Re: circular import Module
Greg Ewing wrote: > Magnus Lycka wrote: > >> Due to the cycle, you can never use file1 without >> file2 or vice versa. Why do you then want it to be >> two different modules instead of one? > > Perhaps because it would then be too big and > unwieldy to maintain? > > Sometimes there are legitimate reasons for > mutually-dependent modules. Agreed, but I think it's important to think about the design if a chunk of code gets that big. Most of the time, things can be organized differently, and there will be maintenance gains from this. If you have mutual dependenices between two modules, they still have to be maintained as a unit. You can't disregard one when you modify the other. To achieve looser (and prefereably not mutual) dependencies between separate modules both makes it easier to maintain the modules and more likely that we will be able to reuse them in other contexts. Sure, there simple are tricks you can use to get away with mutual import dependencies, but if I ran into a mutual dependency between two Python modules, I would first think about their design and try to resolve this dependency issue. I probably wouldn't use a trick such as import in a local scope unless I was in a hurry and needed a quick fix while I was considering a proper design. (I might then be lazy and not resolve the design problem, but it would continue to disturb me...) Composition and inheritance might be used to break out parts of code that doesn't have dependencies to the rest of the code. There are all sorts of design pattern that can resolve mutual dependencies. I often use callbacks to get away from mutual dependencies. E.g. class Adder: def __init__(self, x, y, callback): self.x = x self.y = y self.callback = callback def calc(self): result = self.x + self.y self.callback(result) class Manager: def add(self, x, y): b = B(x, y, self.show) b.calc() def show(self, value): print value This is a silly example of course. Adder.calc could simply have returned the result, but I don't have time to construct anything elaborate now. The point I want to show is that by passing in a callable from Manager to Adder, Adder instances can call certain methods in particular Manager instanes without any dependencies to the module where Manager lives. It just needs to know what kind of parameters it should supply to the callable it was passed. Martin Fowler's book Refactoring shows a lot of examples that are useful when code grows and needs to be divided into smaller chunks. His examples and ideas revolve around Java and C++ though, and it's often much simpler in Python. -- http://mail.python.org/mailman/listinfo/python-list
Re: circular import Module
Magnus Lycka wrote: > Due to the cycle, you can never use file1 without > file2 or vice versa. Why do you then want it to be > two different modules instead of one? Perhaps because it would then be too big and unwieldy to maintain? Sometimes there are legitimate reasons for mutually-dependent modules. -- Greg Ewing, Computer Science Dept, University of Canterbury, Christchurch, New Zealand http://www.cosc.canterbury.ac.nz/~greg -- http://mail.python.org/mailman/listinfo/python-list
Re: circular import Module
On Friday 10 June 2005 07:27 am, Magnus Lycka wrote: > [EMAIL PROTECTED] wrote: > > I have two modules (file1.py and file2.py) > > Is that ok in python (without any weird implication) if my module > > import each other. I mean in module file1.py there exist command import > > file2 and in module file2.py there exist command import file1? > > Even if it works, it gives you a hint of a design > problem that might come back and bite you later. > > If file1 depends on file2 *and* vice versa, it seems > those two modules are tightly coupled. Perhaps they > should be one module rather than two, or perhaps > some redesign should be made to untangle this cycle. True in principle, but what if the resulting module is 1000 lines long? It doesn't happen often, because python programs don't usually get that complicated (e.g. lots of tightly interacting classes), but there are enough exceptions to make it seem desireable to have modules divided only for readability (i.e. logically the same module). I can only think of one way to pull this off at present, though, and that is to create a package: big_package/ __init__.py mod1.py mod2.py mod3.py where __init__.py looks something like: from mod1 import * from mod2 import * from mod3 import * Then you treat "big_package" as the module to import. You'll still have problems with names that need to be global to all three modules (I think they'd have to be in __init__.py or possibly in mod1 in order to work correctly), but it's pretty close. -- Terry Hancock ( hancock at anansispaceworks.com ) Anansi Spaceworks http://www.anansispaceworks.com -- http://mail.python.org/mailman/listinfo/python-list
Re: circular import Module
[EMAIL PROTECTED] wrote: > Hello, > > I have two modules (file1.py and file2.py) > Is that ok in python (without any weird implication) if my module > import each other. I mean in module file1.py there exist command import > file2 and in module file2.py there exist command import file1? Even if it works, it gives you a hint of a design problem that might come back and bite you later. If file1 depends on file2 *and* vice versa, it seems those two modules are tightly coupled. Perhaps they should be one module rather than two, or perhaps some redesign should be made to untangle this cycle. It happens that old Java programmers make one module per class when they start using Python. That's more or less equivalent of never using more than 8.3 characters in filenames in modern operating systems, or to make a detour on your way to work because there used to be a fence blocking the shortest way a long time ago... :) Due to the cycle, you can never use file1 without file2 or vice versa. Why do you then want it to be two different modules instead of one? As others noted, you can usually fix your cycle problems by importing in a local scope, but just because you can, it doesn't mean that you should... -- http://mail.python.org/mailman/listinfo/python-list
Re: circular import Module
Thomas Guettler wrote: > file1.py: > import file2 > > > file2.py: > # import file1 # Does not work! Actually, that *will* work as long as you don't try to use anything from file1 until it has finished being loaded. What won't work is file2.py: from file1 import somename because somename won't yet have been defined in file1 at the time file2 is imported. -- Greg Ewing, Computer Science Dept, University of Canterbury, Christchurch, New Zealand http://www.cosc.canterbury.ac.nz/~greg -- http://mail.python.org/mailman/listinfo/python-list
Re: circular import Module
That's the only way out I found with some module import problem using code generated by wxDesigner. Josef Meile wrote: >>>Circular import does not work on module level, but you can >>>import the module in a method: >>> >>>file1.py: >>>import file2 >>> >>> >>> >>>file2.py: >>># import file1 # Does not work! >>>def foo(): >>>import file1 # Does work >> >> >> Cool idea ! >> >> It works on local namespaces, wich dont cause trouble to the whole >> program. >> >> +1 > Yes, I also think is a good solution. Once I needed to do something like > that and ended by writting a third module with the shared functions that > the two conflicting modules needed. At that time I already knew that one > could import modules from functions; however, I didn't come up with that > solution :-( > > Regards, > Josef -- http://mail.python.org/mailman/listinfo/python-list
Re: circular import Module
>>Circular import does not work on module level, but you can >>import the module in a method: >> >>file1.py: >>import file2 >> >> >> >>file2.py: >># import file1 # Does not work! >>def foo(): >>import file1 # Does work > > > Cool idea ! > > It works on local namespaces, wich dont cause trouble to the whole program. > > +1 Yes, I also think is a good solution. Once I needed to do something like that and ended by writting a third module with the shared functions that the two conflicting modules needed. At that time I already knew that one could import modules from functions; however, I didn't come up with that solution :-( Regards, Josef -- http://mail.python.org/mailman/listinfo/python-list
Re: circular import Module
Hi ! > Circular import does not work on module level, but you can > import the module in a method: > > file1.py: > import file2 > > > > file2.py: > # import file1 # Does not work! > def foo(): > import file1 # Does work Cool idea ! It works on local namespaces, wich dont cause trouble to the whole program. +1 -- Douglas Soares de Andrade http://douglasandrade.cjb.net - dsa at unilestemg.br UnilesteMG - www.unilestemg.br ICQ, MSN = 76277921, douglas at tuxfamily.org -- http://mail.python.org/mailman/listinfo/python-list
Re: circular import Module
Am Wed, 08 Jun 2005 01:11:50 -0700 schrieb [EMAIL PROTECTED]: > Hello, > > I have two modules (file1.py and file2.py) > Is that ok in python (without any weird implication) if my module > import each other. I mean in module file1.py there exist command import > file2 and in module file2.py there exist command import file1? > > This is not working in C#. Circular import does not work on module level, but you can import the module in a method: file1.py: import file2 file2.py: # import file1 # Does not work! def foo(): import file1 # Does work HTH, Thomas -- Thomas Güttler, http://www.thomas-guettler.de/ -- http://mail.python.org/mailman/listinfo/python-list
