You can create a custom action instead of a type:
import argparse
import os
import tempfile
import shutil
import atexit
class readable_dir(argparse.Action):
def __call__(self, parser, namespace, values, option_string=None):
prospective_dir=values
if not os.path.isdir(prospective_dir):
raise argparse.ArgumentTypeError("readable_dir:{0} is not a valid path".format(prospective_dir))
if os.access(prospective_dir, os.R_OK):
setattr(namespace,self.dest,prospective_dir)
else:
raise argparse.ArgumentTypeError("readable_dir:{0} is not a readable dir".format(prospective_dir))
ldir = tempfile.mkdtemp()
atexit.register(lambda dir=ldir: shutil.rmtree(ldir))
parser = argparse.ArgumentParser(description='test', fromfile_prefix_chars="@")
parser.add_argument('-l', '--launch_directory', action=readable_dir, default=ldir)
args = parser.parse_args()
print (args)
But this seems a little fishy to me -- if no directory is given, it passes a non-readable directory which seems to defeat the purpose of checking if the directory is accessible in the first place.
Note that as pointed out in the comments, it might be nicer to
raise argparse.ArgumentError(self, ...) rather than argparse.ArgumentTypeError.
EDIT
As far as I'm aware, there is no way to validate the default argument. I suppose the argparse developers just assumed that if you're providing a default, then it should be valid. The quickest and easiest thing to do here is to simply validate the arguments immediately after you parse them. It looks like, you're just trying to get a temporary directory to do some work. If that's the case, you can use the tempfile module to get a new directory to work in. I updated my answer above to reflect this. I create a temporary directory, use that as the default argument (tempfile already guarantees the directory it creates will be writeable) and then I register it to be deleted when your program exits.