Attempt to parse simple ConditionExpressions in DynamoDB put_item().

Internally these get converted into simpler Expected={} parameters, so
for now only `attribute_exists` and `attribute_not_exists` are supported;
OR is also not suported since `expected` does not support it.

moto/dynamodb2/responses.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 import json
 import six
+import re
 
 from moto.core.responses import BaseResponse
 from moto.core.utils import camelcase_to_underscores
@@ -150,6 +151,26 @@ class DynamoHandler(BaseResponse):
         else:
             expected = None
 
+        # Attempt to parse simple ConditionExpressions into an Expected expression
+        if not expected:
+            condition_expression = self.body.get('ConditionExpression')
+            if condition_expression and 'OR' not in condition_expression:
+                cond_items = [c.strip() for c in condition_expression.split('AND')]
+
+                if cond_items:
+                    expected = {}
+                    overwrite = False
+                    exists_re = re.compile('^attribute_exists\((.*)\)$')
+                    not_exists_re = re.compile('^attribute_not_exists\((.*)\)$')
+
+                for cond in cond_items:
+                    exists_m = exists_re.match(cond)
+                    not_exists_m = not_exists_re.match(cond)
+                    if exists_m:
+                        expected[exists_m.group(1)] = {'Exists': True}
+                    elif not_exists_m:
+                        expected[not_exists_m.group(1)] = {'Exists': False}
+
         try:
             result = dynamodb_backend2.put_item(name, item, expected, overwrite)
         except Exception: