merge.py

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"""Merging of policies."""
 
from __future__ import annotations
 
from typing import cast
 
from .types import CategoryType, PolicyType
 
 
def merge_policies(policies: list[PolicyType]) -> PolicyType:
    """Merge policies."""
    new_policy: dict[str, CategoryType] = {}
    seen: set[str] = set()
    for policy in policies:
        for category in policy:
            if category in seen:
                continue
            seen.add(category)
            new_policy[category] = _merge_policies(
                [policy.get(category) for policy in policies]
            )
    cast(PolicyType, new_policy)
    return new_policy
 
 
def _merge_policies(sources: list[CategoryType]) -> CategoryType:
    """Merge a policy."""
    # When merging policies, the most permissive wins.
    # This means we order it like this:
    # True > Dict > None
    #
    # True: allow everything
    # Dict: specify more granular permissions
    # None: no opinion
    #
    # If there are multiple sources with a dict as policy, we recursively
    # merge each key in the source.
 
    policy: CategoryType = None
    seen: set[str] = set()
    for source in sources:
        if source is None:
            continue
 
        # A source that's True will always win. Shortcut return.
        if source is True:
            return True
 
        assert isinstance(source, dict)
 
        if policy is None:
            policy = cast(CategoryType, {})
 
        assert isinstance(policy, dict)
 
        for key in source:
            if key in seen:
                continue
            seen.add(key)
 
            key_sources = [src.get(key) for src in sources if isinstance(src, dict)]
 
            policy[key] = _merge_policies(key_sources)
 
    return policy