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Hephaestus is a library to statically which parses a SQL query and return an AST.

This is useful for different purposes like:

  • statistic informations, i.e. table or field used, tipe of joins, filter conditions, etc
  • partial or full query comparison
  • query type detection

The SQL grammar is specific for Exasol, but it can be easily ported to support other grammars as well.

Hephaestus's parser is built in Rust using the lalrpop crate for grammar's definition and vervolg used as a base for the SQL grammar.


To install the package you need to install the nightly version of the Rust compiler:

curl -sSf | sh -s -- -y --default-toolchain=nightly

then install the package straight from Github:

pip install git+

Note: until version 0.8 of pyo3 you need to have the python binary available in your PATH; on some systems it will fail because only python3 is present. In this case install the Python 2 package.



Hephaestus exposes a single parse() function which given a SQl string returns a dictionary representing the SQL in an AST format:

>>> import hephaestus
>>> hephaestus.parse("SELECT 1")
{'_type': 'SelectStatement',
 'common': [],
 'expr': {'_type': 'QuerySetExpression',
  'mode': {'_type': 'SelectAll'},
  'columns': {'_type': 'ListResultColumn',
   'values': [{'_type': 'ExprResultColumn',
     'expr': {'_type': 'NumericLiteral', 'num': '1'},
     'rename': None}]},
  'from': [],
  'where_expr': None,
  'group_by': None},
 'order_by': [],
 'limit': None}

Every node of the dictionary has a _type key which indicate the type of the node, i.e. SelectStatement, and a series of keys specific for the node's type which can contains scalars values like str, int, None, a dictionary representing a node or a list of scalar values or nodes.

For a full list of node types see src/ and src/


Parsing the incoming SQL expression is only the first step and its not very useful without the ability to traverse the tree and inspect the nodes.

Instead of writing from scratch the code to traverse the dictionary nested structure by yourself Hephaestus provides you a class to be used to walk through the tree and call a method for every node it encouters during the traverse.

Just inherith from the Visitor class and implement methods in the form of visit_<node_type> where <node_type> is the type of the node in the _type key.

This example shows how to check if the given SQL statement is querying from the DUAL table:

from typing import Dict

from hephaestus.walk import Visitor

class MyVisitor(Visitor):
    has_dual_table = False

    def visit_NamedTableExpression(self, attr: str, node: Dict):
        self.has_dual_table = node["name"] == ["dual"]

visitor = MyVisitor("SELECT 1 FROM dual")

assert visitor.has_dual_table is True

When the NamedTableExpression node is reached the visit_NamedTableExpression method is called and receives the parent node's attribute name attr and the node itself.


There are two set of tests, one for the Rust and one for the Python code.

Note: Unfortunately at the moment it's not possible to execute both test suites with the same Cargo.toml (looks like that the extension-module feature in pyo3 interfere with the way the tests are linked with the Rust library), so it's necessary to switch to a different config before running the them.

To run the Rust tests:

make test_rust

To run the Python tests:

make test_py


If you want to contribute to the Hephaestus project to extend the grammar follow this steps:

  1. switch to a pure Rust Cargo.toml with make rust
  2. write a test in src/tests to cover the cases
  3. extend the grammar in src/sql.lalrpop; please refer to the LALR Book
  4. add the relative AST note to src/
  5. in some cases, like defining a new Rust enumerator, it's necessary to add an explicit conversion form the Rust type to a Python-compatible type; in this add the relevant code into src/


  • not all the grammar of Exasol is supported, most of the functions and statements needs to be implemented
  • despite the fact that the AST returned by the parse() function can be manipulated at will at the moment there's no way to get back a SQL string

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