Refactor out _tofile/_fromfile from DirectoryStore (#503)

* Add `_fromfile` and `_tofile` methods

To make it easier to sub out different ways of reading and writing data,
create to methods to handle reading and writing directly. This way users
don't need to reimplement all of the same logic that `__getitem__` and
`__setitem__` are doing and can focus on simply reading and writing. Can
be useful for trying memmaping for example.

* Use _fromfile/_tofile in __getitem__/__set item__

* Update zarr/storage.py

Co-Authored-By: James Bourbeau <jrbourbeau@users.noreply.github.com>

* Add docstrings to `_fromfile` and `_tofile`

* No need to return in `_tofile`

* Adds release notes entry

Author: jakirkham

docs/release.rst

@@ -40,6 +40,9 @@ Upcoming Release
 * Use ``math.ceil`` for scalars.
   By :user:`John Kirkham <jakirkham>`; :issue:`500`.
 
+* Refactor out ``_tofile``/``_fromfile`` from ``DirectoryStore``.
+  By :user:`John Kirkham <jakirkham>`; :issue:`503`.
+
 .. _release_2.3.2:
 
 2.3.2

zarr/storage.py

@@ -746,12 +746,46 @@ def __init__(self, path, normalize_keys=False):
     def _normalize_key(self, key):
         return key.lower() if self.normalize_keys else key
 
+    def _fromfile(self, fn):
+        """ Read data from a file
+
+        Parameters
+        ----------
+        fn: str
+            Filepath to open and read from.
+
+        Notes
+        -----
+        Subclasses should overload this method to specify any custom
+        file reading logic.
+        """
+        with open(fn, 'rb') as f:
+            return f.read()
+
+    def _tofile(self, a, fn):
+        """ Write data to a file
+
+        Parameters
+        ----------
+        a: array-like
+            Data to write into the file.
+
+        fn: str
+            Filepath to open and write to.
+
+        Notes
+        -----
+        Subclasses should overload this method to specify any custom
+        file writing logic.
+        """
+        with open(fn, mode='wb') as f:
+            f.write(a)
+
     def __getitem__(self, key):
         key = self._normalize_key(key)
         filepath = os.path.join(self.path, key)
         if os.path.isfile(filepath):
-            with open(filepath, 'rb') as f:
-                return f.read()
+            return self._fromfile(filepath)
         else:
             raise KeyError(key)
 
@@ -784,8 +818,7 @@ def __setitem__(self, key, value):
         temp_name = file_name + '.' + uuid.uuid4().hex + '.partial'
         temp_path = os.path.join(dir_path, temp_name)
         try:
-            with open(temp_path, mode='wb') as f:
-                f.write(value)
+            self._tofile(value, temp_path)
 
             # move temporary file into place
             replace(temp_path, file_path)