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<h3>Differences between CPython and Jython</h3>
<P>CPython and Jython are two different implementations of the Python
language. While a <A HREF="http://www.python.org/doc/ref">Language
Reference</A> exists for the Python language, there are a number of
features of the language that are incompletely specified. The
following lists all known differences between the two implementations
of the Python language. These differences range from the trivial --
Jython prints "1.0E20" where CPython prints
"1e+020" -- to the dramatic -- everything in Jython is an
instance of a class. At some point more effort should be made to
separate the interesting differences from the mundane.
<p>Any other differences not listed here can probably be considered a
bug in Jython. Understand of course that CPython and Jython advance
at different paces. All efforts are made to keep the two
implementations in sync, but that's not always possible.
<p><i>This list
has been updated to describe the differences between Jython-2.0 and
CPython 2.0</i>
<h3>Syntax</h3>
<UL>
<LI>Jython has a different interpretation of floating point
literals. CPython doesn't allow 001.1 <I>CPython should be
fixed.</I>
<p><LI>Jython supports continue in a try clause. <I>CPython
should be fixed - but don't hold your breath.</I>
<p><LI>Jython allows keywords to be used as identifier name
in some situations where there are no conflict. This allows
jython to call and override java methods with names like 'print'
and 'exec'. <I>Both behaviors are acceptable.</I>
</UL>
<h3>Standard types, functions and behavior</h3>
<UL>
<LI>Jython have only one string type which support full
two-byte Unicode characters and the functions in the string
module are Unicode-aware. The u"" string modifier is optional
and completely ignored if specified.
CPython-2.0 have two string types, the classic 8-bit string and
and new unicode string which is created with the u"" string modifier.
<I>Both behaviors are acceptable.</I>
<p><li>Jython uses the character properties (isuppercase, isdecimal, ...)
from the java platform. Java uses Unicode-2.0 and not all unicode properties
is available through java.
CPython-2.0 uses Unicode-3.0 and all unicode properties are available.
About 340 of the unicode points have different properties.
<I>Both behaviors are acceptable.</I>
<p><LI>Jython formats floating point numbers differently,
e.g. an upper case 'E' is used, and it switches over to E
notation sooner than CPython does. <I>Both behaviors are
acceptable.</I>
<p><LI>In Jython, 0.1**4 is printed as
1.0000000000000002E-4. In CPython, it is printed
0.00010000000000000005.
<I>Both behaviors are acceptable.</I>
<p><LI>Jython sequences support three argument
slices. i.e. range(3)[::-1] == [2,1,0].
<I>CPython should be fixed.</I>
<p><LI>Every object in Jython is an instance of a class --
there are no types in Jython. i.e. [].__class__ is a sensible
thing to write in Jython. <I>CPython should be fixed - but
don't hold your breath.</I>
<p><LI>Jython file objects are still missing some
functionality -- see todo list in PyFile.java. (Hopefully in
the near future this can be changed to read -- Jython file
objects include the following extra functionality to properly
handle non-ascii files...) <I>Jython should be fixed.</I>
<p><LI>In CPython, range(0.1, 3.2) yields the surprising [0,
1, 2]. Jython does the right thing (reject float arguments).
-- Many other functions taking int arguments have the same
problem. <I>CPython should be fixed, but don't hold your
breath.</I>
<p><LI>The __name__ attribute of built-in extension modules
(e.g. 'sys') is different. <I>Both behaviors are
acceptable.</I>
<p><LI>In many cases, introspection yields different results.
<I>Where appropriate and possible, Jython will adhere to
CPython's introspection behavior. Some differences are
acceptable.</I>
<p><LI>Jython defines __debug__, but always sets it equal to
1. <I>Jython should implement CPython's -O option.</I>
<p><LI>The locals() dictionary in Jython is mutable from
within a function. After "def foo(x=1): locals()['x'] =
2; print x" foo() prints 1 in CPython and 2 in
Jython. Jim thinks that Jython's behavior is better here
-- but the best answer might be that locals() should be
considered a read-only dictionary.<I> Proper behavior here
is still unclear.</I>
<p><LI>Jython doesn't support restricted execution mode and
doesn't have the magic __builtins__ in every namespace.
<I>Jython will probably never support restricted execution
mode -- Java's security model is recommended instead.</I>
<p><LI>Jython uses different values for the IOError
argument. This causes trouble for people who unpack the value
into an (errno, message) tuple. <I>Both behaviors are
acceptable.</I>
<p><LI>Jython code objects are missing other attributes --
co_code, co_consts, co_lnotab, co_names, co_nlocals,
co_stacksize. <I>co_flags is now supported because the Python
debugger requires it. Other attributes will probably never be
supported in Jython due to its implementation of code objects
as compiled Java bytecodes.</I>
<p><li>Accessing, setting, or deleting attributes on built-in
objects may raise <tt>AttributeError</tt> or
<tt>TypeError</tt> differently. <i>This is considered
implementation dependent. In Jython the following rules are
used: when getting a non-existant attribute,
<tt>AttributeError</tt> is raised; when setting or deleting a
readonly attribute, <tt>TypeError</tt> is raised; when setting
or deleting a non-existant attribute, <tt>AttributeError</tt>
is raised. Be aware though currently neither Jython nor
CPython are completely consistent.</i>
<p><li>Function objects do not have writable func_code or
func_defaults attributes. <i>While these are writable in
CPython, I haven't decided whether they should be writable in
Jython.</i>
<p><LI>Jython has "true" garbage collection whereas
CPython uses reference counting. This means that in Jython
users don't need to worry about handling circular references
as these are guaranteed to be collected properly. On the
other hand, users of Jython have no guarantees of when an
object will be finalized -- this can cause problems for people
who use open("foo", 'r').read() excessively. <I>Both
behaviors are acceptable -- and highly unlikely to change.</I>
<p><LI>The dictionaries used by classes, instances, and
modules in Jython are not the same as the dictionaries
created by {}. They are StringMap's which require all of their
keys to be strings. After "class c: pass",
c.__dict__[1] = 2 will work in CPython, but will raise a
"TypeError: keys in namespace must be strings" error
in Jython. <I>Both behaviors are acceptable -- CPython might
adopt Jython's approach in the future for the performance
gains it can provide.</I>
<p><LI>The 'b' (binary) flag parameter to the open(file, flag)
call have a different meaning for Jython. In addition to the
expected platform depending newline translation, the 'b' flag
also controls the unicode/binary translation of characters with
value > 255. When the 'b' flag is specified, only the low-order half
of each unicode character will written to the file and the high-order
byte will be set to zero when reading from a file.
Without the 'b' option, the unicode charecters will the passed
through the default codec before going to/from the file.
<p><LI>The builtin 'chr' function allows values in the range [0..65535].
</UL>
<h3>Extension modules</h3>
<UL>
<LI>Jython supports all Java packages as extension
modules. i.e. from "java.lang import System" will
work in any Jython implementation. <I>This functionality
might be added as an optional extension to some future version
of CPython.</I>
<p><LI>Jython includes the builtin module <tt>jarray</tt> -- which
allows Python programmers to create and manipulate Java array
objects.
<p><LI>Some builtin extension modules don't exist in Jython.
<UL>
<LI>The following are under consideration (working code
would make the decision much easier ;-) -- array, select,
a dbm/gdbm/bsddb style module, Numeric, cmath.
<p><LI>The following are highly unlikely any time soon --
win32com and Tkinter. However, Finn Bock has a JNI implementation called
<a href="http://jTkinter.sourceforge.net/">jTkinter</a> which
supports the full _tkinter API. Very cool stuff!
<p><LI>Let me know if something should be added to this
list.
</UL>
<p><LI>os module
<UL>
<LI>popen() and system() are missing. <I>Jython should be
fixed, patches would be graciously accepted.</I>
<p><LI>os.path.normcase() exists but may not be correctly
implemented. This one is extremely
frustrating as there seems no portable way to
implement in Java.
<p><LI>chmod(), chown(), getpid(), fork(), ... are
missing, stat() exists but is implemented
incompletely. These functions are all very Unix
specific and it is unlikely they will ever be properly
supported in a 100% Pure Java implementation.
<p>Finn Bock has created
a <a href="http://jnios.sourceforge.net/">JNI/C++
implementation for the posix module</a>
</UL>
<p><LI>The socket module is limited.
<p><LI>sys module
<UL>
<p><LI>Jython is still missing exitfunc
<p><LI>Also missing executable, getrefcount,
setcheckinterval which don't make much sense for Jython.
</UL>
<p><LI>thread module
<UL>
<p><LI>CPython's thread modules defines some aliases that
Jython's doesn't. <I>These aliases are considered
obsolete and won't be supported by Jython.</I>
</UL>
<p><li>Harry Mantakos has contributed the underlying md5
implementation, so now the md5 module works out of the box.
<p><li>The time module may produce some different values than with
CPython. This is due to Java 1.1 compatibility, and this
may be improved in later releases.
</UL>
<h3>Interpreter and environment</h3>
<UL>
<p><LI>Jython doesn't catch interrupts. <I>Only fixable with
a GUI console since interrupts are not supported by Java.</I>
<p><LI>Jython doesn't have command line editing. <I>Only
fixable with a GUI console. However, Un*x users can check out
<tt>rlterm</tt> which provides generic GNU Readline support
for any terminal based interactive program. <tt>rlterm</tt>
is part of the
<a href="ftp://ftp-icf.llnl.gov/pub/Yorick/">Yorick</a> package.</I>
<p><LI>Jython should have a feature similar to
$PYTHONSTARTUP, which specifies a script to run at the start
of interactive mode only.
<p><LI>Jython supports
<a href="interpreter.html">different command line options</a> than
CPython, e.g. "-jar" and "-D". It
also has a different convention for indicating non-interactive
standard input (the Java runtime needs help).
</UL>
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