[Bug 1048710] Re: Regression in argparse for both python2.7 and python3.2

[Barry Warsaw]

This is going to affect 3.3 also when Ubuntu gets 3.3 final, since I
just checked upstream hg head and it is broken there, even though it
works in today's python3.3 package.

** Bug watch added: Python Roundup #15906
   http://bugs.python.org/issue15906

** Also affects: python via
   http://bugs.python.org/issue15906
   Importance: Unknown
       Status: Unknown

** Summary changed:

- Regression in argparse for both python2.7 and python3.2
+ Regression in argparse for Python 2.7, 3.2 and 3.3

** Changed in: python2.7 (Ubuntu)
       Status: New => Confirmed

** Changed in: python3.2 (Ubuntu)
       Status: New => Confirmed

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Title:
  Regression in argparse for Python 2.7, 3.2 and 3.3

Status in Python:
  Unknown
Status in “python2.7” package in Ubuntu:
  Confirmed
Status in “python3.2” package in Ubuntu:
  Confirmed

Bug description:
  Using the following example:
  ---
  #!/usr/bin/python

  import argparse

  parser = argparse.ArgumentParser()
  parser.add_argument("--test", dest="test", type=str,
      default=[], action='append')

  args = parser.parse_args()

  args.test.append("something")
  ---

  Then running it with simply "python test.py" (WITHOUT passing --test), you get the following:
  Traceback (most recent call last):
    File "test.py", line 11, in <module>
      args.test.append("something")
  AttributeError: 'str' object has no attribute 'append'

  My understanding of argparse is that when using both default=[] and
  action="append", args.test should always be a list, empty if not
  passing --test or a list containing the --test values if passed. After
  the recent update, it seems to instead default to an empty string.

  This must somehow be related to the type field as for some reason,
  removing "type=str" fixes the issue though I don't really see why and
  it's still a clear regression.

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