celery 运行时 （from . import async, base）SyntaxError: invalid syntax 异常的解决方案
今天，有一个任务需要异步执行。发现 celery 框架不错，于是尝试了一下。
按照官方文档中的示例，执行的时候出现了一个异常：
$ celery -A tasks worker --loglevel=info
[2018-11-16 18:16:52,668: CRITICAL/MainProcess] Unrecoverable error: SyntaxError('invalid syntax', ('/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/backends/redis.py', 22, 19, 'from . import async, base\n'))
Traceback (most recent call last):
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/kombu/utils/objects.py", line 42, in __get__
return obj.__dict__[self.__name__]
KeyError: 'backend'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/worker/worker.py", line 205, in start
self.blueprint.start(self)
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/bootsteps.py", line 115, in start
self.on_start()
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/apps/worker.py", line 139, in on_start
self.emit_banner()
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/apps/worker.py", line 154, in emit_banner
' \n', self.startup_info(artlines=not use_image))),
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/apps/worker.py", line 217, in startup_info
results=self.app.backend.as_uri(),
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/kombu/utils/objects.py", line 44, in __get__
value = obj.__dict__[self.__name__] = self.__get(obj)
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/app/base.py", line 1196, in backend
return self._get_backend()
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/app/base.py", line 914, in _get_backend
self.loader)
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/app/backends.py", line 70, in by_url
return by_name(backend, loader), url
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/app/backends.py", line 50, in by_name
cls = symbol_by_name(backend, aliases)
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/kombu/utils/imports.py", line 56, in symbol_by_name
module = imp(module_name, package=package, **kwargs)
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/importlib/__init__.py", line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
File "<frozen importlib._bootstrap>", line 983, in _find_and_load
File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 724, in exec_module
File "<frozen importlib._bootstrap_external>", line 860, in get_code
File "<frozen importlib._bootstrap_external>", line 791, in source_to_code
File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/celery/backends/redis.py", line 22
from . import async, base
^
SyntaxError: invalid syntax根据错误提示，发现是文件celery/backends/redis.py的22行出现问题：from . import async, base。后来，网上查了一下，发现是 celery 4.2.1 版本有问题。
解决方案
从 master 分支重新获取，执行命令如下：
pip install --upgrade https://github.com/celery/celery/tarball/master
命令执行后，发现版本重置到了 celery-4.2.0。
执行过程：
$ pip install --upgrade https://github.com/celery/celery/tarball/master
Looking in indexes: http://mirrors.aliyun.com/pypi/simple/
Collecting https://github.com/celery/celery/tarball/master
Downloading https://github.com/celery/celery/tarball/master
/ 1.9MB 856kB/s
Requirement already satisfied, skipping upgrade: pytz>dev in /Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages (from celery==4.2.0) (2018.7)
Requirement already satisfied, skipping upgrade: billiard<3.6.0,>=3.5.0.2 in /Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages (from celery==4.2.0) (3.5.0.4)
Requirement already satisfied, skipping upgrade: kombu<5.0,>=4.2.0 in /Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages (from celery==4.2.0) (4.2.1)
Requirement already satisfied, skipping upgrade: amqp<3.0,>=2.1.4 in /Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages (from kombu<5.0,>=4.2.0->celery==4.2.0) (2.3.2)
Requirement already satisfied, skipping upgrade: vine>=1.1.3 in /Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages (from amqp<3.0,>=2.1.4->kombu<5.0,>=4.2.0->celery==4.2.0) (1.1.4)
Building wheels for collected packages: celery
Running setup.py bdist_wheel for celery ... done
Stored in directory: /private/var/folders/nn/nm7dqchn0rzcs0f2l2mpjkq40000gn/T/pip-ephem-wheel-cache-lpfank45/wheels/77/5f/be/3607e48004e6496d18a500339472d42f85c7a2446157726958
Successfully built celery
Installing collected packages: celery
Found existing installation: celery 4.2.1
Uninstalling celery-4.2.1:
Successfully uninstalled celery-4.2.1
Successfully installed celery-4.2.0调用时 redis 异常
另外，在使用 celery + redis 调用时，发生了如下异常：
File "/Users/Allert/.local/share/virtualenvs/testpure-2oTTuBuP/lib/python3.7/site-packages/redis/_compat.py", line 94, in iteritems
return x.iteritems()
AttributeError: 'float' object has no attribute 'iteritems'从错误提示中，可以看到是 redis 中的文件发生异常。
这貌似是 redis 3.0.1 的一个兼容问题。
解决方案：重新安装 2.10.6版本。
命令如下：pip install redis==2.10.6
参考：
https://stackoverflow.com/questions/53322425/celery-critical-mainprocess-unrecoverable-error-attributeerrorfloat-object
https://www.pyget.cn/p/186098