Different kinds of Import Errors

[Thomas Guettler]

If you look at this code, you see there are two kind of ImportErrors:

1. app_name has no attribute or file managment.py: That's OK.
2. managment.py exists, but raises an ImportError: That's not OK: reraise

# Import the 'management' module within each installed app, to register
# dispatcher events.
for app_name in settings.INSTALLED_APPS:
try:
__import__(app_name + '.management', {}, {}, [''])
except ImportError, exc:
if exc.args[0]!='No module named management':
raise
I am searching a better solution, since in a future version of python
the string 'No module namend management' might be changed.

Any better solution?

[kyosohma]

On Nov 27, 7:35 am, Thomas Guettler <[e-mail]:

If you look at this code, you see there are two kind of ImportErrors:

1. app_name has no attribute or file managment.py: That's OK.
2. managment.py exists, but raises an ImportError: That's not OK: reraise

# Import the 'management' module within each installed app, to register
# dispatcher events.
for app_name in settings.INSTALLED_APPS:
try:
__import__(app_name + '.management', {}, {}, [''])
except ImportError, exc:
if exc.args[0]!='No module named management':
raise

I am searching a better solution, since in a future version of python
the string 'No module namend management' might be changed.

Any better solution?

I would assume that in the future versions of Python, it would still
mention the word "management". In that case, you could do something
like this in the except clause:

# untested
args = exc.args[0]
if args.find('management') != -1:
raise
Mike

[Aahz]

In article <a0**********************************@s8g2000prg.g ooglegroups.com>,
<ky******@gmail.comwrote:

>
# untested
args = exc.args[0]
if args.find('management') != -1:
raise

YM

if 'management' in args:
raise

HTH, HAND ;-)
--
Aahz (aa**@pythoncraft.com) <* http://www.pythoncraft.com/

"Typing is cheap. Thinking is expensive." --Roy Smith

[kyosohma]

On Nov 27, 3:50 pm, [e-mail] (Aahz) wrote:

In article <[e-mail]>,

<[e-mail]:

# untested
args = exc.args[0]
if args.find('management') != -1:
raise

YM

if 'management' in args:
raise

HTH, HAND ;-)
--
Aahz ([e-mail]) <* http://www.pythoncraft.com/

"Typing is cheap. Thinking is expensive." --Roy Smith

Oops...I've used this method myself. Your method is definitely nicer
and likelier more "Pythonic" than mine. Live and learn.

Mike

[Graham Dumpleton]

On Nov 28, 12:35 am, Thomas Guettler <[e-mail]:

If you look at this code, you see there are two kind of ImportErrors:

1. app_name has no attribute or file managment.py: That's OK.
2. managment.py exists, but raises an ImportError: That's not OK: reraise

# Import the 'management' module within each installed app, to register
# dispatcher events.
for app_name in settings.INSTALLED_APPS:
try:
__import__(app_name + '.management', {}, {}, [''])
except ImportError, exc:
if exc.args[0]!='No module named management':
raise

I am searching a better solution, since in a future version of python
the string 'No module namend management' might be changed.

Any better solution?

Perhaps check for the presence of the module in sys.modules.

if (app_name + '.management') in sys.modules:
raise

If not there, module couldn't be found. If module there but exception
occurred then some other import related error occurred.

Graham

[Peter Otten]

Thomas Guettler wrote:

If you look at this code, you see there are two kind of ImportErrors:

1. app_name has no attribute or file managment.py: That's OK.
2. managment.py exists, but raises an ImportError: That's not OK: reraise

# Import the 'management' module within each installed app, to register
# dispatcher events.
for app_name in settings.INSTALLED_APPS:
try:
__import__(app_name + '.management', {}, {}, [''])
except ImportError, exc:
if exc.args[0]!='No module named management':
raise
I am searching a better solution, since in a future version of python
the string 'No module namend management' might be changed.

Any better solution?

An alternative way to determine the expected message:

try:
import sys.pink_elephant
except ImportError, e:
expected_message = e.message.replace("pink_elephant", "management")
else:
raise Exception("your python has swallowed a pink elephant")

I'm probably not serious.

Peter

[Thomas Guettler]

Sorry, but this does not work. If there is an ImportError
during importing the existing module, it won't get inserted
into sys.modules. I just tried it with a small example.

An other solution would be to inspect the traceback. If the
app_name+'.management' is in it, it exists.

Graham Dumpleton schrieb:

On Nov 28, 12:35 am, Thomas Guettler <[e-mail]:

>If you look at this code, you see there are two kind of ImportErrors:

1. app_name has no attribute or file managment.py: That's OK.
2. managment.py exists, but raises an ImportError: That's not OK: reraise

# Import the 'management' module within each installed app, to register
# dispatcher events.
for app_name in settings.INSTALLED_APPS:
try:
__import__(app_name + '.management', {}, {}, [''])
except ImportError, exc:
if exc.args[0]!='No module named management':
raise

I am searching a better solution, since in a future version of python
the string 'No module namend management' might be changed.

Any better solution?

Perhaps check for the presence of the module in sys.modules.

if (app_name + '.management') in sys.modules:
raise

If not there, module couldn't be found. If module there but exception
occurred then some other import related error occurred.

Graham

[Graham Dumpleton]

On Dec 1, 12:24 am, Thomas Guettler <[e-mail]:

Sorry, but this does not work. If there is an ImportError
during importing the existing module, it won't get inserted
into sys.modules. I just tried it with a small example.

An other solution would be to inspect the traceback. If the
app_name+'.management' is in it, it exists.

Graham Dumpleton schrieb:

On Nov 28, 12:35 am, Thomas Guettler <[e-mail]:

If you look at this code, you see there are two kind of ImportErrors:

1. app_name has no attribute or file managment.py: That's OK.
2. managment.py exists, but raises an ImportError: That's not OK: reraise

# Import the 'management' module within each installed app, to register
# dispatcher events.
for app_name in settings.INSTALLED_APPS:
try:
__import__(app_name + '.management', {}, {}, [''])
except ImportError, exc:
if exc.args[0]!='No module named management':
raise

I am searching a better solution, since in a future version of python
the string 'No module namend management' might be changed.

Any better solution?

Perhaps check for the presence of the module in sys.modules.

if (app_name + '.management') in sys.modules:
raise

If not there, module couldn't be found. If module there but exception
occurred then some other import related error occurred.

What example did you use to test it? What version of Python are you
using?

Curious, as at least for Python 2.3 it behaves as I described with the
simple examples I tested. I do vaguely remember behaviour related to
this changing though, so possible that newer version works
differently.

First try case of importing module that doesn't exist:

>>import sys
import z

Traceback (most recent call last):
File "<stdin>", line 1, in ?
ImportError: No module named z

>>sys.modules['z']

Traceback (most recent call last):
File "<stdin>", line 1, in ?
KeyError: 'z'

Thus ImportError occurred, and not in sys.modules. Therefore module
must not have been able to be found.

Now lets try existing module, but non ImportError case first:

$ cat a.py
print 'a1'
xxx
print 'a2'

>>import sys
sys.modules['a']

Traceback (most recent call last):
File "<stdin>", line 1, in ?
KeyError: 'a'

>>import a

a1
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "a.py", line 2, in ?
xxx
NameError: name 'xxx' is not defined

>>sys.modules['a']

<module 'a' from 'a.py'>

Thus, although an exception occurred in importing file, the module is
still in sys.modules.

Now lets try for an ImportError for where target module exists:

$ cat a.py
print 'a1'
import z
print 'a2'

>>import sys
import a

a1
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "a.py", line 2, in ?
import z
ImportError: No module named z

>>sys.modules['a']

<module 'a' from 'a.py'>

In this case ImportError occurred but is in sys.modules, thus
ImportError can only have been related to a sub module import as top
level module did actually exist.

I'll have to try this at work next week on newer version of Python
than my Mac has.

Graham

[Thomas Guettler]

Graham Dumpleton schrieb:

What example did you use to test it? What version of Python are you
using?

Yes, this changed. Python 2.3 includes the half imported module. Python 2.4 does not.
But the traceback of the exception contains the needed information:

Here are the two example files:

# foo.py
import sys
print sys.version
try:
import foomod
except ImportError, exc:
mods=sys.modules.keys()
mods.sort()
print mods
import traceback
print traceback.extract_tb(sys.exc_info()[2])

# foomod.py
import does_not_exist

[Gabriel Genellina]

On 1 dic, 02:41, Graham Dumpleton <[e-mail]:

On Dec 1, 12:24 am, Thomas Guettler <[e-mail]:

Sorry, but this does not work. If there is an ImportError
during importing the existing module, it won't get inserted
into sys.modules. I just tried it with a small example.

Curious, as at least for Python 2.3 it behaves as I described with the
simple examples I tested. I do vaguely remember behaviour related to
this changing though, so possible that newer version works
differently.

Yes. It was changed on 2.4. From http://www.python.org/doc/2.4/whatsnew/node12.html:

"Encountering a failure while importing a module no longer leaves a
partially-initialized module object in sys.modules. The incomplete
module object left behind would fool further imports of the same
module into succeeding, leading to confusing errors. (Fixed by Tim
Peters.)"

--
Gabriel Genellina

[Jeremy C B Nicoll]

Thomas Guettler <[e-mail]:

If you look at this code, you see there are two kind of ImportErrors:

1. app_name has no attribute or file managment.py: That's OK.
2. managment.py exists, but raises an ImportError: That's not OK: reraise

# Import the 'management' module within each installed app, to

register

# dispatcher events.
for app_name in settings.INSTALLED_APPS:
try:
__import__(app_name + '.management', {}, {}, [''])
except ImportError, exc:
if exc.args[0]!='No module named management':
raise
I am searching a better solution, since in a future version of python
the string 'No module namend management' might be changed.

Any better solution?

Don't know; I want to ask a possibly related question. My impression is
that while writing and testing something like the above code one has somehow
to know that "ImportError" can be raised, so one can explicitly define how
it is to be treated. How does one find that out?

While in this case, one might hope that an import action could only fail
with ImportError, isn't it possible that - say - one might get an IOError
because of a problem on the module search path ... and maybe there are lots
of other less obvious exceptions that could be raised?

How does one write a try/except piece of code that works (ie traps whatever
exception occurs, though obviously it can't necessarily fix an arbitrary
exception) for any exception, even those not known of in advance by the
author?

--
Jeremy C B Nicoll - my opinions are my own.