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Differences with Python

  • py.js completely ignores old-style classes as well as their lookup details. All py.js types should be considered matching the behavior of new-style classes

  • New types can only have a single base. This is due to py.js implementing its types on top of Javascript’s, and javascript being a single-inheritance language.

    This may change if py.js ever reimplements its object model from scratch.

  • Piggybacking on javascript’s object model also means metaclasses are not available (py.type() is a function)
  • A python-level function (created through py.PY_def()) set on a new type will not become a method, it’ll remain a function.
  • py.PY_parseArgs() supports keyword-only arguments (though it’s a Python 3 feature)
  • Because the underlying type is a javascript String, there currently is no difference between py.str() and py.unicode(). As a result, there also is no difference between __str__() and __unicode__().

Unsupported features

These are Python features which are not supported at all in py.js, usually because they don’t make sense or there is no way to support them

  • The __delattr__, __delete__ and __delitem__: as py.js only handles expressions and these are accessed via the del statement, there would be no way to call them.
  • __del__ the lack of cross-platform GC hook means there is no way to know when an object is deallocated.
  • __slots__ are not handled
  • Dedicated (and deprecated) slicing special methods are unsupported

Missing features

These are Python features which are missing because they haven’t been implemented yet:

  • Class-binding of descriptors doesn’t currently work.
  • Instance and subclass checks can’t be customized
  • “poor” comparison methods (__cmp__ and __rcmp__) are not supported and won’t be falled-back to.
  • __coerce__ is currently supported
  • Context managers are not currently supported
  • Unbound methods are not supported, instance methods can only be accessed from instances.