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Differences between CPython and Jython

CPython and Jython are two different implementations of the Python language. While a Language Reference 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.

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.

This list has been updated to describe the differences between Jython-2.0 and CPython 2.0


  • Jython has a different interpretation of floating point literals. CPython doesn't allow 001.1 CPython should be fixed.

  • Jython supports continue in a try clause. CPython should be fixed - but don't hold your breath.

  • 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'. Both behaviors are acceptable.

Standard types, functions and behavior

  • 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. Both behaviors are acceptable.

  • 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. Both behaviors are acceptable.

  • 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. Both behaviors are acceptable.

  • In Jython, 0.1**4 is printed as 1.0000000000000002E-4. In CPython, it is printed 0.00010000000000000005. Both behaviors are acceptable.

  • Jython sequences support three argument slices. i.e. range(3)[::-1] == [2,1,0]. CPython should be fixed.

  • 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. CPython should be fixed - but don't hold your breath.

  • 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...) Jython should be fixed.

  • 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. CPython should be fixed, but don't hold your breath.

  • The __name__ attribute of built-in extension modules (e.g. 'sys') is different. Both behaviors are acceptable.

  • In many cases, introspection yields different results. Where appropriate and possible, Jython will adhere to CPython's introspection behavior. Some differences are acceptable.

  • Jython defines __debug__, but always sets it equal to 1. Jython should implement CPython's -O option.

  • 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. Proper behavior here is still unclear.

  • Jython doesn't support restricted execution mode and doesn't have the magic __builtins__ in every namespace. Jython will probably never support restricted execution mode -- Java's security model is recommended instead.

  • Jython uses different values for the IOError argument. This causes trouble for people who unpack the value into an (errno, message) tuple. Both behaviors are acceptable.

  • Jython code objects are missing other attributes -- co_code, co_consts, co_lnotab, co_names, co_nlocals, co_stacksize. 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.

  • Accessing, setting, or deleting attributes on built-in objects may raise AttributeError or TypeError differently. This is considered implementation dependent. In Jython the following rules are used: when getting a non-existant attribute, AttributeError is raised; when setting or deleting a readonly attribute, TypeError is raised; when setting or deleting a non-existant attribute, AttributeError is raised. Be aware though currently neither Jython nor CPython are completely consistent.

  • Function objects do not have writable func_code or func_defaults attributes. While these are writable in CPython, I haven't decided whether they should be writable in Jython.

  • 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. Both behaviors are acceptable -- and highly unlikely to change.

  • 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. Both behaviors are acceptable -- CPython might adopt Jython's approach in the future for the performance gains it can provide.

  • 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.

  • The builtin 'chr' function allows values in the range [0..65535].

Extension modules

  • Jython supports all Java packages as extension modules. i.e. from "java.lang import System" will work in any Jython implementation. This functionality might be added as an optional extension to some future version of CPython.

  • Jython includes the builtin module jarray -- which allows Python programmers to create and manipulate Java array objects.

  • Some builtin extension modules don't exist in Jython.
    • The following are under consideration (working code would make the decision much easier ;-) -- array, select, a dbm/gdbm/bsddb style module, Numeric, cmath.

    • The following are highly unlikely any time soon -- win32com and Tkinter. However, Finn Bock has a JNI implementation called jTkinter which supports the full _tkinter API. Very cool stuff!

    • Let me know if something should be added to this list.

  • os module
    • popen() and system() are missing. Jython should be fixed, patches would be graciously accepted.

    • 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.

    • 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.

      Finn Bock has created a JNI/C++ implementation for the posix module

  • The socket module is limited.

  • sys module

    • Jython is still missing exitfunc

    • Also missing executable, getrefcount, setcheckinterval which don't make much sense for Jython.

  • thread module

    • CPython's thread modules defines some aliases that Jython's doesn't. These aliases are considered obsolete and won't be supported by Jython.

  • Harry Mantakos has contributed the underlying md5 implementation, so now the md5 module works out of the box.

  • 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.

Interpreter and environment

  • Jython doesn't catch interrupts. Only fixable with a GUI console since interrupts are not supported by Java.

  • Jython doesn't have command line editing. Only fixable with a GUI console. However, Un*x users can check out rlterm which provides generic GNU Readline support for any terminal based interactive program. rlterm is part of the Yorick package.

  • Jython should have a feature similar to $PYTHONSTARTUP, which specifies a script to run at the start of interactive mode only.

  • Jython supports different command line options than CPython, e.g. "-jar" and "-D". It also has a different convention for indicating non-interactive standard input (the Java runtime needs help).