Title: Differences between CPython and Jython

<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 &quot;1.0E20&quot; where CPython prints

&quot;1e+020&quot; -- 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 &quot;def foo(x=1): locals()['x'] =

2; print x&quot; 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 &quot;true&quot; 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.&nbsp; 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(&quot;foo&quot;, '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 &quot;class c: pass&quot;,

c.__dict__[1] = 2 will work in CPython, but will raise a

&quot;TypeError: keys in namespace must be strings&quot; 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 &quot;java.lang import System&quot; 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. &quot;-jar&quot; and &quot;-D&quot;. It

also has a different convention for indicating non-interactive

standard input (the Java runtime needs help).

</UL>