* Add support for empty strings in non-key attributes
https://github.com/spulec/moto/issues/3339
* Nose, not pytest
* Revert "Nose, not pytest"
This reverts commit 5a3cf6c887dd9fafa49096c82cfa3a3b7f91d224.
* PUT is default action
Part of structured approach for UpdateExpressions:
1) Expression gets parsed into a tokenlist (tokenized)
2) Tokenlist get transformed to expression tree (AST)
3) The AST gets validated (full semantic correctness) -> this commit
4) AST gets processed to perform the update
This commit uses the AST to perform validation. Validation makes sure the
nodes encounterd have valid values and they will also resolve values for
references that refer to item state or values passed into the expression.
