ImportError: ArrowSerializer imported unconditionally but pyarrow not in deps

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Bug Description

When installing cachekit without pyarrow, importing the package fails:

ImportError: pyarrow is not installed. ArrowSerializer requires the [data] extra: pip install 'cachekit[data]'

Root Cause

cachekit/serializers/__init__.py line 7 unconditionally imports ArrowSerializer:

from .arrow_serializer import ArrowSerializer

But pyarrow is not in the required dependencies - it's supposed to be optional.

Expected Behavior

import cachekit should work without pyarrow installed. ArrowSerializer should be lazily imported or guarded.

Reproduction

pip install cachekit  # or uv add cachekit
python -c "from cachekit import cache"
# ImportError: pyarrow is not installed...

Environment

• cachekit version: 0.3.0
• Python: 3.13
• OS: Linux

Suggested Fix

Option 1: Lazy import in __init__.py:

def __getattr__(name):
    if name == "ArrowSerializer":
        from .arrow_serializer import ArrowSerializer
        return ArrowSerializer
    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")

Option 2: Guard with try/except:

try:
    from .arrow_serializer import ArrowSerializer
except ImportError:
    ArrowSerializer = None  # type: ignore

Option 3: Add pyarrow to required deps (not recommended - adds 150MB).