Python: How do I create a reference to a reference?

Question

Hi,

I am traditionally a Perl and C++ programmer, so apologies in advance if I am misunderstanding something trivial about Python!

I would like to create a reference to a reference. Huh? Ok. All objects in Python are actually references to the real object. So, how do I create a reference to this reference?

Why do I need/want this? I am overriding sys.stdout and sys.stderr to create a logging library. I would like a (second-level) reference to sys.stdout.

If I could create a reference to a reference, then I could create a generic logger class where the init function receives a reference to a file handle reference that will be overrided, e.g., sys.stdout or sys.stderr. Currently, I must hard-code both values.

Cheers, Kevin

Answer 1

This can't be done. Pass the desired attribute as a string and use getattr() and setattr().

Answer 2

You can't create references to references in python. You can however override the stderr and stdout files with custom classes with write etc methods to allow your own logging systems:

import sys

class MyLogger:
    def __init__(self, f):
        self.f = f

    def __getattr__(self, name):
        # forward e.g. flush() calls to the original file
        return getattr(self.f, name)

    def write(self, data):
        # log the data here!
        # ...

        # And write to the original file
        self.f.write(data)

sys.stdout = MyLogger(sys.stdout)
sys.stderr = MyLogger(sys.stderr)

Answer 3

Easier done than said:

ostream = sys.stdout
print >> ostream, 'hi mom!'
ostream = sys.stderr
print >> ostream, 'hi mom!'
ostream = open('mylog.txt', 'a')
...

And look at the standard logging module when you have some more Python under your belt.

This answer was based on the presumption, from the level of the question, of what was really needed. The concept of a reference to a reference is not needed in Python, you can multiplex through a list or a dict if you want:

outputs = [sys.stderr, my_open_file_object_which_is_really_a_reference]
print >> outputs[0], 'hi dad!'
outputs = {'terminal': sys.stderr, 'logfile': file_object}
print >> outputs['logfile'], 'goodbye world!'

and so on.

Answer 4

Firstly, there is already a logging module, so you probably should just use that. Secondly, while there is no such thing as a reference to a reference, you can achieve that indirection via a wrapper or function. For example, if you were to create a getter and setter function for assigning the object, like:

 class StdOutWrapper(object):
      def __init__(self):
          self.original = sys.stdout

      @property
      def value(self):
          return sys.stdout

      @value.setter
      def value(self,val):
          sys.stdout = val

You could then pass an instance of this object to your logger to assign/dereference sys.stdout.

Answer 5

As the other answers say, there is no true "references of references" in python, but there are ways of getting nearly the same effect:

>>> reference1 = "Some Data"
>>> reference2 = (reference1,)
>>> def f(data):
    print data

>>> f(reference2)
('Some Data',)
>>> f(*reference2)
Some Data