Cycles between package imports
Hi I'm a tad confused over a problem involving cycles between packages. Assume the following sets of files:: driver.py a/__init__.py a/alice.py b/__init__.py b/bob.py Basically, two packages a and b. Driver simply imports one of the two. This is the file that gets run:: , (driver.py) | | import a.alice | ` The package initialization files (__init__.py) both contain nothing. a.alice imports b.bob, and b.bob imports a.alice. The following works fine: , (a/alice.py) | | import b.bob | ` , (b/bob.py) | | import a.alice | ` However, if I change b/bob.py to import using "from" syntax, i.e.: , (b/bob.py) | | from a import alice | ` It does not work anymore:: Traceback (most recent call last): File "driver.py", line 3, in ? import a.alice File "/tmp/experiments/import-cycle/a/alice.py", line 9, in ? import b.bob File "/tmp/experiments/import-cycle/b/bob.py", line 9, in ? from a import alice ImportError: cannot import name alice So cycles between packages do not behave as cycles between modules--there is a subtlety. At the time that b/bob.py attempts to import a.alice, a.alice is still being imported (i.e. executed), and there is a difference between import a.alice and from a import alice I don't see why the reference to module a.alice could not be available via the "from" syntax, even if it is still incompletely initialized at the time of import. Can anyone shed some light onto this? Is there a rule for determining when a module becomes available to import from a package using the "from" syntax? -- http://mail.python.org/mailman/listinfo/python-list
Re: Cycles between package imports
Martin Blais wrote: > Hi > > I'm a tad confused over a problem involving cycles between > packages. [lengthy example snipped] > > > I don't see why the reference to module a.alice could not be > available via the "from" syntax, even if it is still incompletely > initialized at the time of import. > > Can anyone shed some light onto this? Is there a rule for > determining when a module becomes available to import from a > package using the "from" syntax? It's really easy to see if you trace out, in detail, the exact order in which things happen and when each object is initialized and shows up in the respective module's namespace. The general rule is: don't do that. It doesn't work, and the hoops you have to go through to force it to work are so complex and bizzare that they're not worth it. Redesign the modules so you don't have cyclic dependencies. John Roth -- http://mail.python.org/mailman/listinfo/python-list
Re: Cycles between package imports
On 18 Jun 2006 05:25:14 -0700, John Roth <[EMAIL PROTECTED]> wrote: > Martin Blais wrote: > > Hi > > > > I'm a tad confused over a problem involving cycles between > > packages. > > [lengthy example snipped] > > > > > > > I don't see why the reference to module a.alice could not be > > available via the "from" syntax, even if it is still incompletely > > initialized at the time of import. > > > > Can anyone shed some light onto this? Is there a rule for > > determining when a module becomes available to import from a > > package using the "from" syntax? > > It's really easy to see if you trace out, in detail, the exact > order in which things happen and when each object is initialized and > shows up in the respective module's namespace. No it's not, at least I don't see it. The order in which the symbols appear is different whether you're using the simple "import" form or the "from ... import" form. This is exactly what the question is about. > The general rule is: don't do that. It doesn't work, and the > hoops you have to go through to force it to work are so > complex and bizzare that they're not worth it. Redesign > the modules so you don't have cyclic dependencies. This is a matter completely aside the question. Whether it's worth it or not depends on the specific case --not included in the example-- and in the case where it shows up in my code, removing the cycle actually made sense (it does, most of the time, but not always). Thanks. cheers, -- http://mail.python.org/mailman/listinfo/python-list
Re: Cycles between package imports
In article <[EMAIL PROTECTED]>, "Martin Blais" <[EMAIL PROTECTED]> wrote: >On 18 Jun 2006 05:25:14 -0700, John Roth <[EMAIL PROTECTED]> wrote: > >> The general rule is: don't do that. It doesn't work, and the >> hoops you have to go through to force it to work are so >> complex and bizzare that they're not worth it. Redesign >> the modules so you don't have cyclic dependencies. > >This is a matter completely aside the question. No it isn't. I agree with John Roth. >Whether it's worth it >or not depends on the specific case --not included in the example-- >and in the case where it shows up in my code, removing the cycle >actually made sense (it does, most of the time, but not always). See, even you are unable to come up with an example where a cyclic import makes sense. In general, the complications they introduce are simply not worth it. This was discovered decades ago, during the development of languages like Ada and Modula-2 which could do automatic initialization and finalization of library modules arranged by the compiler/linker system. -- http://mail.python.org/mailman/listinfo/python-list
Re: Cycles between package imports
On 6/22/06, Lawrence D'Oliveiro <[EMAIL PROTECTED]> wrote: > In article <[EMAIL PROTECTED]>, > "Martin Blais" <[EMAIL PROTECTED]> wrote: > > >On 18 Jun 2006 05:25:14 -0700, John Roth <[EMAIL PROTECTED]> wrote: > > > >> The general rule is: don't do that. It doesn't work, and the > >> hoops you have to go through to force it to work are so > >> complex and bizzare that they're not worth it. Redesign > >> the modules so you don't have cyclic dependencies. > > > >This is a matter completely aside the question. > > No it isn't. I agree with John Roth. See below. > >Whether it's worth it > >or not depends on the specific case --not included in the example-- > >and in the case where it shows up in my code, removing the cycle > >actually made sense (it does, most of the time, but not always). > > See, even you are unable to come up with an example where a cyclic > import makes sense. This is irrelevant. I'm not interested in discussing nor debating the merits of prohibiting or allowing cyclic imports with you nor with anybody else, you're wasting your time. The fact of the matter is that Python DOES allow cyclic imports, and the way that they behave depends on the particular syntax used to perform the import. This very specific particular item is what the original question was about. If you want to discuss whether Python should disallow cyclic imports to prevent users to using them, please start your own thread. cheers, > In general, the complications they introduce are simply not worth it. "worth it". Subjective. Whatever. -- http://mail.python.org/mailman/listinfo/python-list
