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Pressing <enter> produces no output with cmd.py whenever ^D has been entered in; after I enter in ^D it gets into an infinite loop spewing out messages via cmd.Cmd.default() about EOF because self.lastcmd isn't being reset properly.
The attached patch fixes that usability nit to be more intuitive (I debated about changing 'EOF' to 0x10, but that's probably less intuitive..).

Could you provide an step-by-step procedure to try this?.

Sure -- the repro steps are pretty easy:
1. Startup an interactive python shell.
2. Enter in the following code:
{{{
import cmd
class Foo(cmd.Cmd):
 def do_bar(self, arg):
 """bar"""
Foo().cmdloop()
}}}
3. You will be greeted with the standard `(Cmd)' prompt.
4. Press enter. Nothing will be printed out (it will continue on to the next line).
5. Enter in Control-D, you will be greeted with `Unknown syntax: EOF' error message from cmd.Cmd.default(..).
6. Press enter multiple times.
Expected output:
- Since a do_EOF handler isn't installed in 5., I expect to see the `Unknown syntax: EOF' error print out on the console. I don't want to delve into whether or not this should or shouldn't be printed out in this ticket.
- Every time I press <enter> I should be greeted with the prompt, and not the `Unknown syntax: EOF` error.
Actual output:
- I see `Unknown syntax: EOF' error, and every time I press <enter> it displays the error beforementioned error.
Example:
{{{
>>> import cmd
>>> class Foo(cmd.Cmd):
... def do_bar(self, arg):
... """bar"""
... 
>>> Foo().cmdloop()
(Cmd) 
(Cmd) 
bar help 
(Cmd) help bar
bar
(Cmd) ^D*** Unknown syntax: EOF
(Cmd) 
*** Unknown syntax: EOF
(Cmd) 
*** Unknown syntax: EOF
}}}

New changeset 5910c385fab6 by Jesus Cea in branch '2.7':
Close #13500: Hitting EOF gets cmd.py into a infinite EOF on return loop
http://hg.python.org/cpython/rev/5910c385fab6
New changeset b6b4d74b8d42 by Jesus Cea in branch '3.2':
Close #13500: Hitting EOF gets cmd.py into a infinite EOF on return loop
http://hg.python.org/cpython/rev/b6b4d74b8d42
New changeset 70ba352f9586 by Jesus Cea in branch 'default':
MERGE: Close #13500: Hitting EOF gets cmd.py into a infinite EOF on return loop
http://hg.python.org/cpython/rev/70ba352f9586 

Garrett, please verify the fix.

I believe the commit would have needed a regression test.

I'll verify the fix in another day or two.
FWIW unless python is willing to import pexpect, or provide an equivalent, I'm not sure how items like this which require interactive input can be run via the python project testing framework.

> I'm not sure how items like this which require interactive input can
> be run via the python project testing framework.
Replace sys.stdin with a custom object (a stub) and you can control input (see the Inputs class used in Lib/packaging/tests/test_create.py or Lib/packaging/tests/test_command_register.py). The readline module would not be used however, so that can’t be tested.

Ok. I'll see if I can provide a unittest for this by the 12th.

Here's a unittest patch for the py3k branch.
{{{
1 items passed all tests:
 32 tests in test.test_cmd.samplecmdclass
32 tests in 19 items.
32 passed and 0 failed.
Test passed.
doctest (test.test_cmd) ... 32 tests with zero failures
test_file_with_missing_final_nl (__main__.TestAlternateInput) ... ok
test_input_reset_at_EOF (__main__.TestAlternateInput) ... ok
----------------------------------------------------------------------
Ran 2 tests in 0.000s
OK
}}}

New changeset 5af5e6b2c053 by Jesus Cea in branch 'default':
Test for issue #13500
http://hg.python.org/cpython/rev/5af5e6b2c053 

New changeset 0d442e166c8f by Jesus Cea in branch '2.7':
Test for issue #13500
http://hg.python.org/cpython/rev/0d442e166c8f
New changeset 3a40af30449e by Jesus Cea in branch '3.2':
Test for issue #13500
http://hg.python.org/cpython/rev/3a40af30449e
New changeset ad204ed6ac51 by Jesus Cea in branch 'default':
MERGE: Test for issue #13500
http://hg.python.org/cpython/rev/ad204ed6ac51 

New changeset 2909e60e7e13 by Jesus Cea in branch '2.7':
Fix Test for issue #13500
http://hg.python.org/cpython/rev/2909e60e7e13