Ensure `import urllib3` can succeed even if trio and twisted aren't installed #42

oliland · 2018-04-30T15:34:42Z

This is ~~a draft~~ an attempt at resolving #25

This ensures that we never directly import trio / twisted until we need them. We do this by adding some scary logic to _backends.

Looking for feedback on the import mechanism.

oliland · 2018-04-30T15:40:24Z

To reiterate - this is very much a draft - just looking for feedback on the approach before I go about fixing any broken tests.

pquentin

Awesome, I think this is going in the good direction! I personally like the string approach. Of course, for an important API question like this, I'll wait for the approval of @njsmith (and tests) before merging. :)

I added some thoughts below about the discovery of new backends and error handling, which is an important part of the design here. Please tell me what you think. Thanks!

pquentin · 2018-05-01T03:38:27Z

urllib3/_backends/__init__.py

+        try:
+            load_backend_fn()
+            output.append(backend_name)
+        except:  # noqa


You probably want at least except Exception in order to avoid catching KeyboardInterrupt exceptions.

Any good reason not to simply use ImportError? I guess the import can fail with different errors, but as a user, I would like to know if a dependency is half-installed.

Am I missing something?

That sounds like a good idea - thanks!

pquentin · 2018-05-01T03:45:22Z

urllib3/_backends/__init__.py

+        return BACKENDS[backend_name]()(**_backend_args)
+    else:
+        valid_backends = ", ".join(sorted(BACKENDS.keys()))
+        raise ValueError("Backend must be one of: {}".format(valid_backends))


Another option would be to use a static list of backends in the else clause, and let the loading of a backend fail if the dependency is not installed. Which might be more intuitive?

I'm not sure I follow - can you rephrase your suggestion?

This else block is raising an error if the user specifies an unknown backend. At this point, I'm assuming that we will have a finite list of backends in urllib3 and will not allow users to write their own without adding them upstream - maybe we want to reconsider this, though.

Sorry, this was not easy to understand. I'll try to be clearer this time.

So I had two suggestions. The first one, above, was simply about replacing except by except Exception or except ImportError.

This suggestion would have more impact. I was wondering about hard coding the valid backends (currently that would be trio, twisted and sync) and always report them as available in the ValueError raised in the else clause of load_backend. I think this would help with the discovery of new backends. Now, if a user tries to use such a backend but the import fails, I don't know if we should catch the ImportError exception or let it propagate.

I'm really thinking aloud here: I don't know if that would be better or not!

@pquentin Apologies - I think I've misled you. The available_backends function is not used - the error I am raising contains all "known" backends.

I added available_backends to assist in debugging from a repl.

~~I'm going to remove the available_backends function as it serves no purpose beyond debugging, and probably wouldn't be exposed as a user visible API.~~ Looks like we're on the same page now.

Not sure how I managed to misread this. Sorry!

I think we can leave available_backends as it is.

Sorry for the back and forth on this. But it didn’t make sense for me to implement available_backends just yet without agreeing on some kind of user interface for it. I’d like to do that separately.

We also need to be careful with that functionality - as it will import all of the backends, it’s probably something that we don’t want to encourage users to call.

oliland · 2018-05-01T14:23:54Z

OK, brace yourselves. This change has uncovered a race in the import logic which means that two modules do not consider two different imports of the LoopAbort class to be equivalent, meaning that the exception cannot be caught across the backend / urllib frontier.

Steps to reproduce:

check out the original bleach-spike branch and add some instrumentation to both sync_backend.py and connection.py - something like

print(LoopAbort, id(LoopAbort))

run this test. observe that the id of the LoopAbort exception is the same in both modules

pytest test/with_dummyserver/test_no_ssl.py -v
<class 'urllib3._backends._common.LoopAbort'> 140369638052840
<class 'urllib3._backends._common.LoopAbort'> 140369638052840

check out this branch and add the above instrumentation. note that the id of LoopAbort in each module is now different, which fails our tests as the LoopAbort exception is no longer catchable as it is not considered equivalent.

pytest test/with_dummyserver/test_no_ssl.py -v
<class 'urllib3._backends._common.LoopAbort'> 140625114597352
<class 'urllib3._backends._common.LoopAbort'> 140625077300856

This still happens if we switch out our raise LoopAbort to be raise LoopAbort() and still occurs when we change the import semantics slightly (e.g. by importing from absolute v.s. relative paths).

python wtf?

oliland · 2018-05-01T14:33:26Z

I've added some code that documents the issue in 1a73eff - @pquentin any ideas?

FWIW, I've had a scan of http://python-notes.curiousefficiency.org/en/latest/python_concepts/import_traps.html and don’t immediately spot any bad practices, but could well be missing something.

I speculate that our __init__.py was previously short circuiting some of our tests which have import blockers, and that they are now confusing the import system.

njsmith · 2018-05-01T19:40:34Z

Regarding the public API, see: #25 (comment)

oliland · 2018-05-02T00:03:30Z

TIL something about Python.

This:

from .sync_backend import SyncBackend


def load_sync_backend():
    return SyncBackend

Is subtly different to this:

def load_sync_backend():
    from .sync_backend import SyncBackend
    return SyncBackend

I believe that in the second case, I am triggering a variant of the Double Import Trap. While the repro is different, the outcome is that I have two copies of the same module.

Two copies of the same module means I have two copies of the LoopAbort class, which can never be equivalent as their type() is different (and I believe this code would be used to check for Exception equality, as the BaseException class doesn't declare a rich comparison function).

Tl;dr: This means that we can’t catch LoopAbort once we re-import it, as the raised exception is of a different type than the type specified in the ‘except’ block.

Perhaps reassuringly, this also fails:

from .sync_backend import SyncBackend


def load_sync_backend():
    from .sync_backend import SyncBackend
    return SyncBackend

I'll need to think quite deeply about how to solve this, as ideally we would allow ourselves to load backends dynamically, but we will need to acknowledge that they cannot communicate using types (including Exceptions) as their types will be considered different if we load them in a subsequent run of the import system.

pquentin · 2018-05-02T06:14:55Z

Wow, nice detective work here! 💯

Would adding a rich comparison function to LoopAbort work?

njsmith · 2018-05-02T06:31:23Z

except blocks use isinstance, not ==, so I don't think rich comparison will help. I guess some __subclasscheck__ metaclass hackery would be possible in theory, but... we should figure out the actual problem :-).

This looks really weird to me, in particular this bit:

<class 'urllib3._backends._common.LoopAbort'> 140625114597352
<class 'urllib3._backends._common.LoopAbort'> 140625077300856

I really don't think this is supposed to be possible!

From the double-import trap link: "The reason this is problematic is that every module in that directory is now potentially accessible under two different names", but in the above, the two different LoopAbort objects both think that they're in the urllib3._backends._common module, and if they have the same name then it's supposed to mean that it has just one entry in sys.modules and only gets imported once. And importing inside a function or at the top-level shouldn't change this...

@oliland I don't suppose you've cut down a minimal reproducer?

To pick some random folks who I know know a lot about the import system: @ncoghlan @brettcannon Do you have any idea how in Python 3.6 you could end up with two different imports of the same module under the same name?

njsmith · 2018-05-02T07:14:16Z

Here's another oddity: when I add a print statement to urllib3/_backends/_common.py, it only seems to execute once, suggesting that it actually only gets imported once. Yet I can still reproduce the issue. So where's this other LoopAbort even coming from?

I thought this must mean that there's another copy of _common.py somehow on the PYTHONPATH, like the one in the build/ directory or something, but when I ran strace -e file -f pytest ..., then grepped the output for _common, I get:

4070  stat("/home/njs/src/urllib3/urllib3/_backends/_common.py", {st_mode=S_IFREG|0644, st_size=1069, ...}) = 0
4070  stat("/home/njs/src/urllib3/urllib3/_backends/_common.py", {st_mode=S_IFREG|0644, st_size=1069, ...}) = 0
4070  openat(AT_FDCWD, "/home/njs/src/urllib3/urllib3/_backends/__pycache__/_common.cpython-35.pyc", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
4070  openat(AT_FDCWD, "/home/njs/src/urllib3/urllib3/_backends/_common.py", O_RDONLY|O_CLOEXEC) = 8
4070  stat("/home/njs/src/urllib3/urllib3/_backends/_common.py", {st_mode=S_IFREG|0644, st_size=1069, ...}) = 0
4070  stat("/home/njs/src/urllib3/urllib3/_backends/_common.py", {st_mode=S_IFREG|0644, st_size=1069, ...}) = 0
4070  openat(AT_FDCWD, "/home/njs/src/urllib3/urllib3/_backends/__pycache__/_common.cpython-35.pyc", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
4070  openat(AT_FDCWD, "/home/njs/src/urllib3/urllib3/_backends/_common.py", O_RDONLY|O_CLOEXEC) = 17
4070  stat("/home/njs/src/urllib3/urllib3/_backends/_common.py", {st_mode=S_IFREG|0644, st_size=1069, ...}) = 0
4070  lstat("/home/njs/src/urllib3/urllib3/_backends/_common.py", {st_mode=S_IFREG|0644, st_size=1069, ...}) = 0

If we ignore the stats and focus on the openats, we see that this python process is in fact opening /home/njs/src/urllib3/urllib3/_backends/_common.py twice, and no other files named _common.py at all.

So where the heck does this other LoopAbort even come from?

ncoghlan · 2018-05-02T11:08:24Z

My initial suspicion would be that you might be running into one of the subtle race conditions [1] that have affected the per-module import locks at various points in time, and are hence getting two concurrent imports of the "urllib3._backends._common" module. However, that would only be a plausible explanation if the error was coming up on 3.6.2, and everything worked fine on 3.6.3 and later.

[1] https://bugs.python.org/issue30891, https://bugs.python.org/issue30876, https://bugs.python.org/issue31070

oliland · 2018-05-02T15:41:41Z

@njsmith good points. As you've pointed out that both imports should resolve to the same key in sys.modules, I've unhidden my initial findings as they are not fully resolved.

I've really struggled to repro this outside of the urllib3 repository. The only other thing I've noticed is that only the id of the LoopAbort class is different, the id's of sys.modules["urllib3._backends._common"] appears to be the same.

There's tons of other weird stuff, hard to tell if relevant or not. Adding another line to consume_bytes just above raise LoopAbort appears to fix the problem:

context['h11_response'] = event
from .._backends._common import LoopAbort
raise LoopAbort

Which is cool, apart from the fact that this shouldn't be necessary. For bonus points, add these lines:

context['h11_response'] = event
print("about to raise loopabort")
print(id(LoopAbort))
from ._backends._common import LoopAbort
print(id(LoopAbort))
raise LoopAbort

What happens here? You guessed it - an UnboundLocalError is thrown - not on the second print(id()), but the first, which implies that the inline import declaration somehow destroys the outer declaration even before the inline import is executed?!

I'll continue to try to extract a repo, though at this point I think I will need to delete the rest of urllib3 around this code to get there.

oliland · 2018-05-02T16:41:40Z

Having failed to produce a minimal repro, I started to delete the rest of urllib3 around this bug and believe I have found the code responsible for this confusion:

https://github.com/python-trio/urllib3/blob/bleach-spike/test/test_no_ssl.py#L67

Our failing test class in question uses this ModuleStash object which appears to mutate sys.modules.

On setup it iterates through sys.modules and pops any modules related to urllib3, then re-adds them on teardown.

Removing all calls to this ModuleStash object solves the problem and all imports resolve to the same id.

Interestingly, it's possible to make the tests pass 50% of the time if you only remove the line module_stash.stash(). Only by removing both module_stash.stash() and module_stash.pop() is the issue enturely resolved.

There are plenty of caveats in the docs [1][2] around screwing around with sys.modules, so we definitely shouldn't do this if it's purely in the interest of coverage. Python 3 docs come with a particularly suitable warning:

...deleting essential items from the dictionary may cause Python to fail.

so tl;dr: It looks like two ingredients are required to get this crazy behaviour:

mutating sys.modules
imports at runtime instead of import time

[1] https://docs.python.org/3/library/sys.html#sys.modules
[2] https://docs.python.org/3/library/importlib.html#importlib.reload

oliland · 2018-05-02T16:48:31Z

It looks like this crazy module stashing thing is in upstream urllib3, so we should definitely make a case for removing it there.

njsmith · 2018-05-02T17:31:28Z

Wow, yeah, that could certainly produce weird effects. Excellent detective work.

It makes sense to want to test that urllib3 copes with a missing ssl module, because that's a thing that happens. But I think the way I'd test it would be to spawn a child process with a clean state, and try importing there.

I agree that it'd be good to try to fix this upstream. While that's happening, we can mark these tests as skipped, so they don't block progress on the branch.

njsmith · 2018-05-02T17:41:08Z

Btw, I think this is demonstrating a weird quirk of how python works in general:

the inline import declaration somehow destroys the outer declaration even before the inline import is executed?!

When python compiles a function body, it statically looks for any variables that are involved in assignment operations, and marks those as local, throughout the whole function body. This is why if you want to assign to a local from inside a function, you need to say so explicitly:

X = 1
def foo():
    # creates a local, doesn't affect global
    X = 2

def bar():
    # works fine
    print(X)

def baz():
    # fails! Python thinks you're trying to print the local created below!
    print(X)
    X = 2

def quux():
    # tells python that in this function, X refers to the global
    global X
    # now this works
    print(X)
    # and this mutates the global
    X = 2

ncoghlan · 2018-05-03T13:56:38Z

Ah, yeah, doing a forced reload can definitely cause weirdness, especially if other modules are doing "from reloaded.module import attribute" (which is the case for LoopAbort) - even if reloaded.module.attribute gets replaced, other modules will still have references to the original object.

Very nice detective work tracking that down :)

oliland · 2018-05-03T17:03:24Z

test/test_no_ssl.py

@@ -90,6 +90,5 @@ def import_ssl():

        self.assertRaises(ImportError, import_ssl)

-    @pytest.mark.xfail


this test now passes. hooray!

oliland · 2018-05-03T17:03:43Z

test/with_dummyserver/test_no_ssl.py

 import urllib3


 class TestHTTPWithoutSSL(HTTPDummyServerTestCase, TestWithoutSSL):
+
+    @pytest.mark.skip(reason=(


this test is cursed. boo.

oliland · 2018-05-03T17:06:38Z

urllib3/_async/connection.py

@@ -311,8 +311,7 @@ def __init__(self, host, port, backend=None,
                 source_address=None, tunnel_host=None, tunnel_port=None,
                 tunnel_headers=None):
        self.is_verified = False
-
-        self._backend = backend or SyncBackend()
+        self._backend = backend or load_backend("sync")


Question: if this function constantly calls from .sync_backend import SyncBackend - is it constantly re-running the import machinery whenever we create a connection?

If so, we should special case SyncBackend to skip the import machinery in the interests of making the common case more efficient.

Alternatively, we should ensure that users have always initialized a backend by this point (which is how I plan for async backends to work), and just leave the or case for unit tests.

Re-importing an already-imported module is pretty cheap, because python immediately notices that it already has the loaded module cached and uses that. So it's like a few dict lookups.

Ok, also: what we actually want the logic to be here (I think) is:

If this is the sync version of the code, then the backend must be None or "sync" or Backend("sync"). Anything else is an error.

If this is the async version of the code, then the backend must be specified, and must be not the sync backend.

And... we kind of want to do this check inside all of the classes that take backend arguments.

So I suggest adding a normalize_backend function that takes a user backend argument, and returns a valid Backend or else an error.

The trickiest bit is detecting whether we are in sync mode or async mode. Eventually we should add a nice way to do this to the code generation script, but for now one can do:

async def f(): return None obj = f() if obj is None: SYNC_MODE = True else: # prevent unawaited coroutine warning obj.close() SYNC_MODE = False

oliland · 2018-05-03T17:12:25Z

urllib3/_backends/__init__.py

@@ -1,9 +0,0 @@
-from urllib3.packages import six


I don't really see the point in this file any more, so I truncated it. I guess it would make sense to put load_backend in here, and have that be the only interface to this module, but that feels like making a decision.

Additionally, I'm not sure why a private module (_backends) would expose a public interface (via __all__)?

If I did that then probably my logic was, this is a private API from the user point of view (they shouldn't mention _backend at all), and an opaque module from the point of view of the rest of the internal code (they can use the stuff in _backend.__all__, but shouldn't go digging deeper).

This is a pretty fine distinction and not particularly important.

oliland · 2018-05-03T17:13:00Z

urllib3/_backends/__init__.py

-
-__all__ = ['SyncBackend']
-
-if six.PY3:


FWIW, this doesn't seem correct - we should be targeting 3.5+, not 3+ (although maybe twisted is a special case)

oliland · 2018-05-03T17:14:34Z

urllib3/_backends/_loader.py

@@ -0,0 +1,67 @@
+import sys
+
+from ..backends import Backend as UserSpecifiedBackend


I'm trying to call out here that this is a user interface and should be handled appropriately. It's also not a "Backend" - it's a recipe for building one, but our users shouldn't need to think about that.

oliland · 2018-05-03T17:14:45Z

urllib3/_backends/_loader.py

+from ..backends import Backend as UserSpecifiedBackend
+
+
+def check_for_python_3_5():


Is this needed?

I don't think so. This should be taken care of by the higher level backend checking logic, plus the fact that you can't get into the async code at all unless you're on a new enough python.

oliland · 2018-05-03T17:15:46Z

urllib3/_backends/twisted_backend.py

@@ -15,8 +15,8 @@


 class TwistedBackend:
-    def __init__(self, reactor):
-        self._reactor = reactor
+    def __init__(self, reactor=None):


This change attempts to ensure that all of our backends can be initialized without any arguments, which would be nice.

oliland · 2018-05-03T17:16:41Z

urllib3/backends.py

@@ -0,0 +1,9 @@
+class Backend:


This is a user (or downstream repository) interface.

oliland · 2018-05-03T17:21:49Z

@njsmith Thanks for the explaination around variable hoisting! Very useful to know indeed.

Now that we're out of that rats nest, I've addressed @pquentin's comments and made moves towards adding the interface that @njsmith and @kennethreitz iterated on. This is looking a bit more merge-able now IMO. I would appreciate some more feedback, especially around the comments I added.

I'm happy to close this and open another PR if the above context is too distracting.

oliland · 2018-05-03T17:23:38Z

test/test_backends.py

+    )
+    def test_async(self):
+        load_backend("trio")
+        load_backend("twisted")


These tests just assert that the interface doesn't go down in flames - should we be asserting anything about the returned socket wrappers?

Ideally their interface would be defined somewhere and we could play with them a little bit, but that may be better suited to another test case.

Yeah, it'd be good to have some shared tests for basic backend functionality, but I think making it a separate PR would be better. (It would also help a lot to add bleached tests, so we could test both the sync and async backends with the same test code.)

njsmith · 2018-05-03T18:29:11Z

I'll try to take a closer look later, but quick note: currently our async mode requires 3.6+, because it uses an async generator. See the check in urllib3/__init__.py.

njsmith · 2018-05-03T18:30:35Z

Ideally we should consolidate this check somehow. Maybe the tests should check hasattr(urllib3, "AsyncPoolManager")?

njsmith

You'll also want to update the demo/ directory.

Still some work to do here, but it's coming along nicely!

njsmith · 2018-05-04T17:44:33Z

urllib3/backends.py

@@ -0,0 +1,9 @@
+class Backend:
+    """
+    Specifies the desired backend and any argumnets passed to it's constructor.


Typos: "arguments", "its" (this isn't short for "it is", so no apostrophe)

njsmith · 2018-05-04T17:46:55Z

test/test_backends.py

+    )
+    def test_async(self):
+        load_backend("trio")
+        load_backend("twisted")


Yeah, it'd be good to have some shared tests for basic backend functionality, but I think making it a separate PR would be better. (It would also help a lot to add bleached tests, so we could test both the sync and async backends with the same test code.)

njsmith · 2018-05-04T17:50:45Z

test/test_backends.py

+    """
+    def test_dummy(self):
+        with pytest.raises(ImportError):
+            load_backend("dummy")


I would drop the load_dummy stuff from the actual backend code – it's going to create funny coverage issues, and it makes error messages awkward. And I don't think we need to worry too much about testing that if a package is missing then we get an import error. (Or if we do want to test that, we could reuse whatever mechanism we come up with to test the missing ssl case.)

So I'd replace this test with one that checks what happens if someone tries to use some unknown backend.

njsmith · 2018-05-04T17:51:55Z

test/test_backends.py

+    @pytest.mark.skipif(
+        sys.version_info < (3, 5),
+        reason="async backends require Python 3.5 or greater",
+    )


Don't forget to replace this check with not hasattr(urllib3, "AsyncPoolManager")

njsmith · 2018-05-04T18:03:05Z

urllib3/_async/connection.py

@@ -311,8 +311,7 @@ def __init__(self, host, port, backend=None,
                 source_address=None, tunnel_host=None, tunnel_port=None,
                 tunnel_headers=None):
        self.is_verified = False
-
-        self._backend = backend or SyncBackend()
+        self._backend = backend or load_backend("sync")


Ok, also: what we actually want the logic to be here (I think) is:

If this is the sync version of the code, then the backend must be None or "sync" or Backend("sync"). Anything else is an error.

If this is the async version of the code, then the backend must be specified, and must be not the sync backend.

And... we kind of want to do this check inside all of the classes that take backend arguments.

So I suggest adding a normalize_backend function that takes a user backend argument, and returns a valid Backend or else an error.

The trickiest bit is detecting whether we are in sync mode or async mode. Eventually we should add a nice way to do this to the code generation script, but for now one can do:

async def f(): return None obj = f() if obj is None: SYNC_MODE = True else: # prevent unawaited coroutine warning obj.close() SYNC_MODE = False

njsmith · 2018-05-04T18:04:34Z

urllib3/_backends/_loader.py

+from ..backends import Backend as UserSpecifiedBackend
+
+
+def check_for_python_3_5():


I don't think so. This should be taken care of by the higher level backend checking logic, plus the fact that you can't get into the async code at all unless you're on a new enough python.

oliland · 2018-05-05T16:36:57Z

@njsmith comments addressed. This is getting a little complex - I've tried to make the error handling as simple as possible for now, we can decide later how best to help users to recover from these errors.

pquentin · 2018-05-23T10:25:46Z

@njsmith ping

pquentin · 2018-06-18T20:36:37Z

A few things are missing:

fix the tests
fix the async demo
fix the wording in trio backend requires urllib3 to be built with async suppor' errors: urllib3 is always built with sync and async support

pquentin · 2018-06-27T05:43:51Z

Hello! I opened #47 which builds on this PR and addresses the comments.

oliland requested review from pquentin and njsmith April 30, 2018 15:34

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remove xfail from TestImportWithoutSSL

21dfc1c

skip TestHTTPWithoutSSL

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urllib3/backends.py

@@ -0,0 +1,9 @@

class Backend:

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This is a user (or downstream repository) interface.

oliland requested a review from pquentin May 3, 2018 17:16

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oliland added 3 commits May 6, 2018 00:07

fix spelling

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add __eq__ to backend objects for testing

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ebbbf86

oliland force-pushed the bleach-spike-defer-backend-import branch from 26477ec to ebbbf86 Compare May 5, 2018 16:34

oliland requested a review from njsmith May 5, 2018 16:35

pquentin mentioned this pull request Jun 27, 2018

Take two: Ensure import urllib3 can succeed even if trio and twisted aren't installed #47

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pquentin merged commit ebbbf86 into python-trio:bleach-spike Jun 28, 2018

pquentin mentioned this pull request Nov 30, 2019

Rename inject_into_urllib3/extract_from_urllib3 #171

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pquentin mentioned this pull request Apr 25, 2021

Make S3 support optional elastic/rally#1252

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		@@ -90,6 +90,5 @@ def import_ssl():

		self.assertRaises(ImportError, import_ssl)

		@pytest.mark.xfail

		@@ -0,0 +1,67 @@
		import sys

		from ..backends import Backend as UserSpecifiedBackend

		from ..backends import Backend as UserSpecifiedBackend


		def check_for_python_3_5():

Ensure import urllib3 can succeed even if trio and twisted aren't installed #42

Ensure import urllib3 can succeed even if trio and twisted aren't installed #42

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oliland commented May 5, 2018

pquentin commented May 23, 2018

pquentin commented Jun 18, 2018

pquentin commented Jun 27, 2018

Ensure `import urllib3` can succeed even if trio and twisted aren't installed #42

Ensure `import urllib3` can succeed even if trio and twisted aren't installed #42

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