mypy/mro.py

from typing import Optional, Callable, List

from mypy.nodes import TypeInfo
from mypy.types import Instance
from mypy.typestate import TypeState


def calculate_mro(info: TypeInfo, obj_type: Optional[Callable[[], Instance]] = None) -> None:
    """Calculate and set mro (method resolution order).

    Raise MroError if cannot determine mro.
    """
    mro = linearize_hierarchy(info, obj_type)
    assert mro, "Could not produce a MRO at all for %s" % (info,)
    info.mro = mro
    # The property of falling back to Any is inherited.
    info.fallback_to_any = any(baseinfo.fallback_to_any for baseinfo in info.mro)
    TypeState.reset_all_subtype_caches_for(info)


class MroError(Exception):
    """Raised if a consistent mro cannot be determined for a class."""


def linearize_hierarchy(info: TypeInfo,
                        obj_type: Optional[Callable[[], Instance]] = None) -> List[TypeInfo]:
    # TODO describe
    if info.mro:
        return info.mro
    bases = info.direct_base_classes()
    if (not bases and info.fullname() != 'builtins.object' and
            obj_type is not None):
        # Second pass in import cycle, add a dummy `object` base class,
        # otherwise MRO calculation may spuriously fail.
        # MRO will be re-calculated for real in the third pass.
        bases = [obj_type().type]
    lin_bases = []
    for base in bases:
        assert base is not None, "Cannot linearize bases for %s %s" % (info.fullname(), bases)
        lin_bases.append(linearize_hierarchy(base, obj_type))
    lin_bases.append(bases)
    return [info] + merge(lin_bases)


def merge(seqs: List[List[TypeInfo]]) -> List[TypeInfo]:
    seqs = [s[:] for s in seqs]
    result = []  # type: List[TypeInfo]
    while True:
        seqs = [s for s in seqs if s]
        if not seqs:
            return result
        for seq in seqs:
            head = seq[0]
            if not [s for s in seqs if head in s[1:]]:
                break
        else:
            raise MroError()
        result.append(head)
        for s in seqs:
            if s[0] is head:
                del s[0]