subclassing "file"

[Peter Otten]

Uwe Mayer wrote:

> Hi,
> 
> when extending a build in class, what does the constructor __init__(...)
> have to return?
> and how does the constructor call its base-class construtor? (or is this
> done automatically?)
> 
> I want to derive from "file" to create a class that reads record from a
> binary file:
> 
> class myFile(file):
>         def __init__(self, filename, mode="r", bufsize=-1):
>                 ....?...
> 
> just calling the basename and the constructor does not work:
> 
>>>> f = myFile("testfile")
>>>> f
> <closed file '<uninitialized file>', mode '<uninitialized file>' at ...>
> 
> What am I missing?
> 
> Thanks for your comments
> Ciao
> Uwe

When you don't want to do anything in the constructor __init__(), it
suffices to override the methods of interest, e. g.:

>>> class myfile(file):
...     def write(self, s):
...             file.write(self, s.upper())
...
>>> f = myfile("tmp.txt", "w")
>>> f.write("a foolish consciousness")
>>> f.close()
>>> file("tmp.txt").read()
'A FOOLISH CONSCIOUSNESS'

Otherwise call base.__init__(self, someargs), e. g: