added file to schema

CHANGELOG.md

 # v2.15
 
-* _TBA_
+* added `file` to the schema

docs/configuration/schema.rst

@@ -15,6 +15,8 @@ you should instantiate a Descriptor. Descriptor reflects how your config is nest
 
 .. autoclass:: satella.configuration.schema.String
 
+.. autoclass:: satella.configuration.schema.File
+
 .. autoclass:: satella.configuration.schema.IPv4
 
 .. autoclass:: satella.configuration.schema.List

satella/__init__.py

-__version__ = '2.15a1'
+__version__ = '2.15'

satella/configuration/schema/basic.py

+import os
 import re
 import typing as tp
 
@@ -52,6 +53,21 @@ class String(Descriptor):
     BASIC_MAKER = str
 
 
+
+@staticmethod
+def _make_file(v: str) -> bool:
+    if not os.path.isfile(v):
+        raise ConfigurationValidationError('Expected to find a file under %s'
+                                           % (v,))
+
+
+@register_custom_descriptor('file')
+class File(Descriptor):
+    """This value must be a valid path to a file"""
+
+    BASIC_MAKER = _make_file
+
+
 class Regexp(String):
     """
     Base class for declaring regexp-based descriptors. Overload it's attribute REGEXP. Use as