Airflow initdb fails due to SyntaxError on importing AsyncRetrying - installation

Question

I am new to Airflow. I install it by pip install apache-airflow. When I run command airflow initdb in terminal then I am getting the error below. Where did I go wrong during install, and how can I fix this issue?
aamir#aamir-Inspiron-3542:~$ airflow initdb
[2019-03-30 18:32:27,309] {__init__.py:51} INFO - Using executor SequentialExecutor
DB: sqlite:////home/aamir/airflow/airflow.db
[2019-03-30 18:32:31,790] {db.py:338} INFO - Creating tables
INFO [alembic.runtime.migration] Context impl SQLiteImpl.
INFO [alembic.runtime.migration] Will assume non-transactional DDL.
ERROR [airflow.models.DagBag] Failed to import: /home/aamir/anaconda3/lib/python3.7/site-packages/airflow/example_dags/example_http_operator.py
Traceback (most recent call last):
File "/home/aamir/anaconda3/lib/python3.7/site-packages/airflow/models.py", line 374, in process_file
m = imp.load_source(mod_name, filepath)
File "/home/aamir/anaconda3/lib/python3.7/imp.py", line 171, in load_source
module = _load(spec)
File "<frozen importlib._bootstrap>", line 696, in _load
File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 728, in exec_module
File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
File "/home/aamir/anaconda3/lib/python3.7/site-packages/airflow/example_dags/example_http_operator.py", line 27, in <module>
from airflow.operators.http_operator import SimpleHttpOperator
File "/home/aamir/anaconda3/lib/python3.7/site-packages/airflow/operators/http_operator.py", line 21, in <module>
from airflow.hooks.http_hook import HttpHook
File "/home/aamir/anaconda3/lib/python3.7/site-packages/airflow/hooks/http_hook.py", line 23, in <module>
import tenacity
File "/home/aamir/anaconda3/lib/python3.7/site-packages/tenacity/__init__.py", line 352
from tenacity.async import AsyncRetrying
^
SyntaxError: invalid syntax
Done.

Answer 1

In Python 3.7, async is a reserved keyword, which means it cannot be used in module and variable names. This was valid in prior Python versions, but starting from 3.7, a SyntaxError is raised.
In your case, Airflow comes pre-installed with example DAGs, which were parsed when you ran airflow initdb. Some of those DAGs make use of the SimpleHttpOperator which depends on http_hook.py. That hook furthermore depends on the tenacity library, which tries to import an async module as part of initialization:
from tenacity.async import AsyncRetrying
To fix this, wait for/install Airflow v1.10.3 which updates Tenacity (see AIRFLOW-2876). Alternatively, you can downgrade your Python version. You can see that the import fails using 3.7.3:
$ docker run --rm -it python:3.7
Python 3.7.3 (default, Mar 27 2019, 23:40:30)
>>> from tenacity.async import AsyncRetrying
File "<stdin>", line 1
from tenacity.async import AsyncRetrying
^
SyntaxError: invalid syntax
But it works fine in version 3.6.8:
$ docker run --rm -it python:3.6
Python 3.6.8 (default, Feb 6 2019, 12:07:20)
>>> from tenacity.async import AsyncRetrying
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'tenacity'