Python/Simple Package

Redefining an Object

Scenario: you are defining an object, and want to extend it by adding additional methods.

The trick: in the first file, we define a class called EchoBase with the following line:

class EchoBase(object):

and in subsequent classes, we extend this definition without overwriting it by re-defining EchoBase as follows:

class EchoBase(EchoBase):

Here is a very simple python package to provide an example:

$ tree pkg_test/
pkg_test/
├── __init__.py
├── a.py
├── b.py
└── c.py

4 files


$ cat pkg_test/__init__.py
from .a import *
from .b import *
from .c import *


$ cat pkg_test/a.py
class EchoBase(object):
    def __init__(self):
        pass

    def foo(self):
        print('foo')


$ cat pkg_test/b.py
from .a import EchoBase

class EchoBase(EchoBase):
    def bar(self):
        print('bar')


$ cat pkg_test/c.py
from .b import EchoBase

class EchoBase(EchoBase):
    def buz(self):
        print('buz')

IMPORTANT: Also note that c imports from b, so that their definitions are chained. If c imports from a, it overwrites changes that b makes.

Now what we can do is, from the directory where the pkg_test directory is located, open a python prompt, and create an EchoBase object:

$ python
Python 3.6.3 |Anaconda, Inc.| (default, Oct  6 2017, 12:04:38)
[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

>>> import pkg_test

>>> e = pkg_test.EchoBase()

>>> e.foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'EchoBase' object has no attribute 'foo'

>>> e.bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'EchoBase' object has no attribute 'bar'

>>> e.buz()
buz

Only the method defined in c.py, the last file imported, is defined in the end.