CODEWITHSUNDEEP
python
Adrien Plisson
Adrien Plisson

Reputation: 23303

how to wrap file object read and write operation (which are readonly)?

i am trying to wrap the read and write operation of an instance of a file object (specifically the readline() and write() methods).

normally, i would simply replace those functions by a wrapper, a bit like this:

def log(stream):
    def logwrite(write):
        def inner(data):
            print 'LOG: > '+data.replace('\r','<cr>').replace('\n','<lf>')
            return write(data)
        return inner
    stream.write = logwrite(stream.write)

but the attributes of a file object are read-only ! how could i wrap them properly ?

(note: i am too lazy to wrap the whole fileobject... really, i don't want to miss a feature that i did not wrap properly, or a feature which may be added in a future version of python)

more context :

i am trying to automate the communication with a modem, whose AT command set is made available on the network through a telnet session. once logged in, i shall "grab" the module with which i want to communicate with. after some time without activity, a timeout occurs which releases the module (so that it is available to other users on the network... which i don't care, i am the sole user of this equipment). the automatic release writes a specific line on the session.

i want to wrap the readline() on a file built from a socket (cf. socket.makefile()) so that when the timeout occurs, a specific exception is thrown, so that i can detect the timeout anywhere in the script and react appropriately without complicating the AT command parser...

(of course, i want to do that because the timeout is quite spurious, otherwise i would simply feed the modem with commands without any side effect only to keep the module alive)

(feel free to propose any other method or strategy to achieve this effect)

Upvotes: 2

Views: 6680

Answers (1)

aaronasterling
aaronasterling

Reputation: 71014

use __getattr__ to wrap your file object. provide modified methods for the ones that you are concerned with.

class Wrapped(object):
    def __init__(self, file_):
        self._file = file_

    def write(self, data):
        print 'LOG: > '+data.replace('\r','<cr>').replace('\n','<lf>')
        return self._file.write(data)

    def __getattr__(self, attr):
         return getattr(self._file, attr)

This way, requests for attributes which you don't explicitly provide will be routed to the attribute on the wrapped object and you can just implement the ones that you want

logged = Wrapped(open(filename))

Upvotes: 3

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