Python circular importing

Question

So I m getting this error Traceback  most recent call last     File  quot  Users alex dev runswift utils sim2014 simulator py quot   line 3  in  lt module gt      from world import World   File  quot  Users alex dev runswift utils sim2014 world py quot   line 2  in  lt module gt      from entities field import Field   File  quot  Users alex dev runswift utils sim2014 entities field py quot   line 2  in  lt module gt      from entities goal import Goal   File  quot  Users alex dev runswift utils sim2014 entities goal py quot   line 2  in  lt module gt      from entities post import Post   File  quot  Users alex dev runswift utils sim2014 entities post py quot   line 4  in  lt module gt      from physics import PostBody   File  quot  Users alex dev runswift utils sim2014 physics py quot   line 21  in  lt module gt      from entities post import Post ImportError  cannot import name Post  and you can see that I use the same import statement further up and it works  Is there some unwritten rule about circular importing  How do I use the same class further down the call stack

User · Answer

For those of you who  like me  come to this issue from Django  you should know that the docs provide a solution  https   docs djangoproject com en 1 10 ref models fields  foreignkey      To refer to models defined in another application  you can explicitly specify a model with the full application label  For example  if the Manufacturer model above is defined in another application called production  you   d need to use   class Car models Model       manufacturer   models ForeignKey           production Manufacturer           on delete models CASCADE      This sort of reference can be useful when resolving circular import dependencies between two applications

User · Answer

I was able to import the module within the function  only  that would require the objects from this module   def my func        import Foo     foo instance   Foo

User · Answer

To understand circular dependencies  you need to remember that Python is essentially a scripting language   Execution of statements outside methods occurs at compile time   Import statements are executed just like method calls  and to understand them you should think about them like method calls   When you do an import  what happens depends on whether the file you are importing already exists in the module table   If it does  Python uses whatever is currently in the symbol table   If not  Python begins reading the module file  compiling executing importing whatever it finds there  Symbols referenced at compile time are found or not  depending on whether they have been seen  or are yet to be seen by the compiler   Imagine you have two source files   File X py  def X1      return  x1   from Y import Y2  def X2      return  x2    File Y py  def Y1      return  y1   from X import X1  def Y2      return  y2    Now suppose you compile file X py  The compiler begins by defining the method X1  and then hits the import statement in X py  This causes the compiler to pause compilation of X py and begin compiling Y py   Shortly thereafter the compiler hits the import statement in Y py  Since X py is already in the module table  Python uses the existing incomplete X py symbol table to satisfy any references requested  Any symbols appearing before the import statement in X py are now in the symbol table  but any symbols after are not  Since X1 now appears before the import statement  it is successfully imported  Python then resumes compiling Y py   In doing so it defines Y2 and finishes compiling Y py   It then resumes compilation of X py  and finds Y2 in the Y py symbol table  Compilation eventually completes w o error   Something very different happens if you attempt to compile Y py from the command line  While compiling Y py  the compiler hits the import statement before it defines Y2  Then it starts compiling X py   Soon it hits the import statement in X py that requires Y2  But Y2 is undefined  so the compile fails   Please note that if you modify X py to import Y1  the compile will always succeed  no matter which file you compile   However if you modify file Y py to import symbol X2  neither file will compile   Any time when module X  or any module imported by X might import the current module  do NOT use   from X import Y   Any time you think there may be a circular import you should also avoid compile time references to variables in other modules   Consider the innocent looking code   import X z   X Y   Suppose module X imports this module before this module imports X   Further suppose Y is defined in X after the import statement   Then Y will not be defined when this module is imported  and you will get a compile error   If this module imports Y first  you can get away with it   But when one of your co-workers innocently changes the order of definitions in a third module  the code will break   In some cases you can resolve circular dependencies by moving an import statement down below symbol definitions needed by other modules  In the examples above  definitions before the import statement never fail   Definitions after the import statement sometimes fail  depending on the order of compilation  You can even put import statements at the end of a file  so long as none of the imported symbols are needed at compile time   Note that moving import statements down in a module obscures what you are doing  Compensate for this with a comment at the top of your module something like the following    import X    actual import moved down to avoid circular dependency    In general this is a bad practice  but sometimes it is difficult to avoid

User · Answer

When you import a module  or a member of it  for the first time  the code inside the module is executed sequentially like any other code  e g   it is not treated any differently that the body of a function  An import is just a command like any other  assignment  a function call  def  class   Assuming your imports occur at the top of the script  then here s what s happening    When you try to import World from world  the world script gets executed  The world script imports Field  which causes the entities field script to get executed  This process continues until you reach the entities post script because you tried to import Post The entities post script causes physics module to be executed because it tries to import PostBody Finally  physics tries to import Post from entities post I m not sure whether the entities post module exists in memory yet  but it really doesn t matter  Either the module is not in memory  or the module doesn t yet have a Post member because it hasn t finished executing to define Post Either way  an error occurs because Post is not there to be imported   So no  it s not  working further up in the call stack   This is a stack trace of where the error occurred  which means it errored out trying to import Post in that class  You shouldn t use circular imports  At best  it has negligible benefit  typically  no benefit   and it causes problems like this  It burdens any developer maintaining it  forcing them to walk on egg shells to avoid breaking it  Refactor your module organization

User · Answer

I think the answer by jpmc26  while by no means wrong  comes down too heavily on circular imports  They can work just fine  if you set them up correctly  The easiest way to do so is to use import my module syntax  rather than from my module import some object  The former will almost always work  even if my module included imports us back  The latter only works if my object is already defined in my module  which in a circular import may not be the case  To be specific to your case  Try changing entities post py to do import physics and then refer to physics PostBody rather than just PostBody directly  Similarly  change physics py to do import entities post and then use entities post Post rather than just Post

User · Answer

If you run into this issue in a fairly complex app it can be cumbersome to refactor all your imports  PyCharm offers a quickfix for this that will automatically change all usage of the imported symbols as well

User · Answer

I was using the following   from module import Foo  foo instance   Foo     but to get rid of circular reference I did the following and it worked   import module foo  foo instance   foo Foo

[python] Python circular importing?

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