PEP on path module for standard library

Peter Hansen peter at
Sat Jul 30 19:37:25 CEST 2005

Reinhold Birkenfeld wrote:
> Mike Orr wrote:
>>- Who needs .open()?  open(myPath) is fine.  But it can stay for
>>backward compatibility.
> Right. I think it's nice for people who like a more OO approach.

There's one very good reason to use .open() on a Path object, and in 
fact it's a (for me) strong justification for the existence of Path in 
the standard library (and, even more, for porting everything else in the 
standard library to use Path).

A scattered assortment of module-level global function names, and 
builtins such as open(), make it extraordinarily difficult to do 
effective and efficient automated testing with "mock" objects.  Although 
one can fairly easily shadow the definition of "open" in any given 
module under test, if the code under test instead calls another standard 
library routine which internally uses open(), it becomes necessary to 
replace the builtin definition of open() and affect all code throughout 
the program.

Object-oriented solutions like Path make it near trivial to substitute a 
mock or other specialized object which (duck typing) acts like a Path 
except perhaps for actually writing the file to disk, or whatever other 
difference you like.

I haven't gotten around to doing a mock Path object yet, though given 
the work that's gone into my existing mock FileSystem object which lets 
you replace open() and test code in a fully controlled and safe 
environment, it shouldn't be hard to do.

So, for the PEP, another justification for Path is that its use can 
encourage better use of automated testing techniques and thereby improve 
the quality of Python software, including in the standard library.


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