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I'm currently writing a make in Python. This make should be 
able to handle not only local files, but remote files 
(http, ftp, etc.). One project might have several thousand 
targets, and some of them are remote. I want to be able to 
handle both types in a uniform way, i.e. via 
urllib/urllib2. This means, that I call urllib2.urlopen() 
to get the header information about the last modification 
date, but I don't want to open the file right away. Only 
when the data is required (because the source resource is 
newer than the target) should the file be read.

And this might open the door to making streams that are 
returned from urlopen() writable (simply by using open
(..., "wb") instead of open(..., "rb") when the first write 
is called.

Another possibility might be using urllib.urlretrieve(), but
the API is horrible (one global cleanup function) and not 
supported by urllib2.