New pattern for views that return either JSON or HTML, available for plugins

[simonw]

Can be part of #870 - refactoring existing views to use register_routes().

I'm going to put the new check_permissions() method on BaseView as well. If I want that method to be available to plugins I can do so by turning that BaseView class into a documented API that plugins are encouraged to use themselves.
Originally posted by @simonw in #832 (comment)

[simonw]

The BaseView class does this for Datasette internals at the moment, but I'm not convinced it works as well as it could.

I'd like to turn this into a class that is documented and available to plugins as well.

[simonw]

I think this is blocking #619

[simonw]

This has been blocking things for too long.

If this becomes a documented pattern, things like adding a JSON output to https://github.com/dogsheep/dogsheep-beta becomes easier too.

[simonw]

I came up with a slightly wild idea for this that would involve pytest-style dependency injection.

Prototype here: https://gist.github.com/simonw/496b24fdad44f6f8b7237fe394a0ced7

Copying from my private notes:

Use the lazy evaluated DI mechanism to break up table view into different pieces eg for faceting

Use that to solve JSON vs HTML views

Oh here's an idea: what if the various components of the table view were each defined as async functions.... and then executed using asyncio.gather in order to run the SQL queries in parallel? Then try benchmarking with different numbers of threads?

The async_call_with_arguments function could do this automatically for any awaitable dependencies

This would give me massively parallel dependency injection

(I could build an entire framework around this and call it c64)

Idea: arguments called eg "count" are executed and the result passed to the function. If called count_fn then a reference to the not-yet-called function is passed instead

I'm not going to completely combine the views mechanism and the render hooks. Instead, the core view will define a bunch of functions used to compose the page and the render hook will have conditional access to those functions - which will otherwise be asyncio.gather executed directly by the HTML page version

Using asyncio.gather to execute facets and suggest facets in parallel would be VERY interesting

suggest facets should be VERY cachable - doesn't matter if it's wrong unlike actual facets themselves

What if all Datasette views were defined in terms of dependency injection - and those dependency functions could themselves depend on others just like pytest fixtures. Everything would become composable and async stuff could execute in parallel

FURTHER IDEA: use this for the ?_extra= mechanism as well.

Any view in Datasette can be defined as a collection of named keys. Each of those keys maps to a function or an async function that accepts as input other named keys, using DI to handle them.

The HTML view is a defined function. So are the other outputs.

Default original inputs include “request” and “datasette”.

So… maybe a view function is a class methods that use DI. One of those methods as an .html() method used for the default page.

Output formats are a bit more complicated because they are supposed to be defined separately in plugins. They are unified across query, row and table though.

I’m going to try breaking up the TableView to see what happens.

[simonw]

Another idea I had: a view is a class that takes the datasette instance in its constructor, and defines a __call__ method that accepts a request and returns a response. Except await __call__ looks like it might be a bit messy, discussion in encode/starlette#886

[simonw]

Things this mechanism needs to be able to support:

• Returning a default JSON representation
• Defining "extra" JSON representations blocks, which can be requested using ?_extra=
• Returning rendered HTML, based on the default JSON + one or more extras + a template
• Using Datasette output renderers to return e.g. CSV data
• Potentially also supporting streaming output renderers for streaming CSV/TSV/JSON-nl etc

[20after4]

This sounds really ambitious but also really awesome. I like the idea that basically any piece of a page could be selectively replaced.

It sort of sounds like a python asyncio version of https://github.com/observablehq/runtime

[simonw]

Here's the latest version of my weird dependency injection async class:

import inspect

class AsyncMeta(type):
    def __new__(cls, name, bases, attrs):
        # Decorate any items that are 'async def' methods
        _registry = {}
        new_attrs = {"_registry": _registry}
        for key, value in attrs.items():
            if inspect.iscoroutinefunction(value) and not value.__name__ == "resolve":
                new_attrs[key] = make_method(value)
                _registry[key] = new_attrs[key]
            else:
                new_attrs[key] = value

        # Topological sort of _registry by parameter dependencies
        graph = {
            key: {
                p for p in inspect.signature(method).parameters.keys()
                if p != "self" and not p.startswith("_")
            }
            for key, method in _registry.items()
        }
        new_attrs["_graph"] = graph
        return super().__new__(cls, name, bases, new_attrs)


def make_method(method):
    @wraps(method)
    async def inner(self, **kwargs):
        parameters = inspect.signature(method).parameters.keys()
        # Any parameters not provided by kwargs are resolved from registry
        to_resolve = [p for p in parameters if p not in kwargs and p != "self"]
        missing = [p for p in to_resolve if p not in self._registry]
        assert (
            not missing
        ), "The following DI parameters could not be found in the registry: {}".format(
            missing
        )
        results = {}
        results.update(kwargs)
        results.update(await self.resolve(to_resolve))
        return await method(self, **results)

    return inner


bad = [0]

class AsyncBase(metaclass=AsyncMeta):
    async def resolve(self, names):
        print(" resolve({})".format(names))
        results = {}
        # Resolve them in the correct order
        ts = TopologicalSorter()
        ts2 = TopologicalSorter()
        print(" names = ", names)
        print(" self._graph = ", self._graph)
        for name in names:
            if self._graph[name]:
                ts.add(name, *self._graph[name])
                ts2.add(name, *self._graph[name])
        print(" static_order =", tuple(ts2.static_order()))
        ts.prepare()
        while ts.is_active():
            print("  is_active, i = ", bad[0])
            bad[0] += 1
            if bad[0] > 20:
                print("    Infinite loop?")
                break
            nodes = ts.get_ready()
            print("  Do nodes:", nodes)
            awaitables = [self._registry[name](self, **{
                k: v for k, v in results.items() if k in self._graph[name]
            }) for name in nodes]
            print("  awaitables: ", awaitables)
            awaitable_results = await asyncio.gather(*awaitables)
            results.update({
                p[0].__name__: p[1] for p in zip(awaitables, awaitable_results)
            })
            print(results)
            for node in nodes:
                ts.done(node)

        return results

Example usage:

class Foo(AsyncBase):
    async def graa(self, boff):
        print("graa")
        return 5
    async def boff(self):
        print("boff")
        return 8
    async def other(self, boff, graa):
        print("other")
        return 5 + boff + graa

foo = Foo()
await foo.other()

Output:

 resolve(['boff', 'graa'])
 names =  ['boff', 'graa']
 self._graph =  {'graa': {'boff'}, 'boff': set(), 'other': {'graa', 'boff'}}
 static_order = ('boff', 'graa')
  is_active, i =  0
  Do nodes: ('boff',)
  awaitables:  [<coroutine object Foo.boff at 0x10bd81a40>]
 resolve([])
 names =  []
 self._graph =  {'graa': {'boff'}, 'boff': set(), 'other': {'graa', 'boff'}}
 static_order = ()
boff
{'boff': 8}
  is_active, i =  1
  Do nodes: ('graa',)
  awaitables:  [<coroutine object Foo.graa at 0x10d66b340>]
 resolve([])
 names =  []
 self._graph =  {'graa': {'boff'}, 'boff': set(), 'other': {'graa', 'boff'}}
 static_order = ()
graa
{'boff': 8, 'graa': 5}
other
18

[simonw]

This code is really fiddly. I just got to this version:

import asyncio
from functools import wraps
import inspect

try:
    import graphlib
except ImportError:
    from . import vendored_graphlib as graphlib


class AsyncMeta(type):
    def __new__(cls, name, bases, attrs):
        # Decorate any items that are 'async def' methods
        _registry = {}
        new_attrs = {"_registry": _registry}
        for key, value in attrs.items():
            if inspect.iscoroutinefunction(value) and not value.__name__ == "resolve":
                new_attrs[key] = make_method(value)
                _registry[key] = new_attrs[key]
            else:
                new_attrs[key] = value
        # Gather graph for later dependency resolution
        graph = {
            key: {
                p
                for p in inspect.signature(method).parameters.keys()
                if p != "self" and not p.startswith("_")
            }
            for key, method in _registry.items()
        }
        new_attrs["_graph"] = graph
        return super().__new__(cls, name, bases, new_attrs)


def make_method(method):
    @wraps(method)
    async def inner(self, _results=None, **kwargs):
        print("inner - _results=", _results)
        parameters = inspect.signature(method).parameters.keys()
        # Any parameters not provided by kwargs are resolved from registry
        to_resolve = [p for p in parameters if p not in kwargs and p != "self"]
        missing = [p for p in to_resolve if p not in self._registry]
        assert (
            not missing
        ), "The following DI parameters could not be found in the registry: {}".format(
            missing
        )
        results = {}
        results.update(kwargs)
        if to_resolve:
            resolved_parameters = await self.resolve(to_resolve, _results)
            results.update(resolved_parameters)
        return_value = await method(self, **results)
        if _results is not None:
            _results[method.__name__] = return_value
        return return_value

    return inner


class AsyncBase(metaclass=AsyncMeta):
    async def resolve(self, names, results=None):
        print("\n  resolve: ", names)
        if results is None:
            results = {}

        # Resolve them in the correct order
        ts = graphlib.TopologicalSorter()
        for name in names:
            ts.add(name, *self._graph[name])
        ts.prepare()

        async def resolve_nodes(nodes):
            print("    resolve_nodes", nodes)
            print("    (current results = {})".format(repr(results)))
            awaitables = [
                self._registry[name](
                    self,
                    _results=results,
                    **{k: v for k, v in results.items() if k in self._graph[name]},
                )
                for name in nodes
                if name not in results
            ]
            print("    awaitables: ", awaitables)
            awaitable_results = await asyncio.gather(*awaitables)
            results.update(
                {p[0].__name__: p[1] for p in zip(awaitables, awaitable_results)}
            )

        if not ts.is_active():
            # Nothing has dependencies - just resolve directly
            print("    no dependencies, resolve directly")
            await resolve_nodes(names)
        else:
            # Resolve in topological order
            while ts.is_active():
                nodes = ts.get_ready()
                print("    ts.get_ready() returned nodes:", nodes)
                await resolve_nodes(nodes)
                for node in nodes:
                    ts.done(node)

        print("  End of resolve(), returning", results)
        return {key: value for key, value in results.items() if key in names}

With this test:

class Complex(AsyncBase):
    def __init__(self):
        self.log = []

    async def c(self):
        print("LOG: c")
        self.log.append("c")

    async def b(self, c):
        print("LOG: b")
        self.log.append("b")

    async def a(self, b, c):
        print("LOG: a")
        self.log.append("a")

    async def go(self, a):
        print("LOG: go")
        self.log.append("go")
        return self.log


@pytest.mark.asyncio
async def test_complex():
    result = await Complex().go()
    # 'c' should only be called once
    assert result == ["c", "b", "a", "go"]

This test sometimes passes, and sometimes fails!

Output for a pass:

tests/test_asyncdi.py inner - _results= None

  resolve:  ['a']
    ts.get_ready() returned nodes: ('c', 'b')
    resolve_nodes ('c', 'b')
    (current results = {})
    awaitables:  [<coroutine object Complex.c at 0x1074ac890>, <coroutine object Complex.b at 0x1074ac820>]
inner - _results= {}
LOG: c
inner - _results= {'c': None}

  resolve:  ['c']
    ts.get_ready() returned nodes: ('c',)
    resolve_nodes ('c',)
    (current results = {'c': None})
    awaitables:  []
  End of resolve(), returning {'c': None}
LOG: b
    ts.get_ready() returned nodes: ('a',)
    resolve_nodes ('a',)
    (current results = {'c': None, 'b': None})
    awaitables:  [<coroutine object Complex.a at 0x1074ac7b0>]
inner - _results= {'c': None, 'b': None}
LOG: a
  End of resolve(), returning {'c': None, 'b': None, 'a': None}
LOG: go

Output for a fail:

tests/test_asyncdi.py inner - _results= None

  resolve:  ['a']
    ts.get_ready() returned nodes: ('b', 'c')
    resolve_nodes ('b', 'c')
    (current results = {})
    awaitables:  [<coroutine object Complex.b at 0x10923c890>, <coroutine object Complex.c at 0x10923c820>]
inner - _results= {}

  resolve:  ['c']
    ts.get_ready() returned nodes: ('c',)
    resolve_nodes ('c',)
    (current results = {})
    awaitables:  [<coroutine object Complex.c at 0x10923c6d0>]
inner - _results= {}
LOG: c
inner - _results= {'c': None}
LOG: c
  End of resolve(), returning {'c': None}
LOG: b
    ts.get_ready() returned nodes: ('a',)
    resolve_nodes ('a',)
    (current results = {'c': None, 'b': None})
    awaitables:  [<coroutine object Complex.a at 0x10923c6d0>]
inner - _results= {'c': None, 'b': None}
LOG: a
  End of resolve(), returning {'c': None, 'b': None, 'a': None}
LOG: go
F

=================================================================================================== FAILURES ===================================================================================================
_________________________________________________________________________________________________ test_complex _________________________________________________________________________________________________

    @pytest.mark.asyncio
    async def test_complex():
        result = await Complex().go()
        # 'c' should only be called once
>       assert result == ["c", "b", "a", "go"]
E       AssertionError: assert ['c', 'c', 'b', 'a', 'go'] == ['c', 'b', 'a', 'go']
E         At index 1 diff: 'c' != 'b'
E         Left contains one more item: 'go'
E         Use -v to get the full diff

tests/test_asyncdi.py:48: AssertionError
================== short test summary info ================================
FAILED tests/test_asyncdi.py::test_complex - AssertionError: assert ['c', 'c', 'b', 'a', 'go'] == ['c', 'b', 'a', 'go']

I figured out why this is happening.

a requires b and c

b also requires c

The code decides to run b and c in parallel.

If c completes first, then when b runs it gets to use the already-calculated result for c - so it doesn't need to call c again.

If b gets to that point before c does it also needs to call c.

[simonw]

What should be happening here instead is it should resolve the full graph and notice that c is depended on by both b and a - so it should run c first, then run the next ones in parallel.

So maybe the algorithm I'm inheriting from https://docs.python.org/3/library/graphlib.html isn't the correct algorithm?

[simonw]

My goal here is to calculate the most efficient way to resolve the different nodes, running them in parallel where possible.

So for this class:

class Complex(AsyncBase):
    async def d(self):
        pass

    async def c(self):
        pass

    async def b(self, c, d):
        pass

    async def a(self, b, c):
        pass

    async def go(self, a):
        pass

A call to go() should do this:

• c and d in parallel
• b
• a
• go

[simonw]

But that does seem to be the plan that TopographicalSorter provides:

graph = {"go": {"a"}, "a": {"b", "c"}, "b": {"c", "d"}}

ts = TopologicalSorter(graph)
ts.prepare()
while ts.is_active():
    nodes = ts.get_ready()
    print(nodes)
    ts.done(*nodes)

Outputs:

('c', 'd')
('b',)
('a',)
('go',)

Also:

graph = {"go": {"d", "e", "f"}, "d": {"b", "c"}, "b": {"c"}}

ts = TopologicalSorter(graph)
ts.prepare()
while ts.is_active():
    nodes = ts.get_ready()
    print(nodes)
    ts.done(*nodes)

Gives:

('e', 'f', 'c')
('b',)
('d',)
('go',)

I'm confident that TopologicalSorter is the way to do this. I think I need to rewrite my code to call it once to get that plan, then await asyncio.gather(*nodes) in turn to execute it.

[simonw]

New test:

class Complex(AsyncBase):
    def __init__(self):
        self.log = []

    async def d(self):
        await asyncio.sleep(random() * 0.1)
        print("LOG: d")
        self.log.append("d")

    async def c(self):
        await asyncio.sleep(random() * 0.1)
        print("LOG: c")
        self.log.append("c")

    async def b(self, c, d):
        print("LOG: b")
        self.log.append("b")

    async def a(self, b, c):
        print("LOG: a")
        self.log.append("a")

    async def go(self, a):
        print("LOG: go")
        self.log.append("go")
        return self.log


@pytest.mark.asyncio
async def test_complex():
    result = await Complex().go()
    # 'c' should only be called once
    assert tuple(result) in (
        # c and d could happen in either order
        ("c", "d", "b", "a", "go"),
        ("d", "c", "b", "a", "go"),
    )

And this code passes it:

import asyncio
from functools import wraps
import inspect

try:
    import graphlib
except ImportError:
    from . import vendored_graphlib as graphlib


class AsyncMeta(type):
    def __new__(cls, name, bases, attrs):
        # Decorate any items that are 'async def' methods
        _registry = {}
        new_attrs = {"_registry": _registry}
        for key, value in attrs.items():
            if inspect.iscoroutinefunction(value) and not value.__name__ == "resolve":
                new_attrs[key] = make_method(value)
                _registry[key] = new_attrs[key]
            else:
                new_attrs[key] = value
        # Gather graph for later dependency resolution
        graph = {
            key: {
                p
                for p in inspect.signature(method).parameters.keys()
                if p != "self" and not p.startswith("_")
            }
            for key, method in _registry.items()
        }
        new_attrs["_graph"] = graph
        return super().__new__(cls, name, bases, new_attrs)


def make_method(method):
    parameters = inspect.signature(method).parameters.keys()

    @wraps(method)
    async def inner(self, _results=None, **kwargs):
        print("\n{}.{}({}) _results={}".format(self, method.__name__, kwargs, _results))

        # Any parameters not provided by kwargs are resolved from registry
        to_resolve = [p for p in parameters if p not in kwargs and p != "self"]
        missing = [p for p in to_resolve if p not in self._registry]
        assert (
            not missing
        ), "The following DI parameters could not be found in the registry: {}".format(
            missing
        )

        results = {}
        results.update(kwargs)
        if to_resolve:
            resolved_parameters = await self.resolve(to_resolve, _results)
            results.update(resolved_parameters)
        return_value = await method(self, **results)
        if _results is not None:
            _results[method.__name__] = return_value
        return return_value

    return inner


class AsyncBase(metaclass=AsyncMeta):
    async def resolve(self, names, results=None):
        print("\n  resolve: ", names)
        if results is None:
            results = {}

        # Come up with an execution plan, just for these nodes
        ts = graphlib.TopologicalSorter()
        to_do = set(names)
        done = set()
        while to_do:
            item = to_do.pop()
            dependencies = self._graph[item]
            ts.add(item, *dependencies)
            done.add(item)
            # Add any not-done dependencies to the queue
            to_do.update({k for k in dependencies if k not in done})

        ts.prepare()
        plan = []
        while ts.is_active():
            node_group = ts.get_ready()
            plan.append(node_group)
            ts.done(*node_group)

        print("plan:", plan)

        results = {}
        for node_group in plan:
            awaitables = [
                self._registry[name](
                    self,
                    _results=results,
                    **{k: v for k, v in results.items() if k in self._graph[name]},
                )
                for name in node_group
            ]
            print("    results = ", results)
            print("    awaitables: ", awaitables)
            awaitable_results = await asyncio.gather(*awaitables)
            results.update(
                {p[0].__name__: p[1] for p in zip(awaitables, awaitable_results)}
            )

        print("  End of resolve(), returning", results)
        return {key: value for key, value in results.items() if key in names}

[simonw]

Wrote a TIL about what I learned using TopologicalSorter: https://til.simonwillison.net/python/graphlib-topologicalsorter

[simonw]

I'm going to continue working on this in a PR.

[simonw]

I shipped that code as a new library, asyncinject: https://pypi.org/project/asyncinject/ - I'll open a new PR to attempt to refactor TableView to use it.

[simonw]

I'm going to build a brand new implementation of the TableView class that doesn't subclass BaseView at all, instead using asyncinject. If I'm lucky that will clean up the grungiest part of the codebase.

I can maybe even run the tests against old TableView and TableView2 to check that they behave the same.

[simonw]

New plan: I'm going to build a brand new implementation of TableView starting out as a plugin, using the register_routes() plugin hook.

It will reuse the existing HTML template but will be a completely new Python implementation, based on asyncinject.

I'm going to start by just getting the table to show up on the page - then I'll add faceting, suggested facets, filters and so-on.

Bonus: I'm going to see if I can get it to work for arbitrary SQL queries too (stretch goal).

[simonw]

Development % cookiecutter gh:simonw/datasette-plugin
You've downloaded /Users/simon/.cookiecutters/datasette-plugin before. Is it okay to delete and re-download it? [yes]: yes
plugin_name []: table-new
description []: New implementation of TableView, see https://github.com/simonw/datasette/issues/878
hyphenated [table-new]: 
underscored [table_new]: 
github_username []: simonw
author_name []: Simon Willison
include_static_directory []:  
include_templates_directory []: 

[simonw]

That's annoying: it looks like plugins can't use register_routes() to over-ride default routes within Datasette itself. This didn't work:

from datasette.utils.asgi import Response
from datasette import hookimpl
import html


async def table(request):
    return Response.html("Hello from {}".format(
        html.escape(repr(request.url_vars))
    ))


@hookimpl
def register_routes():
    return [
        (r"/(?P<db_name>[^/]+)/(?P<table_and_format>[^/]+?$)", table),
    ]

I'll use a /t/ prefix for the moment, but this is probably something I'll fix in Datasette itself later.

[simonw]

Problem: the fancy asyncinject stuff inteferes with the fancy Datasette thing that introspects view functions to look for what parameters they take:

class Table(asyncinject.AsyncInjectAll):
    async def view(self, request):
        return Response.html("Hello from {}".format(
            html.escape(repr(request.url_vars))
        ))


@hookimpl
def register_routes():
    return [
        (r"/t/(?P<db_name>[^/]+)/(?P<table_and_format>[^/]+?$)", Table().view),
    ]

This failed with error: "Table.view() takes 1 positional argument but 2 were given"

So I'm going to use AsyncInject and have the view function NOT use the @inject decorator.

[simonw]

This is working!

from datasette.utils.asgi import Response
from datasette import hookimpl
import html
from asyncinject import AsyncInject, inject


class Table(AsyncInject):
    @inject
    async def database(self, request):
        return request.url_vars["db_name"]

    @inject
    async def main(self, request, database):
        return Response.html("Database: {}".format(
            html.escape(database)
        ))

    async def view(self, request):
        return await self.main(request=request)


@hookimpl
def register_routes():
    return [
        (r"/t/(?P<db_name>[^/]+)/(?P<table_and_format>[^/]+?$)", Table().view),
    ]

This project will definitely show me if I actually like the asyncinject patterns or not.

[simonw]

This exercise is proving so useful in getting my head around how the enormous and complex TableView class works again.

Here's where I've got to now - I'm systematically working through the variables that are returned for HTML and for JSON copying across code to get it to work:

from datasette.database import QueryInterrupted
from datasette.utils import escape_sqlite
from datasette.utils.asgi import Response, NotFound, Forbidden
from datasette.views.base import DatasetteError
from datasette import hookimpl
from asyncinject import AsyncInject, inject
from pprint import pformat


class Table(AsyncInject):
    @inject
    async def database(self, request, datasette):
        # TODO: all that nasty hash resolving stuff can go here
        db_name = request.url_vars["db_name"]
        try:
            db = datasette.databases[db_name]
        except KeyError:
            raise NotFound(f"Database '{db_name}' does not exist")
        return db

    @inject
    async def table_and_format(self, request, database, datasette):
        table_and_format = request.url_vars["table_and_format"]
        # TODO: be a lot smarter here
        if "." in table_and_format:
            return table_and_format.split(".", 2)
        else:
            return table_and_format, "html"

    @inject
    async def main(self, request, database, table_and_format, datasette):
        # TODO: if this is actually a canned query, dispatch to it

        table, format = table_and_format

        is_view = bool(await database.get_view_definition(table))
        table_exists = bool(await database.table_exists(table))
        if not is_view and not table_exists:
            raise NotFound(f"Table not found: {table}")

        await check_permissions(
            datasette,
            request,
            [
                ("view-table", (database.name, table)),
                ("view-database", database.name),
                "view-instance",
            ],
        )

        private = not await datasette.permission_allowed(
            None, "view-table", (database.name, table), default=True
        )

        pks = await database.primary_keys(table)
        table_columns = await database.table_columns(table)

        specified_columns = await columns_to_select(datasette, database, table, request)
        select_specified_columns = ", ".join(
            escape_sqlite(t) for t in specified_columns
        )
        select_all_columns = ", ".join(escape_sqlite(t) for t in table_columns)

        use_rowid = not pks and not is_view
        if use_rowid:
            select_specified_columns = f"rowid, {select_specified_columns}"
            select_all_columns = f"rowid, {select_all_columns}"
            order_by = "rowid"
            order_by_pks = "rowid"
        else:
            order_by_pks = ", ".join([escape_sqlite(pk) for pk in pks])
            order_by = order_by_pks

        if is_view:
            order_by = ""

        nocount = request.args.get("_nocount")
        nofacet = request.args.get("_nofacet")

        if request.args.get("_shape") in ("array", "object"):
            nocount = True
            nofacet = True

        # Next, a TON of SQL to build where_params and filters and suchlike
        # skipping that and jumping straight to...
        where_clauses = []
        where_clause = ""
        if where_clauses:
            where_clause = f"where {' and '.join(where_clauses)} "

        from_sql = "from {table_name} {where}".format(
            table_name=escape_sqlite(table),
            where=("where {} ".format(" and ".join(where_clauses)))
            if where_clauses
            else "",
        )
        from_sql_params ={}
        params = {}
        count_sql = f"select count(*) {from_sql}"
        sql_no_order_no_limit = (
            "select {select_all_columns} from {table_name} {where}".format(
                select_all_columns=select_all_columns,
                table_name=escape_sqlite(table),
                where=where_clause,
            )
        )

        page_size = 100
        offset = " offset 0"

        sql = "select {select_specified_columns} from {table_name} {where}{order_by} limit {page_size}{offset}".format(
            select_specified_columns=select_specified_columns,
            table_name=escape_sqlite(table),
            where=where_clause,
            order_by=order_by,
            page_size=page_size + 1,
            offset=offset,
        )

        # Fetch rows
        results = await database.execute(sql, params, truncate=True)
        columns = [r[0] for r in results.description]
        rows = list(results.rows)

        # Fetch count
        filtered_table_rows_count = None
        if count_sql:
            try:
                count_rows = list(await database.execute(count_sql, from_sql_params))
                filtered_table_rows_count = count_rows[0][0]
            except QueryInterrupted:
                pass


        vars = {
            "json": {
                # THIS STUFF is from the regular JSON
                "database": database.name,
                "table": table,
                "is_view": is_view,
                # "human_description_en": human_description_en,
                "rows": rows[:page_size],
                "truncated": results.truncated,
                "filtered_table_rows_count": filtered_table_rows_count,
                # "expanded_columns": expanded_columns,
                # "expandable_columns": expandable_columns,
                "columns": columns,
                "primary_keys": pks,
                # "units": units,
                "query": {"sql": sql, "params": params},
                # "facet_results": facet_results,
                # "suggested_facets": suggested_facets,
                # "next": next_value and str(next_value) or None,
                # "next_url": next_url,
                "private": private,
                "allow_execute_sql": await datasette.permission_allowed(
                    request.actor, "execute-sql", database, default=True
                ),
            },
            "html": {
                # ... this is the HTML special stuff
                # "table_actions": table_actions,
                # "supports_search": bool(fts_table),
                # "search": search or "",
                "use_rowid": use_rowid,
                # "filters": filters,
                # "display_columns": display_columns,
                # "filter_columns": filter_columns,
                # "display_rows": display_rows,
                # "facets_timed_out": facets_timed_out,
                # "sorted_facet_results": sorted(
                #     facet_results.values(),
                #     key=lambda f: (len(f["results"]), f["name"]),
                #     reverse=True,
                # ),
                # "show_facet_counts": special_args.get("_facet_size") == "max",
                # "extra_wheres_for_ui": extra_wheres_for_ui,
                # "form_hidden_args": form_hidden_args,
                # "is_sortable": any(c["sortable"] for c in display_columns),
                # "path_with_replaced_args": path_with_replaced_args,
                # "path_with_removed_args": path_with_removed_args,
                # "append_querystring": append_querystring,
                "request": request,
                # "sort": sort,
                # "sort_desc": sort_desc,
                "disable_sort": is_view,
                # "custom_table_templates": [
                #     f"_table-{to_css_class(database)}-{to_css_class(table)}.html",
                #     f"_table-table-{to_css_class(database)}-{to_css_class(table)}.html",
                #     "_table.html",
                # ],
                # "metadata": metadata,
                # "view_definition": await db.get_view_definition(table),
                # "table_definition": await db.get_table_definition(table),
            },
        }

        # I'm just trying to get HTML to work for the moment
        if format == "json":
            return Response.json(dict(vars, locals=locals()), default=repr)
        else:
            return Response.html(repr(vars["html"]))

    async def view(self, request, datasette):
        return await self.main(request=request, datasette=datasette)


@hookimpl
def register_routes():
    return [
        (r"/t/(?P<db_name>[^/]+)/(?P<table_and_format>[^/]+?$)", Table().view),
    ]


async def check_permissions(datasette, request, permissions):
    """permissions is a list of (action, resource) tuples or 'action' strings"""
    for permission in permissions:
        if isinstance(permission, str):
            action = permission
            resource = None
        elif isinstance(permission, (tuple, list)) and len(permission) == 2:
            action, resource = permission
        else:
            assert (
                False
            ), "permission should be string or tuple of two items: {}".format(
                repr(permission)
            )
        ok = await datasette.permission_allowed(
            request.actor,
            action,
            resource=resource,
            default=None,
        )
        if ok is not None:
            if ok:
                return
            else:
                raise Forbidden(action)


async def columns_to_select(datasette, database, table, request):
    table_columns = await database.table_columns(table)
    pks = await database.primary_keys(table)
    columns = list(table_columns)
    if "_col" in request.args:
        columns = list(pks)
        _cols = request.args.getlist("_col")
        bad_columns = [column for column in _cols if column not in table_columns]
        if bad_columns:
            raise DatasetteError(
                "_col={} - invalid columns".format(", ".join(bad_columns)),
                status=400,
            )
        # De-duplicate maintaining order:
        columns.extend(dict.fromkeys(_cols))
    if "_nocol" in request.args:
        # Return all columns EXCEPT these
        bad_columns = [
            column
            for column in request.args.getlist("_nocol")
            if (column not in table_columns) or (column in pks)
        ]
        if bad_columns:
            raise DatasetteError(
                "_nocol={} - invalid columns".format(", ".join(bad_columns)),
                status=400,
            )
        tmp_columns = [
            column for column in columns if column not in request.args.getlist("_nocol")
        ]
        columns = tmp_columns
    return columns

[simonw]

OK, I managed to get a table to render! Here's the code I used - I had to copy a LOT of stuff. https://gist.github.com/simonw/281eac9c73b062c3469607ad86470eb2

I'm going to move this work into a new, separate issue.

[simonw]

I'm going to see if I can come up with the simplest possible version of this pattern for the /-/metadata and /-/metadata.json page, then try it for the database query page, before tackling the much more complex table page.

[simonw]

On revisiting https://gist.github.com/simonw/281eac9c73b062c3469607ad86470eb2 a few months later I'm having second thoughts about using @inject on the main() method.

But I still like the pattern as a way to resolve more complex cases like "to generate GeoJSON of the expanded view with labels, the label expansion code needs to run once at some before the GeoJSON formatting code does".

So I'm going to stick with it a tiny bit longer, but maybe try to make it a lot more explicit when it's going to happen rather than having the main view methods themselves also use async DI.