python - Why can't objects instantiated during __init__ see their creator? -

python - Why can't objects instantiated during __init__ see their creator? -

well i'm new so, oop , python too, please gentle ;)

i've looked threads , explainations related scoping issue elsewhere , haven't found any. grateful assistance.

sample code:

class zeus(object): def __init__(self): self.name='zeus' self.maze=maze() self.maze.get_zeus_name_1() self.maze.get_zeus_name_2(self) self.get_name_1() self.get_name_2(self) def get_name_1(self): try: print zeus.name except: print "impossible!?" def get_name_2(self,scope): print scope.name class maze(object): def get_zeus_name_1(self): try: print zeus.name except: print "can't done!?" def get_zeus_name_2(self,scope): print scope.name zeus=zeus() print 'now external calls:' zeus.maze.get_zeus_name_1() zeus.maze.get_zeus_name_2(zeus) zeus.get_name_1() zeus.get_name_2(zeus)

output:

can't done!? zeus impossible!? zeus external calls: zeus zeus zeus zeus

during instantiation of zeus, if __init__ method creates instance of class, maze, new instance not able access creator object, zeus (unless self passed it). (additionally, if __init__ method calls method within own class, get_name_1, method cannot access objects attributes either (unless self passed it).)

however, after objects both instantiated, sec object, maze can recognise , access creator object, zeus.

this behaviour has caused me confusion , difficulties, working on code initialised , run __init__ sequence - suppose improve avoid that..

my questions:

why case? are problems can arise through best avoided leaving instantiation, out of __init__ calls? are there farther design implications? passing self new instance, seems though create problems, due self-referencing, should avoided? other interesting implications/ missing something?

thanks help.

get_zeus_name_1() trying access global variable zeus, has not yet been initialised @ moment when get_zeus_name_1() called.

get_zeus_name_2() takes argument (scope) , accesses that, works. doesn't seek access global variable zeus.

same story get_name_1() , get_name_2().

i think key point understand how python executes line:

zeus=zeus()

this line says python: execute method zeus() (which name __init__(self) method in zeus class), , assign object returned method global variable zeus.

python first executes method zeus() , then, after init method has finished executing , returned object-instance of zeus class, python assigns object global variable zeus. global variable zeus has not been defined until after init method finishes, get_name_1() , get_zeus_name_1() cannot access it.

get_name_2() , get_zeus_name_2() access same object-instance of zeus class get_name_1() , get_zeus_name_1() seek access, access via parameter passed them, not via global variable, don't run problem.

here simpler illustration demonstrates same problem:

>>> class foo: ... def __init__(self): ... self.name = 'foobar' ... print self.name ... print foo.name ... >>> foo = foo() foobar traceback (most recent phone call last): file "<stdin>", line 1, in <module> file "<stdin>", line 5, in __init__ nameerror: global name 'foo' not defined >>>

the print self.name line works fine (equivalent get_name_2() , get_zeus_name_2()), print foo.name line crashes (equivalent get_name_2() , get_zeus_name_2()).

python oop instantiation init