opencypher 0.3.0

# opencypher

[OpenCypher](https://www.opencypher.org/) AST and Builder API

OpenCypher defines an _abstract syntax tree_ based on the published OpenCypher
[EBNF grammar](https://s3.amazonaws.com/artifacts.opencypher.org/cypher.ebnf)
and a builder-oriented API for constructing Cypher queries.

OpenCypher leans heavily on Python 3.7 `dataclasses` and `typing`.


## Setup

pip install opencypher


## Usage

The core API exposes a fluent builder interface for constructing queries and patterns:

from opencypher.api import match, node

query = match(
node("person", "Person").rel_in().node("pet", "Pet")
).ret(
"person",
"pet",
)

print(query) # MATCH (person:Person)-[]->(pet :Pet) RETURN person, pet

The builder supports chaining patterns and chaining clauses; queries may terminate on either
a return statement (`.ret()`) or on any updating clause (e.g. `create()`, `delete()`, `merge()`,
and so forth.

from opencypher.api import match, node

query = match(
node("alice", "Person", {"name": "Alice"}),
).match(
node("bob", "Person", {"name": "Bob"}),
).merge(
node("bob").rel_in(types="IS_FRIENDS_WITH").node("alice"),
).merge(
node("alice").rel_in(types="IS_FRIENDS_WITH").node("bob"),
)

The resulting `Cypher` query object integrates with -- but does not depend on -- the
[Neo4J Python driver](https://github.com/neo4j/neo4j-python-driver):

from neo4j import GraphDatabase

driver = GraphDatabase.driver("bolt://localhost:7687", auth=("neo4j", "password"))

with driver.session() as session:
session.run(str(query), dict(query))


## Caveats

Some compromises have been made with respect to the completeness of the AST:

1. The grammar for expressions (and literals, atoms, etc.) has been deliberately simplified
and reduces to `str` in many cases. The expression grammar is likely to get more complete
over time.

2. Some forms of argumementation (e.g. `Set` items) are not easy to construct using the builder
API (although these remain available within the AST).

3. Several top level query clauses are not yet implemented, including:

- `CALL`
- `WITH`

4. Parameters do not automatically generate unique identifiers/prefixes. Parameter names will be
derived from variable names where known, but no fallback exists yet for anonyomous pattern terms.

5. Parameters do not support numeric values (`$1`); symbolic names (`$foo`) *are* supported.