Base class for file-like objects? (a.k.a "Stream" in Java)

[Asun Friere]

On Jul 25, 8:51 am, Boris Dušek <boris.du... at gmail.com> wrote:

> In Python, I found that "file" objects exist. While specifying
> argument types in Python is not possible as in Java, it is possible to
> check whether an object is an instance of some class and that's what I
> need - I need to check if an argument is a "file"-like object, and if
> yes, behave accordingly, if not, treat the argument as string with
> URL.
>
> But I am afraid there is no such a base class - I tried the following:
>

Yup you are right.  Look.
>>> type(g).mro()
[<type 'instance'>, <type 'object'>]
>>> type(f).mro()
[<type 'file'>, <type 'object'>]

"File-like object" refers the the behaviour of the object, not it's
inheritance.  Python is an equal opportunity employer, we don't ask
and object what race it is, we simply ask "can you do the job?"  This
is known colloquially as "duck-typing."  In other words Python has
'polymorphism by interface,' in contradistinction to Java's
'polymorphism by implementation.'  Until you understand this you will
be trying to write Java in Python, and you will not have much joy.

> P.S.: The code should finally look in esence something like this:
>
> if isinstance(f, file):
>    pass
> elif isinstance(f, string):
>    f = urllib.urlopen(f)
> else:
>    raise "..."
> process_stream(f)

Because of duck-typing, you should almost never use the isintance()
(or type(), hasattr(), or any others that don't immediately come to
mind ...) in actual code. It (usually) breaks Python's polymorphism!
If you find these methods popping up in your code it strongly
indicates that you should be using a try/except statement, if not a
complete change of your code's logic.  Search for LBYL (look before
you leap) vs EAFP (easier to ask forgiveness than permission) for a
full explanation.

I'm not sure why you would ever be sending a file object to urlopen
(or is the test isinstance(f, file) supposed to test for an already
opened url?), but you final code should actually look more like
something along these lines:

try :
  f = urlopen(f)
except AttributeError :
  pass

This is not as elegant as it could be, as it will pass not only on
open files (or urls), but on any type that lacks the .strip method
(ie. it doesn't account for your else condition).  You'd probably have
to catch an exception later when you try to use what should be an open
url, or rewrite your code not to get in this position in the first
place.  But however you refactor your code, it is advisable to
concentrate on what an object can do (and catch an exception where it
fails to perform), rather on the type of the object.