Added LimitedLengthStringField

plenum/common/messages/fields.py

@@ -90,6 +90,22 @@ def _specific_validation(self, val):
             return 'empty string'
 
 
+class LimitedLengthStringField(FieldBase):
+    _base_types = (str,)
+
+    def __init__(self, max_length: int, **kwargs):
+        assert max_length > 0, 'should be greater than 0'
+        super().__init__(**kwargs)
+        self._max_length = max_length
+
+    def _specific_validation(self, val):
+        if not val:
+            return 'empty string'
+        if len(val) > self._max_length:
+            val = val[:100] + ('...' if len(val) > 100 else '')
+            return '{} is longer than {} symbols'.format(val, self._max_length)
+
+
 class SignatureField(FieldBase):
     _base_types = (str, type(None))
     # TODO do nothing because EmptySignature should be raised somehow

plenum/test/input_validation/fields_validation/test_limited_length_string_field.py

@@ -0,0 +1,22 @@
+import pytest
+from plenum.common.messages.fields import LimitedLengthStringField
+
+
+def test_incorrect_max_length():
+    with pytest.raises(Exception):
+        LimitedLengthStringField(max_length=0)
+
+
+def test_empty_string():
+    validator = LimitedLengthStringField(max_length=1)
+    assert validator.validate("")
+
+
+def test_valid_string():
+    validator = LimitedLengthStringField(max_length=1)
+    assert not validator.validate("x")
+
+
+def test_long_string():
+    validator = LimitedLengthStringField(max_length=1)
+    assert validator.validate("xx")