Hi all,
I have a class URI and a bunch of derived sub-classes for example
HttpURI, FtpURI, HttpsURI, etc. (this is an example, I know there is
module urllib & friends, however my actual problem however maps very
well to this example).
Now I want to pass a string to constructor of URI() and get an instance
of one of the subclasses back. For example uri=URI('http://abcd/...')
will make 'uri' an instance of HttpURI class, not instance of URI.
To achieve this I have a list of all subclasses of URI and try to
instantiate one by one in URI.__new__(). In the case I pass e.g. FTP URI
to HttpURI constructor it raises ValueError exception and I want to test
HttpsURI, FtpURI, etc.
For now I have this code:
=====
class URI(object):
def __new__(self, arg):
for subclass in subclasses:
try:
instance = object.__new__(subclass, arg)
return instance
except ValueError, e:
print "Ignoring: %s" % e
raise ValueError("URI format not recognized" % arg)
class HttpURI(URI):
def __init__(self, arg):
if not arg.startswith("http://";):
raise ValueError("%s: not a HTTP URI" % arg)
self._uri = arg
class FtpURI(URI):
def __init__(self, arg):
if not arg.startswith("ftp://";):
raise ValueError("%s: not a FTP URI")
self._uri = arg
subclasses = [HttpURI, FtpURI]
if __name__ == "__main__":
print "Testing HTTP URI"
uri = URI("http://server/path";)
print uri
print "Testing FTP URI"
uri = URI("ftp://server/path";)
print uri
=====
The problem is that ValueError exception raised in HttpURI.__init__() is
not handled in URI.__new__():
-----
~$ ./tst.py
Testing HTTP URI
<__main__.HttpURI object at 0x808572c> # this is good
Testing FTP URI
Traceback (most recent call last):
File "./tst.py", line 35, in <module>
uri = URI("ftp://server/path";)
File "./tst.py", line 18, in __init__
raise ValueError("%s: not a HTTP URI" % arg)
ValueError: ftp://server/path: not a HTTP URI # this is bad
-----
When I change the __init__ methods of subclasses to __new__ I instead get:
-----
./tst.py
Testing HTTP URI
Traceback (most recent call last):
File "./tst.py", line 29, in <module>
uri = URI("http://server/path";)
File "./tst.py", line 7, in __new__
instance = object.__new__(subclass, arg)
TypeError: default __new__ takes no parameters
-----
Does anyone have any hints on how to solve this problem? (other than
using urllib or other standard modules - as I said this is just to
demonstrate the nature of my problem).
Thanks!
GiBo
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