1  import re
   2  from abc import ABCMeta, abstractmethod
   3  from typing import Iterator, Mapping, Optional, Pattern
   4  
   5  _posix_variable: Pattern[str] = re.compile(
   6      r"""
   7      \$\{
   8          (?P<name>[^\}:]*)
   9          (?::-
  10              (?P<default>[^\}]*)
  11          )?
  12      \}
  13      """,
  14      re.VERBOSE,
  15  )
  16  
  17  
  18  class Atom(metaclass=ABCMeta):
  19      def __ne__(self, other: object) -> bool:
  20          result = self.__eq__(other)
  21          if result is NotImplemented:
  22              return NotImplemented
  23          return not result
  24  
  25      @abstractmethod
  26      def resolve(self, env: Mapping[str, Optional[str]]) -> str: ...
  27  
  28  
  29  class Literal(Atom):
  30      def __init__(self, value: str) -> None:
  31          self.value = value
  32  
  33      def __repr__(self) -> str:
  34          return f"Literal(value={self.value})"
  35  
  36      def __eq__(self, other: object) -> bool:
  37          if not isinstance(other, self.__class__):
  38              return NotImplemented
  39          return self.value == other.value
  40  
  41      def __hash__(self) -> int:
  42          return hash((self.__class__, self.value))
  43  
  44      def resolve(self, env: Mapping[str, Optional[str]]) -> str:
  45          return self.value
  46  
  47  
  48  class Variable(Atom):
  49      def __init__(self, name: str, default: Optional[str]) -> None:
  50          self.name = name
  51          self.default = default
  52  
  53      def __repr__(self) -> str:
  54          return f"Variable(name={self.name}, default={self.default})"
  55  
  56      def __eq__(self, other: object) -> bool:
  57          if not isinstance(other, self.__class__):
  58              return NotImplemented
  59          return (self.name, self.default) == (other.name, other.default)
  60  
  61      def __hash__(self) -> int:
  62          return hash((self.__class__, self.name, self.default))
  63  
  64      def resolve(self, env: Mapping[str, Optional[str]]) -> str:
  65          default = self.default if self.default is not None else ""
  66          result = env.get(self.name, default)
  67          return result if result is not None else ""
  68  
  69  
  70  def parse_variables(value: str) -> Iterator[Atom]:
  71      cursor = 0
  72  
  73      for match in _posix_variable.finditer(value):
  74          (start, end) = match.span()
  75          name = match["name"]
  76          default = match["default"]
  77  
  78          if start > cursor:
  79              yield Literal(value=value[cursor:start])
  80  
  81          yield Variable(name=name, default=default)
  82          cursor = end
  83  
  84      length = len(value)
  85      if cursor < length:
  86          yield Literal(value=value[cursor:length])
  87