Implement Const validator to keep data unchanged (#145)

* Implement Const validator to keep data unchanged

* Make pep8 happy

* Fix travis-ci python environments

.travis.yml

@@ -18,12 +18,9 @@ cache:
 install: pip install codecov tox
 
 env:
-  - TOX_ENV=py26
   - TOX_ENV=py27
-  - TOX_ENV=py33
   - TOX_ENV=py34
   - TOX_ENV=py35
-  - TOX_ENV=pypy
   - TOX_ENV=coverage
   - TOX_ENV=pep8
 

README.rst

@@ -162,6 +162,23 @@ Dropping the details, ``Use`` is basically:
             except Exception as e:
                 raise SchemaError('%r raised %r' % (self._callable.__name__, e))
 
+
+Sometimes you need to transform and validate part of data, but keep original data unchanged.
+``Const`` helps to keep your data safe:
+
+.. code:: python
+
+    >> from schema import Use, Const, And, Schema
+
+    >> from datetime import datetime
+
+    >> is_future = lambda date: datetime.now() > date
+
+    >> to_json = lambda v: {"timestamp": v}
+
+    >> Schema(And(Const(And(Use(datetime.fromtimestamp), is_future)), Use(to_json))).validate(1234567890)
+    {"timestamp": 1234567890}
+
 Now you can write your own validation-aware classes and data types.
 
 Lists, similar containers

schema.py

@@ -6,7 +6,7 @@
 
 __version__ = '0.6.6'
 __all__ = ['Schema',
-           'And', 'Or', 'Regex', 'Optional', 'Use', 'Forbidden',
+           'And', 'Or', 'Regex', 'Optional', 'Use', 'Forbidden', 'Const',
            'SchemaError',
            'SchemaWrongKeyError',
            'SchemaMissingKeyError',
@@ -372,6 +372,12 @@ def __init__(self, *args, **kwargs):
         self.key = self._schema
 
 
+class Const(Schema):
+    def validate(self, data):
+        super(Const, self).validate(data)
+        return data
+
+
 def _callable_str(callable_):
     if hasattr(callable_, '__name__'):
         return callable_.__name__

test_schema.py

@@ -1,5 +1,6 @@
 from __future__ import with_statement
 from collections import defaultdict, namedtuple
+from functools import partial
 from operator import methodcaller
 import os
 import re
@@ -8,7 +9,7 @@
 
 from pytest import raises
 
-from schema import (Schema, Use, And, Or, Regex, Optional,
+from schema import (Schema, Use, And, Or, Regex, Optional, Const,
                     SchemaError, SchemaWrongKeyError,
                     SchemaMissingKeyError, SchemaUnexpectedTypeError,
                     SchemaForbiddenKeyError, Forbidden)
@@ -76,6 +77,28 @@ def test_or():
     with SE: Or().validate(2)
 
 
+def test_test():
+    def unique_list(_list):
+        return len(_list) == len(set(_list))
+
+    def dict_keys(key, _list):
+        return list(map(lambda d: d[key], _list))
+
+    schema = (
+        Schema(
+            Const(
+                And(Use(partial(dict_keys, "index")), unique_list))))
+    data = [
+        {"index": 1, "value": "foo"},
+        {"index": 2, "value": "bar"}]
+    assert schema.validate(data) == data
+
+    bad_data = [
+        {"index": 1, "value": "foo"},
+        {"index": 1, "value": "bar"}]
+    with SE: schema.validate(bad_data)
+
+
 def test_regex():
     # Simple case: validate string
     assert Regex(r'foo').validate('afoot') == 'afoot'