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pyrql

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Overview

Resource Query Language (RQL) is a query language designed for use in URIs, with object-style data structures.

This library provides a Python parser that produces output identical to the JavaScript Library, and a query engine that can perform RQL queries on lists of dictionaries.

Installing

pip install pyrql

RQL Syntax

The RQL syntax is a compatible superset of the standard HTML form URL encoding. Simple queries can be written in standard HTML form URL encoding, but more complex queries can be written in a URL friendly query string, using a set of nested operators. For example, querying for a property foo with the value of 3 could be written as:

eq(foo,3)

Or in standard HTML form URL encoding:

foo=3

Both expressions result in the exact same parsed value:

{'name': 'eq', 'args': ['foo', 3]}

Typed Values

The following types are available:

  • string
  • number
  • boolean
  • null
  • epoch
  • date
  • datetime
  • uuid
  • decimal

Numbers, booleans and null are converted automatically to the corresponding Python types. Numbers are converted to float or integer accordingly:

>>> pyrql.parse('ten=10')
{'name': 'eq', 'args': ['ten', 10]}
>>> pyrql.parse('pi=3.14')
{'name': 'eq', 'args': ['pi', 3.14]}
>>> pyrql.parse('mil=1e6')
{'name': 'eq', 'args': ['mil', 1000000.0]}

Booleans and null are converted to booleans and None:

>>> pyrql.parse('a=true')
{'name': 'eq', 'args': ['a', True]}
>>> pyrql.parse('a=false')
{'name': 'eq', 'args': ['a', False]}
>>> pyrql.parse('a=null')
{'name': 'eq', 'args': ['a', None]}

Types can be used explicitly in the form type:value:

>>> pyrql.parse('a=string:1')
{'name': 'eq', 'args': ['a', '1']}

URL encoding

The parser automatically unquotes strings with percent-encoding, but it also accepts characters that would require encoding if submitted on an URI.

>>> pyrql.parse('eq(foo,lero lero)')
{'name': 'eq', 'args': ['foo', 'lero lero']}
>>> pyrql.parse('eq(foo,lero%20lero)')
{'name': 'eq', 'args': ['foo', 'lero lero']}

If that's undesirable, you should verify the URL before calling the parser.

Limitations

The pyrql parser doesn't implement a few redundant details of the RQL syntax, either because the standard isn't clear on what's allowed, or the functionality is already available in a clearer syntax.

The only operator allowed at the query top level is the AND operator, i.e. &. A toplevel or operation using the | operator must be enclosed in parenthesis.

>>> pyrql.parse('(a=1|b=2)')
{'args': [{'args': ['a', 1], 'name': 'eq'}, {'args': ['b', 2], 'name': 'eq'}], 'name': 'or'}

The slash syntax for arrays is not implemented yet and will result in a syntax error. The only valid array syntax is the comma delimited list inside parenthesis:

>>> pyrql.parse('(a,b)=1')
{'args': [('a', 'b'), 1], 'name': 'eq'}

Query Engine

The main use case for the query engine is to allow API clients to perform server-side filtering on large responses on their own. It's an easy drop-in improvement when you want to provide simple querying capabilities on an existing API endpoint without exposing your storage, or reimplementing everything in a more complete querying solution like GraphQL.

The data is fed through the operators in the query from left to right, as a pipeline, where the results of each top-level operator are fed to the next. If you're familiar with MongoDB aggregation pipelines, the query engine follows a similar concept, where each step transforms the current state of the data before being fed to the next step.

The operators can be categorized in three types:

  • Filtering operators, which filter the data, like comparison and membership operators.
  • Transforming operators, which transform all the data at once, like select, sort and aggregate.
  • Aggregation operators, which reduce all data to a single value, like sum and min.

See the reference below for all operators and the equivalent Python code.

Example

For example, if you have a Flask API with an endpoint exposing tasks, like this:

@app.route('/api/v1/tasks')
def get_user_tasks():
    tasks = [task.to_dict() for task in Task.get_all()]
    return jsonify(tasks)

Adding pyrql query support is straightforward:

from pyrql import Query
from urllib.parse import unquote

@app.route('/api/v1/tasks')
def get_user_tasks():
    tasks = [task.to_dict() for task in Task.get_all()]

    query_string = unquote(request.query_string.decode(request.charset))
    query = Query(tasks).query(query_string)

    return jsonify(query.all())

And now the endpoint supports the RQL syntax. For sake of example, let's consider a typical tasks response is similar to the following:

[
    {
    "status": "PENDING",
    "name": "Update mobile app",
    "due_date": "2022-02-01T15:00:00",
    "completed_date": null,
    "tags": ["development", "easy"],
    "assigned_to": null,
    "hours_budgeted": 4,
    "hours_spent": 0
    },
    {
    "status": "COMPLETED",
    "name": "Design new frontend",
    "due_date": "2022-01-28T14:00:00",
    "completed_date": "2022-01-27T12:17:00"
    "tags": ["design", "medium"],
    "assigned_to": "Bill",
    "hours_budgeted": 8,
    "hours_spent": 6
    },
    ...
]

If an API client wants to retrieve only tasks in the PENDING status, the simple equality comparison is supported with standard query strings:

GET /api/v1/asks?state=PENDING

Or with the RQL syntax:

GET /api/v1/tasks?eq(state,PENDING)

Let's say the client wants tasks in the PENDING state which contain the easy tag:

GET /api/v1/tasks?eq(state,PENDING)&contains(tags,easy)

It can also perform simple aggregations, like adding up all hours spent by completed tasks, for each assigned user:

GET /api/v1/tasks?eq(state,COMPLETED)&ne(assigned_user,null)&aggregate(assigned_to,sum(hours_spent))

Reference Table

RQL Python equivalent Obs.
FILTERING
eq(key,value) [row for row in data if row[key] == value]
ne(key,value) [row for row in data if row[key] != value]
lt(key,value) [row for row in data if row[key] < value]
le(key,value) [row for row in data if row[key] <= value]
gt(key,value) [row for row in data if row[key] > value]
ge(key,value) [row for row in data if row[key] >= value]
in(key,value) [row for row in data if row[key] in value]
out(key,value) [row for row in data if row[key] not in value]
contains(key,value) [row for row in data if value in row[key]]
excludes(key,value) [row for row in data if value not in row[key]]
and(expr1,expr2,...) [row for row in data if expr1 and expr2]
or(expr1,expr2,...) [row for row in data if expr1 or expr2]
TRANSFORMING
select(a,b,c,...) [{a: row[a], b: row[b], c: row[c]} for row in data]
values(a) [row[a] for row in data]
limit(count,start?) data[start:count]
sort(key) sorted(data, key=lambda row: row[key])
sort(-key) sorted(data, key=lambda row: row[key], reverse=True)
distinct() list(set(data)) Unlike set, RQL preserves order.
first() data[0]
one() data[0] Raises RQLQueryError if len(data) != 1
aggregate(key,agg1(a),agg2(b),...) See below
unwind(key) [{**row, key: item} for row in data for item in row[key]]
AGGREGATION
sum(key) sum([row[key] for row in data])
mean(key) statistics.mean([row[key] for row in data])
max(key) max([row[key] for row in data])
min(key) min([row[key] for row in data])
count() len(data)

The aggregate operator can't be summarized in a readable one-liner. It accepts a key, and any number of aggregation operators. All the data is grouped by the key value, aggregated by each aggregation operator, and a new list is built with the results and key value.