Namespaces

A namespace is a collection of currently defined symbolic names and information about an object. You can think of a namespace as a dictionary in which the keys are the object names and the values are the objects themselves. Each of these key-value pairs assigns a name to the corresponding object.

Namespaces are one honking great idea – let’s do more of those!

– The Zen of Python, by Tim Peters

Python now uses namespaces extensively. We have already learnt about some of them in function variables: local, global and non-local variables.

If you are in the method of a class, you have direct access

  1. to the local namespace with the parameters and variables declared in this method,

  2. the global namespace with functions and variables declared at module level, and

  3. the built-in namespace with the built-in functions and built-in exceptions.

These three namespaces are searched in this order.

To explain the different namespaces in more detail in our example, we have extended our existing module to make it clear what can be accessed within a method: form_ns.py.

You can get an overview of the methods that are available in a namespace with

65    def namespaces(self):
66        print("Builtin namespace:", dir(__builtins__))
67        print("Global namespace:", list(globals().keys()))
68        print("Superclass namespace:", dir(Form))
69        print("Class namespace:", dir(Circle))
70        print("Instance namespace:", dir(self))
71        print("Local namespace:", list(locals().keys()))

>>> import form_ns
>>> c1 = form_ns.Circle()
>>> c1.namespaces()
['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'BaseExceptionGroup', 'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning', 'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError', 'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning', 'EOFError', 'Ellipsis', 'EncodingWarning', 'EnvironmentError', 'Exception', 'ExceptionGroup', 'False', 'FileExistsError', 'FileNotFoundError', 'FloatingPointError', 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError', 'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'ModuleNotFoundError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError', 'PythonFinalizationError', 'RecursionError', 'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning', 'StopAsyncIteration', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'ZeroDivisionError', '_', '_IncompleteInputError', '__build_class__', '__debug__', '__doc__', '__import__', '__loader__', '__name__', '__package__', '__spec__', 'abs', 'aiter', 'all', 'anext', 'any', 'ascii', 'bin', 'bool', 'breakpoint', 'bytearray', 'bytes', 'callable', 'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'divmod', 'enumerate', 'eval', 'exec', 'exit', 'filter', 'float', 'format', 'frozenset', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass', 'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview', 'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property', 'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars', 'zip']
Global namespace: ['__name__', '__doc__', '__package__', '__loader__', '__spec__', '__file__', '__cached__', '__builtins__', 'Form', 'Square', 'Circle']
Superclass namespace: ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'move']
Class namespace: ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'circles', 'circumference', 'circumferences', 'diameter', 'instance_variables', 'move', 'namespaces', 'pi']
Instance namespace: ['_Circle__diameter', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'circles', 'circumference', 'circumferences', 'diameter', 'instance_variables', 'move', 'namespaces', 'pi', 'x', 'y']
Local namespace: ['self']

Via the self variable you also have access to

  1. the namespace of the instance with

    • instance variables

    • private instance variables and

    • instance variables of the superclass,

  2. the namespace of the class with

    • methods,

    • class variables,

    • private methods and

    • private class variables and

  3. the namespace of the superclass with

    • methods of the superclass and

    • class variables of the superclass.

These three namespaces are also searched in this order.

You can now analyse the namespace of the instance with the method instance_variables, for example:

73    def instance_variables(self):
74        print(
75            "Instance variables self.__diameter, self.x, self.y:",
76            self.__diameter,
77            self.x,
78            self.y,
79        )

>>> import form_ns
>>> c1 = form_ns.Circle()
>>> c1.instance_variables()
Instance variables self.__diameter, self.x, self.y: 1 0 0

Note

While you can access the move method of the superclass form with self, private instance variables, private methods and private class variables of the superclass are not accessible in this way.

If you only want to change instances of a certain class, you can do this with the garbage collector, for example:

>>> import forms
>>> c1 = forms.Circle()
>>> c2 = forms.Circle(2, 3, 4)
>>> s1 = forms.Square(5, 6, 7)
>>> import gc
>>> for obj in gc.get_objects():
...     if isinstance(obj, forms.Circle):
...         obj.move(3, 0)
...
>>> c1.x, c1.y
(3, 0)
>>> c2.x, c2.y
(6, 4)
>>> s1.x, s1.y
(6, 7)