Add argparse-checker for file allowing empty

Often one needs to give as argument an existing file, so we have a check for
that, that can be used with argparse. However, sometimes you want to give
users the option to not give a filename at all, which is what this new
checker allows.

diff --git a/src/argparse_helpers.py b/src/argparse_helpers.py
index e18a1c2..91e64d2 100644 (file)
--- a/src/argparse_helpers.py
+++ b/src/argparse_helpers.py
@@ -79,3 +79,12 @@ def existing_file(filename):
     if isfile(filename):
         return filename
     raise ArgumentTypeError('{} is not an existing file'.format(filename))
+
+
+def existing_file_or_empty(filename=''):
+    """
+    Like :py:func:`existing_file` but accepts empty filename (returns '' then).
+    """
+    if not filename.strip():
+        return ''
+    return existing_file(filename)