Better DDB expressions support2: ExpressionTree

Part of structured approach for UpdateExpressions:
 1) Expression gets parsed into a tokenlist (tokenized)
 2) Tokenlist get transformed to expression tree (AST) -> This commit
 3) The AST gets validated (full semantic correctness)
 4) AST gets processed to perform the update

This commit uses the tokenlist to build an expression tree. This tree is not
yet used. Still it allows to raise additional Validation Exceptions which
previously were missed silently therefore it allows tests to catch these type of
ValidationException. For that reason DDB UpdateExpressions will be parsed
already. It also makes sure we won't break existing tests.

One of the existing tests had to be changed in order to still pass:
 - test_dynamodb_table_with_range_key.test_update_item_with_expression

This test passed in a numeric literal which is not supported by DynamoDB
and with the current tokenization it would get the same error as in AWS
DynamoDB.

tests/test_dynamodb2/test_dynamodb.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals, print_function
 
+import re
 from decimal import Decimal
 
 import six
@@ -4177,3 +4178,70 @@ def test_gsi_verify_negative_number_order():
     [float(item["gsiK1SortKey"]) for item in resp["Items"]].should.equal(
         [-0.7, -0.6, 0.7]
     )
+
+
+def assert_raise_syntax_error(client_error, token, near):
+    """
+    Assert whether a client_error is as expected Syntax error. Syntax error looks like: `syntax_error_template`
+
+    Args:
+        client_error(ClientError): The ClientError exception that was raised
+        token(str): The token that ws unexpected
+        near(str): The part in the expression that shows where the error occurs it generally has the preceding token the
+        optional separation and the problematic token.
+    """
+    syntax_error_template = (
+        'Invalid UpdateExpression: Syntax error; token: "{token}", near: "{near}"'
+    )
+    expected_syntax_error = syntax_error_template.format(token=token, near=near)
+    assert client_error.response["Error"]["Code"] == "ValidationException"
+    assert expected_syntax_error == client_error.response["Error"]["Message"]
+
+
+@mock_dynamodb2
+def test_update_expression_with_numeric_literal_instead_of_value():
+    """
+    DynamoDB requires literals to be passed in as values. If they are put literally in the expression a token error will
+    be raised
+    """
+    dynamodb = boto3.client("dynamodb", region_name="eu-west-1")
+
+    dynamodb.create_table(
+        TableName="moto-test",
+        KeySchema=[{"AttributeName": "id", "KeyType": "HASH"}],
+        AttributeDefinitions=[{"AttributeName": "id", "AttributeType": "S"}],
+    )
+
+    try:
+        dynamodb.update_item(
+            TableName="moto-test",
+            Key={"id": {"S": "1"}},
+            UpdateExpression="SET MyStr = myNum + 1",
+        )
+        assert False, "Validation exception not thrown"
+    except dynamodb.exceptions.ClientError as e:
+        assert_raise_syntax_error(e, "1", "+ 1")
+
+
+@mock_dynamodb2
+def test_update_expression_with_multiple_set_clauses_must_be_comma_separated():
+    """
+    An UpdateExpression can have multiple set clauses but if they are passed in without the separating comma.
+    """
+    dynamodb = boto3.client("dynamodb", region_name="eu-west-1")
+
+    dynamodb.create_table(
+        TableName="moto-test",
+        KeySchema=[{"AttributeName": "id", "KeyType": "HASH"}],
+        AttributeDefinitions=[{"AttributeName": "id", "AttributeType": "S"}],
+    )
+
+    try:
+        dynamodb.update_item(
+            TableName="moto-test",
+            Key={"id": {"S": "1"}},
+            UpdateExpression="SET MyStr = myNum Mystr2 myNum2",
+        )
+        assert False, "Validation exception not thrown"
+    except dynamodb.exceptions.ClientError as e:
+        assert_raise_syntax_error(e, "Mystr2", "myNum Mystr2 myNum2")