[#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.

[SophieTech88]

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.

@xunliu Thank you so much. It is a great help.

[mchades]

what are the codes for?

[xunliu]

@mchades The clients/client-python/tests/unittests/test_gvfs_with_local.py report waring C0302: Too many lines in module (1102/1100) (too-many-lines), So I fixed it.

[mchades]

The type definition logic LGTM

[noidname01]

For Singleton Implementation in Python, as I used in implementing Error Handlers before, there's another approach we can consider, we can initialize the instance in separate module, and import wherever needed. This ensure the instance is Singleton.

docs need to be updated

[xunliu]

@SophieTech88 Thank you for your contributions.
LGTM