Cleaner mechanism for handling custom errors

[simonw]

This code is pretty messy:

datasette/datasette/app.py

Lines 245 to 265 in 0abd3ab

	if shape == 'object':
	error = None
	if 'primary_keys' not in data:
	error = '_shape=object is only available on tables'
	else:
	pks = data['primary_keys']
	if not pks:
	error = '_shape=object not available for tables with no primary keys'
	else:
	object_rows = {}
	for row in data['rows']:
	pk_string = path_from_row_pks(row, pks, not pks)
	object_rows[pk_string] = row
	data['rows'] = object_rows
	if error:
	data = {
	'ok': False,
	'error': error,
	'database': name,
	'database_hash': hash,
	}

Instead, it would be nice if I could raise an exception that would be converted into the appropriate JSON or HTML error message, with a corresponding HTTP code.

[earthboundkid]

You could try pulling out a validate query strings method. If it fails validation build the error object from the message. If it passes, you only need to go down a happy path.

[simonw]

This is harder than I thought, because the _shape= logic actually runs AFTER the main block of code which is set up to catch exceptions - this code here:

datasette/datasette/app.py

Lines 200 to 216 in 0abd3ab

	try:
	response_or_template_contexts = await self.data(
	request, name, hash, **kwargs
	)
	if isinstance(response_or_template_contexts, response.HTTPResponse):
	return response_or_template_contexts
	else:
	data, extra_template_data, templates = response_or_template_contexts
	except (sqlite3.OperationalError, InvalidSql) as e:
	data = {
	'ok': False,
	'error': str(e),
	'database': name,
	'database_hash': hash,
	}
	status_code = 400
	templates = ['error.html']

[simonw]

I can clean this up further with the mechanism I'm using for #184