separate relative path and pytex path

PyTeX/paths/relative_path.py

@@ -0,0 +1,54 @@
+import os
+from pathlib import Path, WindowsPath, PosixPath
+
+from PyTeX.config import GlobalPyTeXConfig
+from PyTeX.paths import PyTeXRootDirType
+
+
+class RelativePath(Path):
+    """
+    Represents a path that knows of its corresponding root directory
+    """
+    def __new__(cls, pytex_root_dir_type: PyTeXRootDirType, *args, **kwargs):
+        if cls is RelativePath:
+            cls = RelativeWindowsPath if os.name == 'nt' else RelativePosixPath
+        self = cls._from_parts(args, init=False)
+        if not self._flavour.is_supported:
+            raise NotImplementedError("cannot instantiate %r on your system"
+                                      % (cls.__name__,))
+        self._init()
+        self.__init(pytex_root_dir_type)
+        return self
+
+    def __init(self, pytex_root_dir_type: PyTeXRootDirType):
+        self._pytex_root_dir_type: PyTeXRootDirType = pytex_root_dir_type
+        self._root_dir: Path = {
+            PyTeXRootDirType.BUILD: GlobalPyTeXConfig.build_root(),
+            PyTeXRootDirType.PYTEX_SOURCE: GlobalPyTeXConfig.source_root(),
+            PyTeXRootDirType.DOC: GlobalPyTeXConfig.doc_root(),
+            PyTeXRootDirType.TEX_SOURCE: GlobalPyTeXConfig.tex_root()
+        }[self._pytex_root_dir_type]
+        try:
+            self._relative_path = self.relative_to(self._root_dir)
+        except ValueError as e:
+            raise ValueError  # TODO
+
+    @property
+    def root_dir_type(self) -> PyTeXRootDirType:
+        return self._pytex_root_dir_type
+
+    @property
+    def root_dir(self) -> Path:
+        return self._root_dir
+
+    @property
+    def relative_path(self):
+        return self._relative_path
+
+
+class RelativeWindowsPath(RelativePath, WindowsPath):
+    pass
+
+
+class RelativePosixPath(RelativePath, PosixPath):
+    pass