add optional validation to ValueType

* add optional parameter for ValueType: validator
used for checking, if a value meets a criteria (e.g. is dict)
+ InternalParameter, which is not exported and can hold any python value

Change-Id: If39a7a4a8019f2aa1a930e42cbef4fca59163b78
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/30787
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Alexander Zaft <a.zaft@fz-juelich.de>

frappy/datatypes.py

@@ -28,8 +28,8 @@
 import sys
 from base64 import b64decode, b64encode
 
-from frappy.errors import WrongTypeError, RangeError, \
-    ConfigError, ProgrammingError, ProtocolError, DiscouragedConversion
+from frappy.errors import ConfigError, DiscouragedConversion, \
+    ProgrammingError, ProtocolError, RangeError, WrongTypeError
 from frappy.lib import clamp, generalConfig
 from frappy.lib.enum import Enum
 from frappy.parse import Parser
@@ -1161,11 +1161,35 @@ class DataTypeType(DataType):
 
 
 class ValueType(DataType):
-    """validates any python value"""
+    """Can take any python value.
+
+    The optional (callable) validator can be used to restrict values to a
+    certain type.
+    For example using `ValueType(dict)` would ensure only values that can be
+    turned into a dictionary can be used in this instance, as the conversion
+    `dict(value)` is called for validation.
+
+    Notes:
+    The validator must either accept a value by returning it or the converted value,
+    or raise an error.
+    """
+    def __init__(self, validator=None):
+        super().__init__()
+        self.validator = validator
+
     def __call__(self, value):
-        """accepts any type -> no conversion"""
+        """accepts any type -> default is no conversion"""
+        if self.validator:
+            try:
+                return self.validator(value)
+            except Exception as e:
+                raise ConfigError('Validator %s raised %r for value %s' \
+                                           % (self.validator, e, value)) from e
         return value
 
+    def copy(self):
+        return ValueType(self.validator)
+
     def export_value(self, value):
         """if needed, reformat value for transport"""
         return value