Adds filetype documentation and up the version number

Author: ESultanik

docs/filetypes.rst

@@ -3,3 +3,50 @@
 Defining New Filetypes
 ======================
 
+Implementing support for a new Graphtage filetype entails extending the :class:`graphtage.Filetype` class. Subclassing :class:`graphtage.Filetype` automatically registers it with Graphtage.
+
+Filetype Matching
+-----------------
+
+Input files are matched to an associated :class:`graphtage.Filetype` using MIME types. Each :class:`graphtage.Filetype` registers one or more MIME types for which it will be responsible. Input file MIME types are classified using the :mod:`mimetypes` module. Sometimes a filetype does not have a standardized MIME type or is not properly classified by the :mod:`mimetypes` module. For example, Graphtage's :class:`graphtage.pickle.Pickle` filetype has neither. You can add support for such a filetype as follows:
+
+.. code-block:: python
+
+    import mimetypes
+
+    if '.pkl' not in mimetypes.types_map and '.pickle' not in mimetypes.types_map:
+        mimetypes.add_type('application/x-python-pickle', '.pkl')
+        mimetypes.suffix_map['.pickle'] = '.pkl'
+
+Implementing a New Filetype
+---------------------------
+
+With the MIME type registered, here is a sketch of how one might define the Pickle filetype:
+
+.. code-block:: python
+
+    from graphtage import BuildOptions, Filetype, Formatter, TreeNode
+
+    class Pickle(Filetype):
+        def __init__(self):
+            super().__init__(
+                "pickle",                      # a unique identifier
+                "application/python-pickle",   # the primary MIME type
+                "application/x-python-pickle"  # an optional secondary MIME type
+            )
+
+        def build_tree(self, path: str, options: Optional[BuildOptions] = None) -> TreeNode:
+            # return the root node of the tree built from the given pickle file
+
+        def build_tree_handling_errors(self, path: str, options: Optional[BuildOptions] = None) -> Union[str, TreeNode]:
+            # the same as the build_tree() function,
+            # but on error return a string containing the error message
+            #
+            # for example:
+            try:
+                return self.build_tree(path=path, options=options)
+            except PickleDecodeError as e:
+                return f"Error deserializing {os.path.basename(path)}: {e!s}"
+
+        def get_default_formatter(self) -> GraphtageFormatter:
+            # return the formatter associated with this file type

graphtage/version.py

@@ -59,7 +59,7 @@ def git_branch() -> Optional[str]:
 """
 
 
-__version__: Tuple[Union[int, str], ...] = (0, 3, 0)
+__version__: Tuple[Union[int, str], ...] = (0, 3, 1)
 
 if DEV_BUILD:
     branch_name = git_branch()