feat(context): Add priority_select for budget-aware section selection

sdk/python/feast/context/__init__.py

@@ -0,0 +1,59 @@
+"""Token-budget-aware helpers for assembling LLM context in feature views.
+
+Plain functions over immutable values, so an OnDemandFeatureView calling them
+gets the same prompt offline and online.
+"""
+
+from feast.context.errors import (
+    ContextError,
+    RequiredSectionError,
+    TokenBudgetExceededError,
+    TokenizerNotFoundError,
+    TokenizerUnavailableError,
+)
+from feast.context.priority_select import (
+    DEFAULT_SEPARATOR,
+    Priority,
+    PrioritySpec,
+    Section,
+    SectionOrder,
+    SectionSpec,
+    Selection,
+    priority_select,
+)
+from feast.context.token_budget import TokenBudget
+from feast.context.tokenizer import (
+    ApproximateTokenizer,
+    Cl100kBaseTokenizer,
+    O200kBaseTokenizer,
+    TiktokenTokenizer,
+    Tokenizer,
+    TokenizerName,
+    TokenizerSpec,
+    get_tokenizer,
+)
+
+__all__ = [
+    "ApproximateTokenizer",
+    "Cl100kBaseTokenizer",
+    "ContextError",
+    "DEFAULT_SEPARATOR",
+    "O200kBaseTokenizer",
+    "Priority",
+    "PrioritySpec",
+    "RequiredSectionError",
+    "Section",
+    "SectionOrder",
+    "SectionSpec",
+    "Selection",
+    "TiktokenTokenizer",
+    "TokenBudget",
+    "TokenBudgetExceededError",
+    "Tokenizer",
+    "TokenizerName",
+    "TokenizerNotFoundError",
+    "TokenizerSpec",
+    "TokenizerUnavailableError",
+    "get_tokenizer",
+    "priority_select",
+]

sdk/python/feast/context/errors.py

@@ -0,0 +1,84 @@
+from typing import Iterable
+
+from fastapi import status as HttpStatusCode
+
+from feast.errors import FeastError
+
+
+class ContextError(FeastError):
+    """Base class for all feast.context failures."""
+
+
+class TokenizerNotFoundError(ContextError):
+    def __init__(self, name: str, available: Iterable[str]):
+        super().__init__(
+            f"Unknown tokenizer '{name}'. Built-in tokenizers: "
+            f"{', '.join(sorted(available))}. For your own, pass the class "
+            f"path of a Tokenizer subclass, e.g. 'my_pkg.MyTokenizer'."
+        )
+
+    def http_status_code(self) -> int:
+        return HttpStatusCode.HTTP_400_BAD_REQUEST
+
+
+class TokenizerUnavailableError(ContextError):
+    """A known tokenizer cannot be loaded.
+
+    Covers a missing dependency, and one present but unable to load its
+    encoding — tiktoken fetching a BPE file on a host with no network, say.
+    """
+
+    def __init__(self, name: str, reason: str, install_hint: str):
+        super().__init__(
+            f"Tokenizer '{name}' is unavailable: {reason}. Install its "
+            f"dependencies with: {install_hint}"
+        )
+        self.tokenizer_name = name
+
+    def http_status_code(self) -> int:
+        return HttpStatusCode.HTTP_400_BAD_REQUEST
+
+
+class TokenBudgetExceededError(ContextError):
+    def __init__(
+        self,
+        requested: int,
+        remaining: int,
+        max_tokens: int,
+        *,
+        message: str = "",
+    ):
+        super().__init__(
+            message
+            or (
+                f"Cannot consume {requested} tokens: only {remaining} of "
+                f"{max_tokens} tokens remain in the budget."
+            )
+        )
+        self.requested = requested
+        self.remaining = remaining
+        self.max_tokens = max_tokens
+
+    def http_status_code(self) -> int:
+        return HttpStatusCode.HTTP_400_BAD_REQUEST
+
+
+class RequiredSectionError(TokenBudgetExceededError):
+    """A section that must not be dropped does not fit the budget.
+
+    Raised instead of quietly returning a prompt missing its instructions:
+    the caller has to widen the budget or shorten the section.
+    """
+
+    def __init__(self, section: str, requested: int, remaining: int, max_tokens: int):
+        super().__init__(
+            requested,
+            remaining,
+            max_tokens,
+            message=(
+                f"Section {section} is critical and cannot be dropped, but it "
+                f"needs {requested} tokens with only {remaining} of "
+                f"{max_tokens} left in the budget."
+            ),
+        )
+        self.section = section