Python types for polyglot objects

[antonykamp]

This PR was developed during the seminar "Softwaredesign" at the Hasso-Plattner Institute Potsdam
Authors: @Olliwehr, @antonykamp
Supervisors: @timfel @JensLincke

What did we change?

This PR introduces the CRUD operations for a register of Python types as equivalents for Java classes.
In the following example, we want to use instances of java.util.ArrayList in the Python code. To have a Python idiomatic interface, we register with polyglot.register_java_interop_type or @polyglot.java_interop_type, a Python type jlist equivalent to java.util.ArrayList.

import java
import polyglot

class jList(__graalpython__.ForeignType):
    def append(self, element):
        self.add(element)

polyglot.register_java_interop_type('java.util.ArrayList', jList)
l = java.type('java.util.ArrayList')()
assert isinstance(l, jList) # == true

We used the existing interoperability extension API as reference.

New operations

We introduced CRUD operations to insert, update, delete, and get mappings and integrated the first three operations into the polyglot module.

Register type mappings with polyglot.register_java_interop_type(javaClassNameAsString, pythonType) or @polyglot.java_interop_type(javaClassNameAsString).

Edit mappings by calling the same function and passing the flag overwrite=True. Otherwise, when registering the same java class two times, it'll throw an Error INTEROP_TYPE_ALREADY_REGISTERED.

Remove a mapping by calling remove_java_interop_type(javaClassNameAsString). If the class is not registered, it will throw an Error INTEROP_TYPE_NOT_REGISTERED.

Whenever we call getClassNode, we look for a registered type for the class. Alternatively, we return Foreign.

How did we change it?

We introduced a WeakHashMap in the Python context. In the map, the keys are string representations of the Java classes, and the values are Python types. In getClassNode, we look the Java classes up in the map. If the key is not found, we return ForeignType as the default Python type.

Additionally, we introduced:

• register_java_interop_type function,
• java_interop_type decorator and
• remove_java_interop_type function

to the polyglot module.

Future work

We identified two possible future work items:

• support multi-inheritance in Python types,
• support Java interfaces

Support multi-inheritance in Python types

In Python types, e.g., list, some functions are based on "base functions." The "base function" is building the base for other functions. It would be helpful to inherit from these types, implement the "base function" and get some tasks for free:

class jList(__graalpython__.ForeignType, list):
    def append(self, element):
        self.add(element)

Currently, that's not possible. We receive a "Layout Conflict Error" because ForeignType has some predefined attributes and functions.

TypeError: multiple bases have instance lay-out conflict

We recommend splitting ForeignType into:

• ForeignType with the current behavior and
• ForeignTypeBase without the predefined attributes and functions.

If we'd inherit from ForeignTypeBase and list now, we probably wouldn't receive the "Layout Conflict Error" but some functions for free.

Support java interfaces

If we want the Python idiomatic interface for different list classes (ArrayList, LinkedList, etc.), we'd register (and unregister) the same Python type repeatedly.

The idea is that we only register a jlist type for the interface java.util.List, and it's applied to ArrayList, LinkedList, and... by default.

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[timfel]

@Sowasvonbot will integrate this in the next months.