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import contextlib
import enum
import os
from collections.abc import Generator
from collections.abc import MutableMapping
from typing import NamedTuple
from typing import Union

_Unset = enum.Enum('_Unset', 'UNSET')
UNSET = _Unset.UNSET

class Var(NamedTuple):
    name: str
    default: str = ''

SubstitutionT = tuple[Union[str, Var], ...]
ValueT = Union[str, _Unset, SubstitutionT]
PatchesT = tuple[tuple[str, ValueT], ...]

def format_env(parts: SubstitutionT, env: MutableMapping[str, str]) -> str:
    return ''.join(
        env.get(part.name, part.default) if isinstance(part, Var) else part
        for part in parts
    )

@contextlib.contextmanager
def envcontext(
        patch: PatchesT,
        _env: MutableMapping[str, str] | None = None,
) -> Generator[None]:
    """In this context, `os.environ` is modified according to `patch`.

`patch` is an iterable of 2-tuples (key, value):
        `key`: string
        `value`:
            - string: `environ[key] == value` inside the context.
            - UNSET: `key not in environ` inside the context.
            - template: A template is a tuple of strings and Var which will be
              replaced with the previous environment
    """
    env = os.environ if _env is None else _env
    before = dict(env)

for k, v in patch:
        if v is UNSET:
            env.pop(k, None)
        elif isinstance(v, tuple):
            env[k] = format_env(v, before)
        else:
            env[k] = v

try:
        yield
    finally:
        env.clear()
        env.update(before)