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Python get parent class

I have a Maze class, and I have a MazeTilePlacer (MTP) class. The MTP class instances will be created inside the Maze class, and need access to a variable named emptytiles in the maze class, defined as self.emptytiles = []. How can I access the parent object from the MTP instance to access this variable?

You can handle this by giving each instance of MazeTilePlacer a reference to the same emptytiles list as your Maze object has. The Maze object can then delegate the responsibility of placing tiles to the other objects.
class Maze:
def __init__(self):
self.emptytiles = []
def place_tiles(self):
placer = MazeTilePlacer(emptytiles=self.emptytiles)
placer.do_placements()
class MazeTilePlacer:
def __init__(self, emptytiles):
self.emptytiles = emptytiles
def do_placements(self):
self.emptytiles.append('Another tile my Sir')
maze = Maze()
maze.place_tiles()
print(maze.emptytiles)
If you want to have multiple tile placers, you can all give them the same reference for emptytiles. If you need these to run in parallel and manipulate the emptytiles structure in a thread safe manner (i.e. where you need to have multiple accesses without anyone changing the structure under you - for example pop, do something, then append without something being popped or appended in between) you'll have to wrap it in a lock.

Keeping classes loosely coupled and sharing data

I've been working in python on a project where I have a GUI which I split up a bunch of the work between classes. I don't know a lot of the best practices for passing data around between classes, and I've frequently run into the issue, where I have to implement something, or change something for work, and I've resorted to making a lot of the classes objects of another class in order to give it the data I need.
Any ideas or suggests would be greatly appreciated on how to keep my classes independent for later modification and still pass the relevant data around without affecting interfaces too much?
As an example
class Window():
def __init__(self, parent=None):
self.parent = parent
def doStuff(self):
#do work here
class ParseMyWork(Window):
def __init__(self, parent=None):
self.parent=parent
I often find myself doing stuff like the above giving objects to class Window
or simply inheriting everything from them as in ParseMyWork
There must be better and cleaner ways of passing data around without making my classes utterly dependent on eachother, where one little change creates a cascade effect that forces me to make changes in a bunch of other classes.
Any answers to the question don't necessarily have to be in python, but it will be helpful if they are

If I'm understanding your question correctly, I would say that inheritance is not necessary in your case. Why not give ParseMyWork a function for dealing with a specific Window task?
class Window():
def __init__(self, parent=None):
self.parent = parent
def doStuff(self):
#do work here
class ParseMyWork():
def __init__(self, parent=None):
self.parent=parent`
def doWindowActivity(self, window):
window.doStuff
Then you can use the function like this
work_parser = ParseMyWork()
window = Window()
work_parser.doWindowActivity(window);
That way you can use your work_parse instance with any window instance.
Apologies in advance for my Python, it's been a while so if you see any rookie mistakes, do point them out.

Keep it simple.py:
def doStuff(window):
#do work here
return window
def parseStuff(stuff):
pass
really.py:
from simple import doStuff, parseStuff
def really_simple(window):
okay = doStuff(window)
return parseStuff(okay)
don't complicate the class:
from really import really_simple
really_simple(window)
imo: classes are overly complicated objects, and in a lot of cases more confusing than they need to be, plus they hold references and modify stuff, and can be difficult to decouple once they have been tied to other classes. if there isn't a clear reason why a class needs to be used, then it probably doesn't need to be used.

Classes are super powerful, so it's good you're getting started with em.
Discalimer: Haven't worked in python for a while now, so things might not be exact. The general idea still applies though.
Getting into your question now:
I would say the best way to achieve what you want is to create an instance of the first object where you will extract information from.
Now when creating a class, it's vital that you have attributes within them that you will want to be stored within it that you would like to retrieve once the class is instantiated.
For example, using your Window class example above, let's say that you have an attribute called resolution. It would look something like this:
class Window():
def __init__(self, parent = None):
self.parent = None
self.resolution = '40x80'
Now the resolution information associated with your Window class is forever part of any Window class instance. Now, the next step would be to create a get method for resolution. This should be done as follow:
class Window():
def __init__(self, parent = None):
self.parent = None
self.resolution = '40x80'
def getResoultion():
return self.resolution
Now, the reason we created this get method is because we can now set a variable to the information that is returned with it.
So let's say that you have everything associated with your Window class in its own file (let's say the file name is called Window.py). In a separate file (let's call it main.py), you can do the following:
import Window
windowInstance = Window()
windowResolution = windowInstance.getResolution()
If you print out the variable windowResolution, you should get that 40x80 printed out.
Now, as a side note, I do believe it is possible to get the information associated with an attribute with an instance of a class by simply doing something like
windowResolution = windowInstance.resolution
but that is bad practice in general. The reason, in a nutshell, is because you are now exposing attribute names of your class which you do not want to do because it makes it easy for a person outside of your code to learn the name where that information is held and change it. This can then lead to a myriad of other problems when it comes to making an overall program work. That is why it is best practice to use getters and setters. I already showed what getters are. Simply a get method for attributes. Setters, as you can probably assume, allow for one to set the information of an attribute to something else. Now you might say "Gabe, if we can create setter methods, what's the point of it if they just change it". My answer to that is to not give a setter method to all attributes. For attributes you don't mind for a person to change, give it a setter method, but for attributes you do not want any outside users to touch, simply don't create a setter method for it. Same goes with getter methods too. Users don't need to see all of the information of all attributes that makes your program work. Here's a better explanation: https://en.wikipedia.org/wiki/Mutator_method
Now, back to your example. Now let's say you have your ParseMyWork class in its own file like we did with your Window class, and let's say that ParseMyWork needs the resolution info from Window class. You can do the following :
import Window
import ParseMyWork
windowInstance = Window()
windowResolution = windowInstance.getResolution()
parseInstance = ParseMyWork(windowResolution)
This will only pass the window resolution information associated with your Window class. Hope this helps.

How come I cannot access an instance of a class defined globally inside another class?

I'm trying to use the ui object within the Pawn class and I define and initiate ui outside everything, so it is global right? I've looked on here for questions relating to using variables outside classes but they seem to all refer to .self which I used in the UI class when initiating the Chess object.
The # represent bits of code I've cut out to help readability.
class UI:
def ___init__(self):
self.chess = Chess()
# Calls main menu function which goes to a new game option
def newGame(self):
self.chess.startGame()
while len(self.chess.pieces[0] == 2): # Which is index for Kings
# Game, which is where *move* is called
class Chess:
def startGame(self):
self.chessBoard = Grid(9,9)
self.pieces = []
self.pawns = []
for i in range(8):
self.pawns.append(PawnObject(x,y,side))
self.pieces.append(self.pawns)
class Grid:
# Init method
def cellOccupied(self,x,y):
# This function checks if a place is empty
# If empty, return false else, true
class Object:
# Sets x, y, width, height
class ChessPiece:
# Child of Object sets side (black or white)
class PawnObject:
def move(self,newX,newY):
if ui.chess.chessBoard.cellOccupied(newX,newY) == False:
# Move
# More code
ui = UI()
Traceback: https://gyazo.com/33e5b37b5290ff88433b29874c117ad7
Am I doing something blindingly wrong? I think the way I've programmed this all is very inefficient as I am still learning so is this a result of that? Thank you.

The problem is that this cascading series of events all happens inside the initialisation function for UI; one class calls the next, all before the original __init__ has had a chance to return. This means that the line that did that initialisation has not completed, so the ui variable does not exist yet.
You should try and move some of this out of that cascade. In particular, I can't see why the pawns should move as a result of initialising the UI class; that doesn't seem to make sense at all.
You should also consider why you need ui to be a global variable; seems more likely that it should be an attribute of one of the classes, perhaps Grid, and the pawn can reference it via that instance.

You're probably using ui in the body of a class, which is executed the moment the interpreter sees the class (and therefore before ui exists). You can only use it inside methods or functions since those are only executed when they're called. In other words:
class UI:
def open(self):
pass
class Chess:
ui.open() # <--- this causes an error because it happens before the last line does
def startGame(self):
ui.open() # <--- this is OK
ui = UI()
Also, I think your message indicates that you wrote global ui somewhere which is only necessary if you're planning on assigning a new value to ui, i.e. ui = something. If you just want to use it, e.g. ui.open(), you don't need the global declaration. But removing it won't solve your problem.
You also need to correct
chessBoard = Grid(9,9)
to
self.chessBoard = Grid(9,9)
to make chessBoard an attribute of Chess, which you need to be able to say ui.chess.chessBoard.

A private list variable is inadvertently shared between instance objects

I created many instances of a PlotHandler class. An instance must keep it's variables private. But the way I managed them led to a hard to detect problem, a private list variable is shared between instances! And that too without any obvious source for the leak.
My debugging told me that the private member function that modifies the list sees the same list, even if they are different objects.
Is this a "gotcha" problem? What is the best way to troubleshoot this?
Here are the relevant parts (I hope they are!) of the implementation. Please see the ALL-CAPS comments:
The file implementing PlotHandler:
class PlotHandler(wx.Frame):
__crop_section = None
__projection = None
__crop_xcord = None
_band_data = [] #THIS GETS SHARED
def _on_plot_click(self, xcord): #CALLED BY ANOTHER OBJECT
band = self._analyze_band( xcord )
self._band_data.append(band)
...
The parent class that it is managing PlotHandlers:
class MainFrame(wx.Frame):
__close_callback__ = None
_plot_handlers = []
def __init__(self, parent, title):
...
def InitUI(self):
...
img_handler = ImageHandler(panel)
self.img_src.register_callback( img_handler.update_image )
#you need to call PlotHandler(parent, cropped)
img_handler.register_sample_callback( self._create_new_plot_handler )
...
def _create_new_plot_handler(self, cropped_sample ):
self._plot_handlers.append( PlotHandler(self, cropped_sample) ) #CREATE THEM

See this question, this one, and tons of other stuff you can find by googling "Python class variables shared", "Python FAQ class variables", etc.
The short answer is: variables defined directly in the class body are class variables, not instance variables, and are thus shared among instances of the class. If you want instance variables you must assign them from within a method, where you have access to self.

Class attributes are shared between instances. If you want to define an instance attribute (so each object have its own reference to the variable) you have to define it in __init__
class PlotHandler(wx.Frame):
__crop_section = None
__projection = None
__crop_xcord = None
def __init__(self, **kwargs):
self._band_data = [] #THIS IS NOT SHARED

Python-passing variable between classes

I'm trying to create a character generation wizard for a game. In one class I calculate the attributes of the character. In a different class, I'm displaying to the user which specialties are available based on the attributes of the character. However, I can't remember how to pass variables between different classes.
Here is an example of what I have:
class BasicInfoPage(wx.wizard.WizardPageSimple):
def __init__(self, parent, title):
wiz.WizardPageSimple.__init__(self, parent)
self.next = self.prev = None
self.sizer = makePageTitle(self, title)
<---snip--->
self.intelligence = self.genAttribs()
class MOS(wx.wizard.WizardPageSimple):
def __init__(self, parent, title):
wiz.WizardPageSimple.__init__(self, parent)
self.next = self.prev = None
self.sizer = makePageTitle(self, title)
def eligibleMOS(self, event):
if self.intelligence >= 12:
self.MOS_list.append("Analyst")
The problem is that I can't figure out how to use the "intelligence" variable from the BasicInfoPage class to the MOS class. I've tried several different things from around the Internet but nothing seems to work. What am I missing?
Edit I realized after I posted this that I didn't explain it that well. I'm trying to create a computer version of the Twilight 2000 RPG from the 1980s.
I'm using wxPython to create a wizard; the parent class of my classes is the Wizard from wxPython. That wizard will walk a user through the creation of a character, so the Basic Information page (class BasicInfoPage) lets the user give the character's name and "roll" for the character's attributes. That's where the "self.intelligence" comes from.
I'm trying to use the attributes created her for a page further on in the wizard, where the user selects the speciality of the character. The specialities that are available depend on the attributes the character has, e.g. if the intelligence is high enough, the character can be an Intel Anaylst.
It's been several years since I've programmed, especially with OOP ideas. That's why I'm confused on how to create what's essentially a global variable with classes and methods.

You may have "Class" and "Instance" confused. It's not clear from your example, so I'll presume that you're using a lot of class definitions and don't have appropriate object instances of those classes.
Classes don't really have usable attribute values. A class is just a common set of definitions for a collection of objects. You should think of of classes as definitions, not actual things.
Instances of classes, "objects", are actual things that have actual attribute values and execute method functions.
You don't pass variables among classes. You pass variables among instances. As a practical matter only instance variables matter. [Yes, there are class variables, but they're a fairly specialized and often confusing thing, best avoided.]
When you create an object (an instance of a class)
b= BasicInfoPage(...)
Then b.intelligence is the value of intelligence for the b instance of BasicInfoPage.
A really common thing is
class MOS( wx.wizard.PageSimple ):
def __init__( self, parent, title, basicInfoPage ):
<snip>
self.basicInfo= basicInfoPage
Now, within MOS methods, you can say self.basicInfo.intelligence because MOS has an object that's a BasicInfoPage available to it.
When you build MOS, you provide it with the instance of BasicInfoPage that it's supposed to use.
someBasicInfoPage= BasicInfoPage( ... )
m= MOS( ..., someBasicInfoPage )
Now, the object m can examine someBasicInfoPage.intelligence

Each page of a Wizard -- by itself -- shouldn't actually be the container for the information you're gathering.
Read up on the Model-View-Control design pattern. Your pages have the View and Control parts of the design. They aren't the data model, however.
You'll be happier if you have a separate object that is "built" by the pages. Each page will set some attributes of that underlying model object. Then, the pages are independent of each other, since the pages all get and set values of this underlying model object.
Since you're building a character, you'd have some class like this
class Character( object ):
def __init__( self ):
self.intelligence= 10
<default values for all attributes.>
Then your various Wizard instances just need to be given the underlying Character object as a place to put and get values.

My problem was indeed the confusion of classes vs. instances. I was trying to do everything via classes without ever creating an actual instance. Plus, I was forcing the "BasicInfoPage" class to do too much work.
Ultimately, I created a new class (BaseAttribs) to hold all the variables I need. I then created in instance of that class when I run the wizard and pass that instance as an argument to the classes that need it, as shown below:
#---Run the wizard
if __name__ == "__main__":
app = wx.PySimpleApp()
wizard = wiz.Wizard(None, -1, "TW2K Character Creation")
attribs = BaseAttribs
#---Create each page
page1 = IntroPage(wizard, "Introduction")
page2 = BasicInfoPage(wizard, "Basic Info", attribs)
page3 = Ethnicity(wizard, "Ethnicity")
page4 = MOS(wizard, "Military Occupational Specialty", attribs)
I then used the information S.Lott provided and created individual instances (if that's what it's called) within each class; each class is accessing the same variables though.
Everything works, as far as I can tell. Thanks.

All you need is a reference. It's not really a simple problem that I can give some one-line solution to (other than a simple ugly global that would probably break something else), but one of program structure. You don't magically get access to a variable that was created on another instance of another class. You have to either give the intelligence reference to MOS, or take it from BasicInfoPage, however that might happen. It seems to me that the classes are designed rather oddly-- an information page, for one thing, should not generate anything, and if it does, it should give it back to whatever needs to know-- some sort of central place, which should have been the one generating it in the first place. Ordinarily, you'd set the variables there, and get them from there. Or at least, I would.
If you want the basic answer of "how do I pass variables between different classes", then here you go, but I doubt it's exactly what you want, as you look to be using some sort of controlling framework:
class Foo(object):
def __init__(self, var):
self.var = var
class Bar(object):
def do_something(self, var):
print var*3
if __name__ == '__main__':
f = Foo(3)
b = Bar()
# look, I'm using the variable from one instance in another!
b.do_something(f.var)

If I understood you correctly, then the answer is: You can't.
intelligence should be an attribute of WizardPageSimple, if you'd want both classes to inherit it.
Depending on your situation, you might try to extract intelligence and related attributes into another baseclass. Then you could inherit from both:
class MOS(wiz.WizardPageSimple, wiz.IntelligenceAttributes): # Or something like that.
In that case you must use the co-operative super. In fact, you should be using it already. Instead of calling
wiz.WizardPageSimple.__init__(self, parent)
call
super(MOS, self).__init__(self, parent)