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Flask import problem with Python 3 and __main__.py

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  Flask import problem with Python 3 and __main__.py Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2014-08-26 16:03 +0000
    Re: Flask import problem with Python 3 and __main__.py Terry Reedy <tjreedy@udel.edu> - 2014-08-26 14:03 -0400
      Re: Flask import problem with Python 3 and __main__.py Jon Ribbens <jon+usenet@unequivocal.co.uk> - 2014-08-26 18:31 +0000

#77048 — Flask import problem with Python 3 and __main__.py

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__)

Running this with 'python3 runflaskapp.py' works fine. 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'. Unfortunately this doesn't
work:

    $ python3 -m flaskapp
     * Running on http://127.0.0.1:5000/
     * Restarting with reloader
    Traceback (most recent call last):
      File "/home/username/src/flaskapp/__main__.py", line 1, in <module>
	from flaskapp import app
    ImportError: No module named 'flaskapp'

Does anyone know why and how to fix it?

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#77062

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)

> 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.

> 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?

 > Unfortunately this doesn't work:

Because x does not get added to sys.path.

>      $ python3 -m flaskapp
>       * Running on http://127.0.0.1:5000/
>       * Restarting with reloader
>      Traceback (most recent call last):
>        File "/home/username/src/flaskapp/__main__.py", line 1, in <module>
> 	from flaskapp import app
>      ImportError: No module named 'flaskapp'
>
> Does anyone know why and how to fix it?

Since flaskapp/__main__.py is found and run, make the change suggested 
above that eliminates the flaskapp import.

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

Pip, and I presume other installers, typically puts startup scripts in a 
directory that is on the system path. For Windows, this is 
pythonxy/Scripts.  But this is more than I would do for most local apps.

-- 
Terry Jan Reedy

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#77063

On 2014-08-26, Terry Reedy <tjreedy@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.

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