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import re
from abc import ABCMeta, abstractmethod
from typing import Iterator, Mapping, Optional, Pattern
_posix_variable: Pattern[str] = re.compile(
r"""
\$\{
(?P<name>[^\}:]*)
(?::-
(?P<default>[^\}]*)
)?
\}
""",
re.VERBOSE,
)
class Atom(metaclass=ABCMeta):
def __ne__(self, other: object) -> bool:
result = self.__eq__(other)
if result is NotImplemented:
return NotImplemented
return not result
@abstractmethod
def resolve(self, env: Mapping[str, Optional[str]]) -> str: ...
class Literal(Atom):
def __init__(self, value: str) -> None:
self.value = value
def __repr__(self) -> str:
return f"Literal(value={self.value})"
def __eq__(self, other: object) -> bool:
if not isinstance(other, self.__class__):
return NotImplemented
return self.value == other.value
def __hash__(self) -> int:
return hash((self.__class__, self.value))
def resolve(self, env: Mapping[str, Optional[str]]) -> str:
return self.value
class Variable(Atom):
def __init__(self, name: str, default: Optional[str]) -> None:
self.name = name
self.default = default
def __repr__(self) -> str:
return f"Variable(name={self.name}, default={self.default})"
def __eq__(self, other: object) -> bool:
if not isinstance(other, self.__class__):
return NotImplemented
return (self.name, self.default) == (other.name, other.default)
def __hash__(self) -> int:
return hash((self.__class__, self.name, self.default))
def resolve(self, env: Mapping[str, Optional[str]]) -> str:
default = self.default if self.default is not None else ""
result = env.get(self.name, default)
return result if result is not None else ""
def parse_variables(value: str) -> Iterator[Atom]:
cursor = 0
for match in _posix_variable.finditer(value):
(start, end) = match.span()
name = match["name"]
default = match["default"]
if start > cursor:
yield Literal(value=value[cursor:start])
yield Variable(name=name, default=default)
cursor = end
length = len(value)
if cursor < length:
yield Literal(value=value[cursor:length])
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