmodule.c: removed lambda (which is now a built-in function);
removed implied lambda for string arg to filter/map/reduce.
* Grammar, graminit.[ch], compile.[ch]: replaced lambda as built-in
function by lambda as grammar entity: instead of "lambda('x: x+1')" you
write "lambda x: x+1".
* Xtmodule.c (checkargdict): return 0, not NULL, for error.
* posixmodule.c: don't prototype getcwd() -- it's not portable...
* mappingobject.c: double-check validity of last_name_char in
dict{lookup,insert,remove}.
* arraymodule.c: need memmove only for non-STDC Suns.
* Makefile: comment out HTML_LIBS and XT_USE by default
* pythonmain.c: don't prototype getopt() -- it's not standardized
* socketmodule.c: cast flags arg to {get,set}sockopt() and addrbuf arg to
recvfrom() to (ANY*).
* pythonrun.c (initsigs): fix prototype, make it static
* intobject.c (LONG_BIT): only #define it if not already defined
* classobject.[ch]: remove all references to unused instance_convert()
* mappingobject.c (getmappingsize): Don't return NULL in int function.
each dir in sys.path, try each possible extension. (Note: C extensions
are loaded before Python modules in the same directory, to allow having
a C version used when dynamic loading is supported and a Python version
as a back-up.)
* import.c (reload_module): test for error from getmodulename()
* moduleobject.c: implement module name as dict entry '__name__' instead
of special-casing it in module_getattr(); this way a module (or
function!) can access its own module name, and programs that know what
they are doing can rename modules.
* stdwinmodule.c (initstdwin): strip ".py" suffix of argv[0].
* {tuple,list,mapping,array}object.c: call printobject with 0 for flags
* compile.c (parsestr): use quote instead of '\'' at one crucial point
* arraymodule.c (array_getattr): Added __members__ attribute
* object.[ch], bltinmodule.c, fileobject.c: changed str() to call
strobject() which calls an object's __str__ method if it has one.
strobject() is also called by writeobject() when PRINT_RAW is passed.
* ceval.c: rationalize code for PRINT_ITEM (no change in function!)
* funcobject.c, codeobject.c: added compare and hash functionality.
Functions with identical code objects and the same global dictionary are
equal. Code objects are equal when their code, constants list and names
list are identical (i.e. the filename and code name don't count).
(hash doesn't work yet since the constants are in a list and lists can't
be hashed -- suppose this should really be done with a tuple now we have
resizetuple!)
setlistslice() can be used to cut the unused part out of a freshly made
slice (as done by bagof()). [needed by the next mod!]
* structural changes to bagof(), map() etc.
* PROTO.h, mymalloc.h: added #ifdefs for TURBOC and GNUC.
* allobjects.h: added #include "rangeobject.h"
* Grammar: added lambda_input; relaxed syntax for exec.
* bltinmodule.c: added bagof, map, reduce, lambda, xrange.
* tupleobject.[ch]: added resizetuple().
* rangeobject.[ch]: new object type to speed up range operations (not
convinced this is needed!!!)
* Grammar: add exec statement; allow testlist in expr statement.
* ceval.c, compile.c, opcode.h: support exec statement;
avoid optimizing locals when it is used
* fileobject.{c,h}: add getfilename() internal function.
shared. The default is to save references to the integers in
the range -1..99. The lower limit can be set by defining
NSMALLNEGINTS (absolute value of smallest integer to be saved)
and NSMALLPOSINTS (1 more than the largest integer to be
saved).
tupleobject.c: Save a reference to the empty tuple to be returned
whenever a tuple of size 0 is requested. Tuples of size 1
upto, but not including, MAXSAVESIZE (default 20) are put in
free lists when deallocated. When MAXSAVESIZE equals 1, only
share references to the empty tuple, when MAXSAVESIZE equals
0, don't include the code at all and revert to the old
behavior.
object.c: Print some more statistics when COUNT_ALLOCS is defined.
without .py file); Bill's dynamic loading for SunOS using shared
libraries.
pwdmodule.c (mkgrent): remove DECREF of uninitialized variable.
classobject.c (instance_getattr): Fix case when class lookup returns
unbound method instead of function.
image objects, and lots of new methods.
* Added counting of allocations and deallocations of builtin types if
COUNT_ALLOCS is defined. Had to move calls to NEWREF down in some
files.
* Bug fix in sorting lists.
objects of its derived classes; allow anything that has an attribute
named "__privileged__" access to anything.
* object.[ch]: added hasattr() -- test whether getattr() will succeed.
* many files: made some functions static; removed "extern int errno;".
* frozenmain.c: fixed bugs introduced on 24 June...
* flmodule.c: remove 1.5 bw compat hacks, add new functions in 2.2a
(and some old functions that were omitted).
* timemodule.c: added MSDOS floatsleep version .
* pgenmain.c: changed exit() to goaway() and added defn of goaway().
* intrcheck.c: add hack (to UNIX only) so interrupting 3 times
will exit from a hanging program. The second interrupt prints
a message explaining this to the user.
Added $(SYSDEF) to its build rule in Makefile.
* cgensupport.[ch], modsupport.[ch]: removed some old stuff. Also
changed files that still used it... And made several things static
that weren't but should have been... And other minor cleanups...
* listobject.[ch]: add external interfaces {set,get}listslice
* socketmodule.c: fix bugs in new send() argument parsing.
* sunaudiodevmodule.c: added flush() and close().
function found as instance data.
* socketmodule.c: added 'flags' argument sendto/recvfrom, rewrite
argument parsing in send/recv.
* More changes related to access (terminology change: owner instead of
class; allow any object as owner; local/global variables are owned
by their dictionary, only class/instance data is owned by the class;
"from...import *" now only imports objects with public access; etc.)
* Added "access *: ...", made access work for class methods.
* Introduced subclass check: make sure that when calling
ClassName.methodname(instance, ...), the instance is an instance of
ClassName or of a subclass thereof (this might break some old code!)
yet). The class is now passed to eval_code and stored in the current
frame. It is also stored in instance method objects. An "unbound"
instance method is now returned when a function is retrieved through
"classname.funcname", which when called passes the class to eval_code.
(1) dictionaries/mappings now have attributes values() and items() as
well as keys(); at the C level, use the new function mappinggetnext()
to iterate over a dictionary.
(2) "class C(): ..." is now illegal; you must write "class C: ...".
(3) Class objects now know their own name (finally!); and minor
improvements to the way how classes, functions and methods are
represented as strings.
(4) Added an "access" statement and semantics. (This is still
experimental -- as long as you don't use the keyword 'access' nothing
should be changed.)
before it.
* ceval.c, object.c: moved testbool() to object.c (now extern visible)
* stringobject.c: fix bugs in and rationalize string resize in formatstring()
* tokenizer.[ch]: fix non-working code for lines longer than BUFSIZ
f_fastlocals in a traceback object (this is a core dump hazard
if there are <nil> entries), but instead eval_code() merges the fast
locals back into the locals dictionary if it looks like the local
variables will be retained. Also, the merge routines save
exceptions since this is sometimes needed (alas!).
* Added id() to bltinmodule.c, which returns an object's address
(identity). Useful to walk arbitrary data structures containing
cycles.
* Added compile() to bltinmodule.c and compile_string() to
pythonrun.[ch]: support to exec/eval arbitrary code objects. The
code that defaults globals and locals is moved from run_node in
pythonrun.c (which is now identical to eval_node) to eval_code in
ceval.c. [XXX For elegance a clean-up session is necessary.]
lookup (opcode.h, ceval.[ch], compile.c, frameobject.[ch],
pythonrun.c, import.c). The .pyc MAGIC number is changed again.
Added get_menu_text to flmodule.
* Stubs for faster implementation of local variables (not yet finished)
* Added function name to code object. Print it for code and function
objects. THIS MAKES THE .PYC FILE FORMAT INCOMPATIBLE (the version
number has changed accordingly)
* Print address of self for built-in methods
* New internal functions getattro and setattro (getattr/setattr with
string object arg)
* Replaced "dictobject" with more powerful "mappingobject"
* New per-type functio tp_hash to implement arbitrary object hashing,
and hashobject() to interface to it
* Added built-in functions hash(v) and hasattr(v, 'name')
* classobject: made some functions static that accidentally weren't;
added __hash__ special instance method to implement hash()
* Added proper comparison for built-in methods and functions
* Fixcprt.py: added [-y file] option, do only files younger than file.
* modsupport.[ch]: added vmkvalue().
* intobject.c: use mkvalue().
* stringobject.c: added "formatstring"; renamed string* to string_*;
ceval.c: call formatstring for string % value.
* longobject.c: close memory leak in divmod.
* parsetok.c: set result node to NULL when returning an error.
listfontnames, bitmap ops.
* listobject.c: use mkvalue() when possible; avoid weird error when
calling append() without args.
* modsupport.c: new feature in getargs(): if the format string
contains a semicolor the string after that is used as the error
message instead of "bad argument list (format %s)" when there's an
error.
* various modules: added 1993 to copyright.
* thread.c: added copyright notice.
* ceval.c: minor change to error message for "+"
* stdwinmodule.c: check for error from wfetchcolor
* config.c: MS-DOS fixes (define PYTHONPATH, use DELIM, use osdefs.h)
* Add declaration of inittab to import.h
* sysmodule.c: added sys.builtin_module_names
* xxmodule.c, xxobject.c: fix minor errors
stdwinmodule.c: wsetfont can now return an error
Makefile: add CL_USE and CL_LIB*S; config.c: move CL part around
New things in imgfile; also in Makefile.
longobject.c: fix comparison of negative long ints... [REAL BUG!]
marshal.c: add dumps() and loads() to read/write strings
timemodule.c: make sure there's always a floatsleep()
posixmodule.c: rationalize struct returned by times()
Makefile: add test target, disable imgfile by default
thread.c: Improved coexistance with dl module (sjoerd)
stdwinmodule.c: Change include stdwin.h if macintosh
rotormodule.c: added missing last argument to RTR_?_region calls
confic.c: merged with configmac.c, added 1993 to copyright message
fileobject.c: int compared to NULL in writestring(); change fopenRF ifdef
timemodule.c: simplify times() using mkvalue; include myselect.h
earlier (for sequent).
posixmodule: for sequent, include unistd.h instead of explicit
extern definitions and don't define rename()
Makefile: change misleading/wrong MD5 comments
* posixmodule.c: move extern function declarations to top
* listobject.c: cmp() arguments must be void* if __STDC__
* Makefile, allobjects.h, panelmodule.c, modsupport.c: get rid of
strdup() -- it is a portability risk
* Makefile: enclosed ranlib command in parentheses for Sequent Make
which aborts if the command is not found even if '-' is present
* timemodule.c: time() returns a floating point number, in microsecond
precision if BSD_TIME is defined.
* flmodule.c: added {do,check}_only_forms to fl's list of functions;
and don't print a message when an unknown object is returned.
* pythonrun.c: catch SIGHUP and SIGTERM to do essential cleanup.
* Made jpegmodule.c smaller by using getargs() and mkvalue() consistently.
* Increased parser stack size to 500 in parser.h.
* Implemented custom allocation of stack frames to frameobject.c and
added dynamic stack overflow checks (value stack only) to ceval.c.
(There seems to be a bug left: sometimes stack traces don't make sense.)
sys.stderr or sys.stdin, and to work with any object as long as it has
a write() (respectively readline()) methods. Some functions that took
a FILE* argument now take an object* argument.
* flmodule.c: added some missing functions; changed readonly flags of
some data members based upon FORMS documentation.
* listobject.c: fixed int/long arg lint bug (bites PC compilers).
* several: removed redundant print methods (repr is good enough).
* posixmodule.c: added (still experimental) process group functions.
calls the repr function. When the refcount is bad, don't print
the object at all (chances of crashes).
Changes to checking and printing of references: the consistency
check is somewhat faster; don't print strings referenced once
(most occur in function's name lists).
argument to malloc() (size_t or unsigned int)
* listobject.c: check for overflow of the size of the object,
so things like range(0x7fffffff) will raise MemoryError instead
of calling malloc() with -4 (and then crashing -- malloc's fault)
coercion is now completely generic.
* ceval.c: for instances, don't coerce for + and *; * reverses
arguments if left one is non-instance numeric and right one sequence.
* socketmodule.c: get rid of makepair(); fix makesocketaddr to fix
broken recvfrom()
* socketmodule: get rid of getStrarg()
* ceval.h: move eval_code() to new file eval.h, so compile.h is no
longer needed.
* ceval.c: move thread comments to ceval.h; always make save/restore
thread functions available (for dynloaded modules)
* cdmodule.c, listobject.c: don't include compile.h
* flmodule.c: include ceval.h
* import.c: include eval.h instead of ceval.h
* cgen.py: add forground(); noport(); winopen(""); to initgl().
* bltinmodule.c, socketmodule.c, fileobject.c, posixmodule.c,
selectmodule.c:
adapt to threads (add BGN/END SAVE macros)
* stdwinmodule.c: adapt to threads and use a special stdwin lock.
* pythonmain.c: don't include getpythonpath().
* pythonrun.c: use BGN/END SAVE instead of direct calls; also more
BGN/END SAVE calls etc.
* thread.c: bigger stack size for sun; change exit() to _exit()
* threadmodule.c: use BGN/END SAVE macros where possible
* timemodule.c: adapt better to threads; use BGN/END SAVE; add
longsleep internal function if BSD_TIME; cosmetics
* split pythonmain.c in two: most stuff goes to pythonrun.c, in the library.
* new optional built-in threadmodule.c, build upon Sjoerd's thread.{c,h}.
* new module from Sjoerd: mmmodule.c (dynamically loaded).
* new module from Sjoerd: sv (svgen.py, svmodule.c.proto).
* new files thread.{c,h} (from Sjoerd).
* new xxmodule.c (example only).
* myselect.h: bzero -> memset
* select.c: bzero -> memset; removed global variable
arguments crashed in INCREF() calls which should be XINCREF() calls.
timemodule.c: fix for SEQUENT port (sys/select, struct timezone) by
Jaap Vermeulen
xxobject.c: include modsupport.h
must (pretend to) support all operations except assignments;
if you don't want to support an operation you have to provide
a dummy function that raises an exception...
Guido van Rossum
Barry Warsaw
Jack Jansen
Sjoerd Mullender