Source code for django.test.utils
import collections
import logging
import os
import re
import sys
import time
import warnings
from contextlib import contextmanager
from functools import wraps
from io import StringIO
from itertools import chain
from types import SimpleNamespace
from unittest import TestCase, skipIf, skipUnless
from xml.dom.minidom import Node, parseString
from asgiref.sync import iscoroutinefunction
from django.apps import apps
from django.apps.registry import Apps
from django.conf import UserSettingsHolder, settings
from django.core import mail
from django.core.exceptions import ImproperlyConfigured
from django.core.signals import request_started, setting_changed
from django.db import DEFAULT_DB_ALIAS, connections, reset_queries
from django.db.models.options import Options
from django.template import Template
from django.test.signals import template_rendered
from django.urls import get_script_prefix, set_script_prefix
from django.utils.deprecation import RemovedInDjango50Warning
from django.utils.translation import deactivate
try:
import jinja2
except ImportError:
jinja2 = None
__all__ = (
"Approximate",
"ContextList",
"isolate_lru_cache",
"get_runner",
"CaptureQueriesContext",
"ignore_warnings",
"isolate_apps",
"modify_settings",
"override_settings",
"override_system_checks",
"tag",
"requires_tz_support",
"setup_databases",
"setup_test_environment",
"teardown_test_environment",
)
TZ_SUPPORT = hasattr(time, "tzset")
class Approximate:
def __init__(self, val, places=7):
self.val = val
self.places = places
def __repr__(self):
return repr(self.val)
def __eq__(self, other):
return self.val == other or round(abs(self.val - other), self.places) == 0
class ContextList(list):
"""
A wrapper that provides direct key access to context items contained
in a list of context objects.
"""
def __getitem__(self, key):
if isinstance(key, str):
for subcontext in self:
if key in subcontext:
return subcontext[key]
raise KeyError(key)
else:
return super().__getitem__(key)
def get(self, key, default=None):
try:
return self.__getitem__(key)
except KeyError:
return default
def __contains__(self, key):
try:
self[key]
except KeyError:
return False
return True
def keys(self):
"""
Flattened keys of subcontexts.
"""
return set(chain.from_iterable(d for subcontext in self for d in subcontext))
def instrumented_test_render(self, context):
"""
An instrumented Template render method, providing a signal that can be
intercepted by the test Client.
"""
template_rendered.send(sender=self, template=self, context=context)
return self.nodelist.render(context)
class _TestState:
pass
[docs]def setup_test_environment(debug=None):
"""
Perform global pre-test setup, such as installing the instrumented template
renderer and setting the email backend to the locmem email backend.
"""
if hasattr(_TestState, "saved_data"):
# Executing this function twice would overwrite the saved values.
raise RuntimeError(
"setup_test_environment() was already called and can't be called "
"again without first calling teardown_test_environment()."
)
if debug is None:
debug = settings.DEBUG
saved_data = SimpleNamespace()
_TestState.saved_data = saved_data
saved_data.allowed_hosts = settings.ALLOWED_HOSTS
# Add the default host of the test client.
settings.ALLOWED_HOSTS = [*settings.ALLOWED_HOSTS, "testserver"]
saved_data.debug = settings.DEBUG
settings.DEBUG = debug
saved_data.email_backend = settings.EMAIL_BACKEND
settings.EMAIL_BACKEND = "django.core.mail.backends.locmem.EmailBackend"
saved_data.template_render = Template._render
Template._render = instrumented_test_render
mail.outbox = []
deactivate()
[docs]def teardown_test_environment():
"""
Perform any global post-test teardown, such as restoring the original
template renderer and restoring the email sending functions.
"""
saved_data = _TestState.saved_data
settings.ALLOWED_HOSTS = saved_data.allowed_hosts
settings.DEBUG = saved_data.debug
settings.EMAIL_BACKEND = saved_data.email_backend
Template._render = saved_data.template_render
del _TestState.saved_data
del mail.outbox
[docs]def setup_databases(
verbosity,
interactive,
*,
time_keeper=None,
keepdb=False,
debug_sql=False,
parallel=0,
aliases=None,
serialized_aliases=None,
**kwargs,
):
"""Create the test databases."""
if time_keeper is None:
time_keeper = NullTimeKeeper()
test_databases, mirrored_aliases = get_unique_databases_and_mirrors(aliases)
old_names = []
for db_name, aliases in test_databases.values():
first_alias = None
for alias in aliases:
connection = connections[alias]
old_names.append((connection, db_name, first_alias is None))
# Actually create the database for the first connection
if first_alias is None:
first_alias = alias
with time_keeper.timed(" Creating '%s'" % alias):
# RemovedInDjango50Warning: when the deprecation ends,
# replace with:
# serialize_alias = (
# serialized_aliases is None
# or alias in serialized_aliases
# )
try:
serialize_alias = connection.settings_dict["TEST"]["SERIALIZE"]
except KeyError:
serialize_alias = (
serialized_aliases is None or alias in serialized_aliases
)
else:
warnings.warn(
"The SERIALIZE test database setting is "
"deprecated as it can be inferred from the "
"TestCase/TransactionTestCase.databases that "
"enable the serialized_rollback feature.",
category=RemovedInDjango50Warning,
)
connection.creation.create_test_db(
verbosity=verbosity,
autoclobber=not interactive,
keepdb=keepdb,
serialize=serialize_alias,
)
if parallel > 1:
for index in range(parallel):
with time_keeper.timed(" Cloning '%s'" % alias):
connection.creation.clone_test_db(
suffix=str(index + 1),
verbosity=verbosity,
keepdb=keepdb,
)
# Configure all other connections as mirrors of the first one
else:
connections[alias].creation.set_as_test_mirror(
connections[first_alias].settings_dict
)
# Configure the test mirrors.
for alias, mirror_alias in mirrored_aliases.items():
connections[alias].creation.set_as_test_mirror(
connections[mirror_alias].settings_dict
)
if debug_sql:
for alias in connections:
connections[alias].force_debug_cursor = True
return old_names
def iter_test_cases(tests):
"""
Return an iterator over a test suite's unittest.TestCase objects.
The tests argument can also be an iterable of TestCase objects.
"""
for test in tests:
if isinstance(test, str):
# Prevent an unfriendly RecursionError that can happen with
# strings.
raise TypeError(
f"Test {test!r} must be a test case or test suite not string "
f"(was found in {tests!r})."
)
if isinstance(test, TestCase):
yield test
else:
# Otherwise, assume it is a test suite.
yield from iter_test_cases(test)
def dependency_ordered(test_databases, dependencies):
"""
Reorder test_databases into an order that honors the dependencies
described in TEST[DEPENDENCIES].
"""
ordered_test_databases = []
resolved_databases = set()
# Maps db signature to dependencies of all its aliases
dependencies_map = {}
# Check that no database depends on its own alias
for sig, (_, aliases) in test_databases:
all_deps = set()
for alias in aliases:
all_deps.update(dependencies.get(alias, []))
if not all_deps.isdisjoint(aliases):
raise ImproperlyConfigured(
"Circular dependency: databases %r depend on each other, "
"but are aliases." % aliases
)
dependencies_map[sig] = all_deps
while test_databases:
changed = False
deferred = []
# Try to find a DB that has all its dependencies met
for signature, (db_name, aliases) in test_databases:
if dependencies_map[signature].issubset(resolved_databases):
resolved_databases.update(aliases)
ordered_test_databases.append((signature, (db_name, aliases)))
changed = True
else:
deferred.append((signature, (db_name, aliases)))
if not changed:
raise ImproperlyConfigured("Circular dependency in TEST[DEPENDENCIES]")
test_databases = deferred
return ordered_test_databases
def get_unique_databases_and_mirrors(aliases=None):
"""
Figure out which databases actually need to be created.
Deduplicate entries in DATABASES that correspond the same database or are
configured as test mirrors.
Return two values:
- test_databases: ordered mapping of signatures to (name, list of aliases)
where all aliases share the same underlying database.
- mirrored_aliases: mapping of mirror aliases to original aliases.
"""
if aliases is None:
aliases = connections
mirrored_aliases = {}
test_databases = {}
dependencies = {}
default_sig = connections[DEFAULT_DB_ALIAS].creation.test_db_signature()
for alias in connections:
connection = connections[alias]
test_settings = connection.settings_dict["TEST"]
if test_settings["MIRROR"]:
# If the database is marked as a test mirror, save the alias.
mirrored_aliases[alias] = test_settings["MIRROR"]
elif alias in aliases:
# Store a tuple with DB parameters that uniquely identify it.
# If we have two aliases with the same values for that tuple,
# we only need to create the test database once.
item = test_databases.setdefault(
connection.creation.test_db_signature(),
(connection.settings_dict["NAME"], []),
)
# The default database must be the first because data migrations
# use the default alias by default.
if alias == DEFAULT_DB_ALIAS:
item[1].insert(0, alias)
else:
item[1].append(alias)
if "DEPENDENCIES" in test_settings:
dependencies[alias] = test_settings["DEPENDENCIES"]
else:
if (
alias != DEFAULT_DB_ALIAS
and connection.creation.test_db_signature() != default_sig
):
dependencies[alias] = test_settings.get(
"DEPENDENCIES", [DEFAULT_DB_ALIAS]
)
test_databases = dict(dependency_ordered(test_databases.items(), dependencies))
return test_databases, mirrored_aliases
[docs]def teardown_databases(old_config, verbosity, parallel=0, keepdb=False):
"""Destroy all the non-mirror databases."""
for connection, old_name, destroy in old_config:
if destroy:
if parallel > 1:
for index in range(parallel):
connection.creation.destroy_test_db(
suffix=str(index + 1),
verbosity=verbosity,
keepdb=keepdb,
)
connection.creation.destroy_test_db(old_name, verbosity, keepdb)
def get_runner(settings, test_runner_class=None):
test_runner_class = test_runner_class or settings.TEST_RUNNER
test_path = test_runner_class.split(".")
# Allow for relative paths
if len(test_path) > 1:
test_module_name = ".".join(test_path[:-1])
else:
test_module_name = "."
test_module = __import__(test_module_name, {}, {}, test_path[-1])
return getattr(test_module, test_path[-1])
class TestContextDecorator:
"""
A base class that can either be used as a context manager during tests
or as a test function or unittest.TestCase subclass decorator to perform
temporary alterations.
`attr_name`: attribute assigned the return value of enable() if used as
a class decorator.
`kwarg_name`: keyword argument passing the return value of enable() if
used as a function decorator.
"""
def __init__(self, attr_name=None, kwarg_name=None):
self.attr_name = attr_name
self.kwarg_name = kwarg_name
def enable(self):
raise NotImplementedError
def disable(self):
raise NotImplementedError
def __enter__(self):
return self.enable()
def __exit__(self, exc_type, exc_value, traceback):
self.disable()
def decorate_class(self, cls):
if issubclass(cls, TestCase):
decorated_setUp = cls.setUp
def setUp(inner_self):
context = self.enable()
inner_self.addCleanup(self.disable)
if self.attr_name:
setattr(inner_self, self.attr_name, context)
decorated_setUp(inner_self)
cls.setUp = setUp
return cls
raise TypeError("Can only decorate subclasses of unittest.TestCase")
def decorate_callable(self, func):
if iscoroutinefunction(func):
# If the inner function is an async function, we must execute async
# as well so that the `with` statement executes at the right time.
@wraps(func)
async def inner(*args, **kwargs):
with self as context:
if self.kwarg_name:
kwargs[self.kwarg_name] = context
return await func(*args, **kwargs)
else:
@wraps(func)
def inner(*args, **kwargs):
with self as context:
if self.kwarg_name:
kwargs[self.kwarg_name] = context
return func(*args, **kwargs)
return inner
def __call__(self, decorated):
if isinstance(decorated, type):
return self.decorate_class(decorated)
elif callable(decorated):
return self.decorate_callable(decorated)
raise TypeError("Cannot decorate object of type %s" % type(decorated))
[docs]class override_settings(TestContextDecorator):
"""
Act as either a decorator or a context manager. If it's a decorator, take a
function and return a wrapped function. If it's a contextmanager, use it
with the ``with`` statement. In either event, entering/exiting are called
before and after, respectively, the function/block is executed.
"""
enable_exception = None
def __init__(self, **kwargs):
self.options = kwargs
super().__init__()
def enable(self):
# Keep this code at the beginning to leave the settings unchanged
# in case it raises an exception because INSTALLED_APPS is invalid.
if "INSTALLED_APPS" in self.options:
try:
apps.set_installed_apps(self.options["INSTALLED_APPS"])
except Exception:
apps.unset_installed_apps()
raise
override = UserSettingsHolder(settings._wrapped)
for key, new_value in self.options.items():
setattr(override, key, new_value)
self.wrapped = settings._wrapped
settings._wrapped = override
for key, new_value in self.options.items():
try:
setting_changed.send(
sender=settings._wrapped.__class__,
setting=key,
value=new_value,
enter=True,
)
except Exception as exc:
self.enable_exception = exc
self.disable()
def disable(self):
if "INSTALLED_APPS" in self.options:
apps.unset_installed_apps()
settings._wrapped = self.wrapped
del self.wrapped
responses = []
for key in self.options:
new_value = getattr(settings, key, None)
responses_for_setting = setting_changed.send_robust(
sender=settings._wrapped.__class__,
setting=key,
value=new_value,
enter=False,
)
responses.extend(responses_for_setting)
if self.enable_exception is not None:
exc = self.enable_exception
self.enable_exception = None
raise exc
for _, response in responses:
if isinstance(response, Exception):
raise response
def save_options(self, test_func):
if test_func._overridden_settings is None:
test_func._overridden_settings = self.options
else:
# Duplicate dict to prevent subclasses from altering their parent.
test_func._overridden_settings = {
**test_func._overridden_settings,
**self.options,
}
def decorate_class(self, cls):
from django.test import SimpleTestCase
if not issubclass(cls, SimpleTestCase):
raise ValueError(
"Only subclasses of Django SimpleTestCase can be decorated "
"with override_settings"
)
self.save_options(cls)
return cls
[docs]class modify_settings(override_settings):
"""
Like override_settings, but makes it possible to append, prepend, or remove
items instead of redefining the entire list.
"""
def __init__(self, *args, **kwargs):
if args:
# Hack used when instantiating from SimpleTestCase.setUpClass.
assert not kwargs
self.operations = args[0]
else:
assert not args
self.operations = list(kwargs.items())
super(override_settings, self).__init__()
def save_options(self, test_func):
if test_func._modified_settings is None:
test_func._modified_settings = self.operations
else:
# Duplicate list to prevent subclasses from altering their parent.
test_func._modified_settings = (
list(test_func._modified_settings) + self.operations
)
def enable(self):
self.options = {}
for name, operations in self.operations:
try:
# When called from SimpleTestCase.setUpClass, values may be
# overridden several times; cumulate changes.
value = self.options[name]
except KeyError:
value = list(getattr(settings, name, []))
for action, items in operations.items():
# items my be a single value or an iterable.
if isinstance(items, str):
items = [items]
if action == "append":
value += [item for item in items if item not in value]
elif action == "prepend":
value = [item for item in items if item not in value] + value
elif action == "remove":
value = [item for item in value if item not in items]
else:
raise ValueError("Unsupported action: %s" % action)
self.options[name] = value
super().enable()
class override_system_checks(TestContextDecorator):
"""
Act as a decorator. Override list of registered system checks.
Useful when you override `INSTALLED_APPS`, e.g. if you exclude `auth` app,
you also need to exclude its system checks.
"""
def __init__(self, new_checks, deployment_checks=None):
from django.core.checks.registry import registry
self.registry = registry
self.new_checks = new_checks
self.deployment_checks = deployment_checks
super().__init__()
def enable(self):
self.old_checks = self.registry.registered_checks
self.registry.registered_checks = set()
for check in self.new_checks:
self.registry.register(check, *getattr(check, "tags", ()))
self.old_deployment_checks = self.registry.deployment_checks
if self.deployment_checks is not None:
self.registry.deployment_checks = set()
for check in self.deployment_checks:
self.registry.register(check, *getattr(check, "tags", ()), deploy=True)
def disable(self):
self.registry.registered_checks = self.old_checks
self.registry.deployment_checks = self.old_deployment_checks
def compare_xml(want, got):
"""
Try to do a 'xml-comparison' of want and got. Plain string comparison
doesn't always work because, for example, attribute ordering should not be
important. Ignore comment nodes, processing instructions, document type
node, and leading and trailing whitespaces.
Based on https://github.com/lxml/lxml/blob/master/src/lxml/doctestcompare.py
"""
_norm_whitespace_re = re.compile(r"[ \t\n][ \t\n]+")
def norm_whitespace(v):
return _norm_whitespace_re.sub(" ", v)
def child_text(element):
return "".join(
c.data for c in element.childNodes if c.nodeType == Node.TEXT_NODE
)
def children(element):
return [c for c in element.childNodes if c.nodeType == Node.ELEMENT_NODE]
def norm_child_text(element):
return norm_whitespace(child_text(element))
def attrs_dict(element):
return dict(element.attributes.items())
def check_element(want_element, got_element):
if want_element.tagName != got_element.tagName:
return False
if norm_child_text(want_element) != norm_child_text(got_element):
return False
if attrs_dict(want_element) != attrs_dict(got_element):
return False
want_children = children(want_element)
got_children = children(got_element)
if len(want_children) != len(got_children):
return False
return all(
check_element(want, got) for want, got in zip(want_children, got_children)
)
def first_node(document):
for node in document.childNodes:
if node.nodeType not in (
Node.COMMENT_NODE,
Node.DOCUMENT_TYPE_NODE,
Node.PROCESSING_INSTRUCTION_NODE,
):
return node
want = want.strip().replace("\\n", "\n")
got = got.strip().replace("\\n", "\n")
# If the string is not a complete xml document, we may need to add a
# root element. This allow us to compare fragments, like "<foo/><bar/>"
if not want.startswith("<?xml"):
wrapper = "<root>%s</root>"
want = wrapper % want
got = wrapper % got
# Parse the want and got strings, and compare the parsings.
want_root = first_node(parseString(want))
got_root = first_node(parseString(got))
return check_element(want_root, got_root)
class CaptureQueriesContext:
"""
Context manager that captures queries executed by the specified connection.
"""
def __init__(self, connection):
self.connection = connection
def __iter__(self):
return iter(self.captured_queries)
def __getitem__(self, index):
return self.captured_queries[index]
def __len__(self):
return len(self.captured_queries)
@property
def captured_queries(self):
return self.connection.queries[self.initial_queries : self.final_queries]
def __enter__(self):
self.force_debug_cursor = self.connection.force_debug_cursor
self.connection.force_debug_cursor = True
# Run any initialization queries if needed so that they won't be
# included as part of the count.
self.connection.ensure_connection()
self.initial_queries = len(self.connection.queries_log)
self.final_queries = None
request_started.disconnect(reset_queries)
return self
def __exit__(self, exc_type, exc_value, traceback):
self.connection.force_debug_cursor = self.force_debug_cursor
request_started.connect(reset_queries)
if exc_type is not None:
return
self.final_queries = len(self.connection.queries_log)
class ignore_warnings(TestContextDecorator):
def __init__(self, **kwargs):
self.ignore_kwargs = kwargs
if "message" in self.ignore_kwargs or "module" in self.ignore_kwargs:
self.filter_func = warnings.filterwarnings
else:
self.filter_func = warnings.simplefilter
super().__init__()
def enable(self):
self.catch_warnings = warnings.catch_warnings()
self.catch_warnings.__enter__()
self.filter_func("ignore", **self.ignore_kwargs)
def disable(self):
self.catch_warnings.__exit__(*sys.exc_info())
# On OSes that don't provide tzset (Windows), we can't set the timezone
# in which the program runs. As a consequence, we must skip tests that
# don't enforce a specific timezone (with timezone.override or equivalent),
# or attempt to interpret naive datetimes in the default timezone.
requires_tz_support = skipUnless(
TZ_SUPPORT,
"This test relies on the ability to run a program in an arbitrary "
"time zone, but your operating system isn't able to do that.",
)
@contextmanager
def extend_sys_path(*paths):
"""Context manager to temporarily add paths to sys.path."""
_orig_sys_path = sys.path[:]
sys.path.extend(paths)
try:
yield
finally:
sys.path = _orig_sys_path
@contextmanager
def isolate_lru_cache(lru_cache_object):
"""Clear the cache of an LRU cache object on entering and exiting."""
lru_cache_object.cache_clear()
try:
yield
finally:
lru_cache_object.cache_clear()
@contextmanager
def captured_output(stream_name):
"""Return a context manager used by captured_stdout/stdin/stderr
that temporarily replaces the sys stream *stream_name* with a StringIO.
Note: This function and the following ``captured_std*`` are copied
from CPython's ``test.support`` module."""
orig_stdout = getattr(sys, stream_name)
setattr(sys, stream_name, StringIO())
try:
yield getattr(sys, stream_name)
finally:
setattr(sys, stream_name, orig_stdout)
def captured_stdout():
"""Capture the output of sys.stdout:
with captured_stdout() as stdout:
print("hello")
self.assertEqual(stdout.getvalue(), "hello\n")
"""
return captured_output("stdout")
def captured_stderr():
"""Capture the output of sys.stderr:
with captured_stderr() as stderr:
print("hello", file=sys.stderr)
self.assertEqual(stderr.getvalue(), "hello\n")
"""
return captured_output("stderr")
def captured_stdin():
"""Capture the input to sys.stdin:
with captured_stdin() as stdin:
stdin.write('hello\n')
stdin.seek(0)
# call test code that consumes from sys.stdin
captured = input()
self.assertEqual(captured, "hello")
"""
return captured_output("stdin")
@contextmanager
def freeze_time(t):
"""
Context manager to temporarily freeze time.time(). This temporarily
modifies the time function of the time module. Modules which import the
time function directly (e.g. `from time import time`) won't be affected
This isn't meant as a public API, but helps reduce some repetitive code in
Django's test suite.
"""
_real_time = time.time
time.time = lambda: t
try:
yield
finally:
time.time = _real_time
def require_jinja2(test_func):
"""
Decorator to enable a Jinja2 template engine in addition to the regular
Django template engine for a test or skip it if Jinja2 isn't available.
"""
test_func = skipIf(jinja2 is None, "this test requires jinja2")(test_func)
return override_settings(
TEMPLATES=[
{
"BACKEND": "django.template.backends.django.DjangoTemplates",
"APP_DIRS": True,
},
{
"BACKEND": "django.template.backends.jinja2.Jinja2",
"APP_DIRS": True,
"OPTIONS": {"keep_trailing_newline": True},
},
]
)(test_func)
class override_script_prefix(TestContextDecorator):
"""Decorator or context manager to temporary override the script prefix."""
def __init__(self, prefix):
self.prefix = prefix
super().__init__()
def enable(self):
self.old_prefix = get_script_prefix()
set_script_prefix(self.prefix)
def disable(self):
set_script_prefix(self.old_prefix)
class LoggingCaptureMixin:
"""
Capture the output from the 'django' logger and store it on the class's
logger_output attribute.
"""
def setUp(self):
self.logger = logging.getLogger("django")
self.old_stream = self.logger.handlers[0].stream
self.logger_output = StringIO()
self.logger.handlers[0].stream = self.logger_output
def tearDown(self):
self.logger.handlers[0].stream = self.old_stream
class isolate_apps(TestContextDecorator):
"""
Act as either a decorator or a context manager to register models defined
in its wrapped context to an isolated registry.
The list of installed apps the isolated registry should contain must be
passed as arguments.
Two optional keyword arguments can be specified:
`attr_name`: attribute assigned the isolated registry if used as a class
decorator.
`kwarg_name`: keyword argument passing the isolated registry if used as a
function decorator.
"""
def __init__(self, *installed_apps, **kwargs):
self.installed_apps = installed_apps
super().__init__(**kwargs)
def enable(self):
self.old_apps = Options.default_apps
apps = Apps(self.installed_apps)
setattr(Options, "default_apps", apps)
return apps
def disable(self):
setattr(Options, "default_apps", self.old_apps)
class TimeKeeper:
def __init__(self):
self.records = collections.defaultdict(list)
@contextmanager
def timed(self, name):
self.records[name]
start_time = time.perf_counter()
try:
yield
finally:
end_time = time.perf_counter() - start_time
self.records[name].append(end_time)
def print_results(self):
for name, end_times in self.records.items():
for record_time in end_times:
record = "%s took %.3fs" % (name, record_time)
sys.stderr.write(record + os.linesep)
class NullTimeKeeper:
@contextmanager
def timed(self, name):
yield
def print_results(self):
pass
def tag(*tags):
"""Decorator to add tags to a test class or method."""
def decorator(obj):
if hasattr(obj, "tags"):
obj.tags = obj.tags.union(tags)
else:
setattr(obj, "tags", set(tags))
return obj
return decorator
@contextmanager
def register_lookup(field, *lookups, lookup_name=None):
"""
Context manager to temporarily register lookups on a model field using
lookup_name (or the lookup's lookup_name if not provided).
"""
try:
for lookup in lookups:
field.register_lookup(lookup, lookup_name)
yield
finally:
for lookup in lookups:
field._unregister_lookup(lookup, lookup_name)
