CODEWITHSUNDEEP
pythoninheritance
Tarik
Tarik

Reputation: 81721

Instantiating a Python Class without Construction Function and Inheritance

In the code following, OP created a class with inheriting Template class class BatchRename(Template): and at the bottom of the code, he is instantiating the class as t = BatchRename(fmt) albeit there is no construction method (def __init__():). As I understand inheritance in Python, there might be something wrong with this code but I am new to Python so I might be missing something. Could anyone explain me this?

Little code explaining: The OP shows how to use custom delimiters in Templates and he is doing a Batch Renaming on some files in the example. He is creating a custom class which inherits from Template to change the delimeter name(field,property,attribute) and use the inner functionality at the same time.

>>> import time, os.path
>>> photofiles = ['img_1074.jpg', 'img_1076.jpg', 'img_1077.jpg']
>>> class BatchRename(Template):
...     delimiter = '%'
>>> fmt = raw_input('Enter rename style (%d-date %n-seqnum %f-format):  ')
Enter rename style (%d-date %n-seqnum %f-format):  Ashley_%n%f

>>> t = BatchRename(fmt)
>>> date = time.strftime('%d%b%y')
>>> for i, filename in enumerate(photofiles):
...     base, ext = os.path.splitext(filename)
...     newname = t.substitute(d=date, n=i, f=ext)
...     print '{0} --> {1}'.format(filename, newname)

img_1074.jpg --> Ashley_0.jpg
img_1076.jpg --> Ashley_1.jpg
img_1077.jpg --> Ashley_2.jpg

I took this code from: http://docs.python.org/tutorial/stdlib2.html#templating

Upvotes: 1

Views: 348

Answers (2)

César
César

Reputation: 10119

The class BatchRename doesn't need to have a __init__ at all because it's inheriting that from the super class Template. Example:

class A:
    def __init__(self):
        self.age = 27

class B(A):
    pass # Does nothing

>>> A().age
27
>>> B().age # Calling __init__ from super class A
27

Upvotes: 2

Fred Foo
Fred Foo

Reputation: 363587

You don't need an __init__ in every Python class; it's optional and only called when present. The actual constructor for new-style Python classes is __new__, which is inherited from object:

>>> class Foo(object):
...  pass
...
>>> Foo()
<__main__.Foo object at 0x021757F0>
>>> Foo(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object.__new__() takes no parameters

If a class inherits from a class that does provide __init__, that function is inherited along, just like any other method. This is different from, say, C++, where constructors are treated as being special and are not inherited.

Upvotes: 3

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