Flask import problem with Python 3 and __main__.py

[Jon Ribbens]

On 2014-08-26, Terry Reedy <tjreedy at udel.edu> wrote:
> On 8/26/2014 12:03 PM, Jon Ribbens wrote:
>> Flask suggests the following file layout:
>>
>>      runflaskapp.py
>>      flaskapp/
>>          __init__.py
>>
>> runflaskapp.py contains:
>>
>>      from flaskapp import app
>>      app.run(debug=True)
>>
>> flaskapp/__init__.py contains:
>>
>>      from flask import Flask
>>      app = Flask(__name__)
>
> Unless there is something else in flaskapp, this seems senseless.  Why 
> not runflaskapp.py:
>
> from flask import Flask
> app = Flask(__name__)
> app.run(debug=True)

Because that's not how Flask apps work. I am showing a minimal test
case, obviously for any real app not only would __init__.py contain
more code, but there would be other files inside flaskapp/ too.
Then when deployed, 'runflaskapp.py' would either be changed or go
away entirely and the web server would just be pointed at 'flaskapp'.

>> Running this with 'python3 runflaskapp.py' works fine.
>
> You are either giving this in directory 'x' containing runflaskapp.py or 
> given a longer pathname. In either case, directory 'x' get prepended to 
> sys.path, so that 'import flaskapp' finds flaskapp in x.

Well, as I understand it actually the empty string is in sys.path,
which is taken by Python to mean 'the current directory'.

>> However it seems to me that a more Python3onic way of doing this
>> would be to rename 'runflaskapp.py' as 'flaskapp/__main__.py'
>> and then run the whole thing as 'python3 -m flaskapp'.
>
> In what directory?

In the same directory as above, i.e. the one containing 'flaskapp'.
It clearly does find 'flaskapp' initially, otherwise I would get
a different error message "/usr/bin/python3: No module named flaskapp".

> > Unfortunately this doesn't work:
>
> Because x does not get added to sys.path.

No, but the current directory does (effectively).

> Or put flaskapp in site_packages, which is on the import search path .

That's no use for development though.

The important part of my question is "why is running __main__.py
from inside flaskapp/ somehow different to running runflaskapp.py
from the parent directory?" It's probably a fairly Flask-specific
question.