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Hi,
when a default is specified for a file argument that is open with writing permission (FileType('w')), the default file is always created even if the argument is specified in the command line. 
For example he code:
import argparse 
parser = argparse.ArgumentParser() 
parser.add_argument( 
"-o", 
default = 'fake', 
dest = 'OutputFile', 
type = argparse.FileType('w') 
) 
args = parser.parse_args() 
will create the empty file "fake" even if the -o option is given. 
The value inside the code of args.Outputfile is not affected.
Paolo

I have the same issue with default here with 2.7.
Fortunately I have my own type function so I can prevent is by changing my internal state. This is however only a workaround for real bug in the argparse.

Happens in 3.2, too...
Would be nice to see that fixed.
Stefan

I think Issue 12776, which delays type conversions on defaults, should solve this problem, right? If you agree, could you add your code here as a test case to that issue and mark this as duplicate?

Steven - 12776 indeed fixes this issue. I applied the patch from it to a build of todays checkout, verified that my simple test script worked, then wrote some test cases for test_argparse.
I've uploaded the patch for that test to both issues. I can't close this as a duplicate, though.

New changeset 1b614921aefa by R David Murray in branch '3.2':
#12776,#11839: call argparse type function only once.
http://hg.python.org/cpython/rev/1b614921aefa
New changeset 74f6d87cd471 by R David Murray in branch 'default':
Merge #12776,#11839: call argparse type function only once.
http://hg.python.org/cpython/rev/74f6d87cd471
New changeset 62b5667ef2f4 by R David Murray in branch '2.7':
#12776,#11839: call argparse type function only once.
http://hg.python.org/cpython/rev/62b5667ef2f4