[#5200]Update the Column data type for python client

[SophieTech88]

What changes were proposed in this pull request?

1. Implement type.java as type.py in Python client
2. Implement types.java as types.py in Python client
3. Create a test_types.py in unit tests.

Why are the changes needed?

We need to support the column data type in python client

Fix: #5200

Does this PR introduce any user-facing change?

No

How was this patch tested?

Need to add the unit tests

[jerryshao]

@mchades can you please help to review?

[jerryshao]

@noidname01 , would you please also help to review this PR? Thanks a lot.

[SophieTech88]

When I double check the types folder (gravitino/tree/main/api/src/main/java/org/apache/gravitino/rel/types/) I find it has another file Decimal.java, so just update a new commit to convert decimal.java to decimal.py with unit test in python client folder.

[mchades]

I'm not sure if these codes can guarantee the singleton of _instance under race conditions in Python, can you help confirm it? @xunliu @noidname01

[SophieTech88]

Thank you for catching that! You’re absolutely correct—this implementation does not currently handle race conditions, and under high concurrency, there’s a chance multiple threads could attempt to create the singleton instance simultaneously, breaking the singleton pattern.

Solution: Adding Thread Safety with a Lock
To handle this, we can use threading.Lock, which ensures that only one thread can enter the critical section where the instance is created.

Here is the example:

class template:
    _instance = None
    _lock = threading.Lock()  # A lock to ensure thread safety

    @classmethod
    def get(cls):
        with cls._lock:  # Ensures only one thread can access this block at a time
            if cls._instance is None:
                cls._instance = cls()
        return cls._instance

[mchades]

Lock makes things more complex, is there another simple common method?

[SophieTech88]

Another option is to use __new__():

class Types:

    class NullType(Type):
        _instance = None

        @classmethod
        def get(cls):
            if cls._instance is None:
                cls._instance = cls.__new__(cls)
            return cls._instance

logic here:

• get() checks if _instance is None.
• If _instance is None, cls.new(cls) is called to create a new instance of NullType without directly calling the constructor init.
• get() then returns the instance. Subsequent calls to get() will return the existing _instance.

[jerryshao]

Does Python support singleton?

[SophieTech88]

Yes, Python does support the Singleton pattern, which ensures that only one instance of a class is created. While Python does not enforce singletons natively, you can implement it in several ways.

[xunliu]

hi @SophieTech88
I help you improve this PR, please help me review my change parts.

1. We can use __new__() implement singleton.
2. Added some return type and function param type.
3. Added some class and function comments.

[mchades]

should return _instance directly?

[xunliu]

return cls() will called __new__() function.