Import Problems

[Bill Jackson]

Once again, I am having issues with imports...

Until now, I thought the general guidelines were to rarely use 'from x
import y' syntax, except when you really want to copy names over.
However, I have run into issues by following this guideline. So...

1) What is going wrong in the example below?
2) What is a good way to handle imports for packages w/subdirectories?

Here is a sample directory structure:

importtest/
__init__.py
test2/
__init__.py
someclass.py
mytest.py


Here are the definitions of the files:

# importtest/__init__.py
from test2 import *

# importtest/test2/__init__.py
from someclass import *
from test2 import *

# importtest/test2/someclass.py
class SomeClass:
pass

# importtest/test2/mytest.py
import importtest
print importtest.Some Class


On import I get the following:

>>import importtest

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "/home/user/lib/python/importtest/__init__.py", line 1, in ?
from test2 import *
File "/home/user/lib/python/importtest/test2/__init__.py", line 2, in ?
from mytest import *
File "/home/user/lib/python/importtest/test2/mytest.py", line 3, in ?
print importtest.Some Class
AttributeError: 'module' object has no attribute 'SomeClass'

>>>

The problem seems to be an 'order' issue. importtest/test2/__init__.py
has loaded someclass.py, but it doesn't seem to have copied its contents
into importtest/__init__.py.... perhaps it is because it hasn't finished
all of its imports. Is this correct?

So what is a good way to deal with this? In files which contain
implementations , I thought it was best not to use 'from x import y', but
this seems to be the only way to get this to work:

# importtest/test2/mytest.py
from someclass import SomeClass
print SomeClass

Is this the guideline?
Use 'from x import y' for modules within your package.
Use 'import y' for modules outside your package.

Help.

[Bill Jackson]

Re: Import Problems

Bill Jackson wrote the following on 04/27/2007 12:49 PM:

# importtest/test2/__init__.py
from someclass import *
from test2 import *

Sorry typo here:

# importtest/test2/__init__.py
from someclass import *
from mytest import *

[Robert Kern]

Re: Import Problems

Bill Jackson wrote:

Once again, I am having issues with imports...
>
Until now, I thought the general guidelines were to rarely use 'from x
import y' syntax, except when you really want to copy names over.

No, the guideline is to not use "from x import *" except at the interactive
prompt and occasionally in __init__.py's to "copy names over" as you put it.

However, I have run into issues by following this guideline. So...
>
1) What is going wrong in the example below?
2) What is a good way to handle imports for packages w/subdirectories?
>
Here is a sample directory structure:
>
importtest/
__init__.py
test2/
__init__.py
someclass.py
mytest.py
>
>
Here are the definitions of the files:
>
# importtest/__init__.py
from test2 import *
>
# importtest/test2/__init__.py
from someclass import *
from test2 import *
>
# importtest/test2/someclass.py
class SomeClass:
pass
>
# importtest/test2/mytest.py
import importtest
print importtest.Some Class
>
>
On import I get the following:
>

>>import importtest

Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "/home/user/lib/python/importtest/__init__.py", line 1, in ?
from test2 import *
File "/home/user/lib/python/importtest/test2/__init__.py", line 2, in ?
from mytest import *
File "/home/user/lib/python/importtest/test2/mytest.py", line 3, in ?
print importtest.Some Class
AttributeError: 'module' object has no attribute 'SomeClass'

>>>

>
>
The problem seems to be an 'order' issue. importtest/test2/__init__.py
has loaded someclass.py, but it doesn't seem to have copied its contents
into importtest/__init__.py.... perhaps it is because it hasn't finished
all of its imports. Is this correct?

You are right that it is an order issue. The line "from test2 import *" hasn't
finished executing by the time "print importtest.Some Class" is executed because
the former is trying to execute the latter.

So what is a good way to deal with this? In files which contain
implementations , I thought it was best not to use 'from x import y', but
this seems to be the only way to get this to work:

My recommendation is that inside packages, be as specific as is feasible with
your imports, even if you expose the symbols at a higher level for external
callers. If you have a module whose contents are being exposed in your
__init__.py, never rely on the contents of that __init__.py in that code.
Instead, go straight to the real modules that implement the symbols that you
need. So mytest.py should read:

from importtest.test 2.someclass import SomeClass
print SomeClass

or

from importtest.test 2 import someclass
print someclass.SomeC lass

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco