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:

tests/test_dynamodb2/test_dynamodb_table_with_range_key.py

@@ -1016,6 +1016,63 @@ def test_boto3_conditions():
     results['Count'].should.equal(1)
 
 
+@mock_dynamodb2
+def test_boto3_put_item_with_conditions():
+    import botocore
+    dynamodb = boto3.resource('dynamodb', region_name='us-east-1')
+
+    # Create the DynamoDB table.
+    table = dynamodb.create_table(
+        TableName='users',
+        KeySchema=[
+            {
+                'AttributeName': 'forum_name',
+                'KeyType': 'HASH'
+            },
+            {
+                'AttributeName': 'subject',
+                'KeyType': 'RANGE'
+            },
+        ],
+        AttributeDefinitions=[
+            {
+                'AttributeName': 'forum_name',
+                'AttributeType': 'S'
+            },
+            {
+                'AttributeName': 'subject',
+                'AttributeType': 'S'
+            },
+        ],
+        ProvisionedThroughput={
+            'ReadCapacityUnits': 5,
+            'WriteCapacityUnits': 5
+        }
+    )
+    table = dynamodb.Table('users')
+
+    table.put_item(Item={
+        'forum_name': 'the-key',
+        'subject': '123'
+    })
+
+    table.put_item(
+        Item={
+            'forum_name': 'the-key-2',
+            'subject': '1234',
+        },
+        ConditionExpression='attribute_not_exists(forum_name) AND attribute_not_exists(subject)'
+    )
+
+    table.put_item.when.called_with(
+        Item={
+            'forum_name': 'the-key',
+            'subject': '123'
+        },
+        ConditionExpression='attribute_not_exists(forum_name) AND attribute_not_exists(subject)'
+    ).should.throw(botocore.exceptions.ClientError)
+
+
 def _create_table_with_range_key():
     dynamodb = boto3.resource('dynamodb', region_name='us-east-1')