Python readline Bug

Source

Problem

In the Python interpreter whenever I hit enter it would show a blank line without a prompt and I'd have to press enter again to see the prompt. When I exited the interpreter the terminal would not show anything I typed. Pressing the return key would show my shell prompt on the same line. Up arrow or down arrow keys did not make any difference. reset was a last resort.

For example,

Python 3.4.3 (default, Aug 26 2015, 18:29:14)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.56)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> abs(1-0)
>>> 1

>>> ^D>>>
~ $ ~ $ ~ $ reset

Investigation

I uninstalled all packages in MacPorts and then only installed Python 3.4.

~ $ sudo port install python34
~ $ port installed
The following ports are currently installed:
  bzip2 @1.0.6_0 (active)
  expat @2.1.0_0 (active)
  gettext @0.19.6_0 (active)
  libedit @20150325-3.1_0 (active)
  libiconv @1.14_0 (active)
  ncurses @6.0_0 (active)
  openssl @1.0.2d_0 (active)
  python3_select @0.0_1 (active)
  python34 @3.4.3_5 (active)
  python_select @0.3_5 (active)
  sqlite3 @3.8.11.1_0 (active)
  xz @5.2.1_0 (active)
  zlib @1.2.8_0 (active)

My repro steps for the symptoms.

~ $ which -a python3.4
/opt/local/bin/python3.4
~ $ python3.4
Python 3.4.3 (default, Aug 26 2015, 18:29:14)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.56)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> abs(1-0)
>>> 1

>>> ^D>>>
~ $ ~ $ ~ $ reset
~ $

Solution

A lot of DuckDuckGo searching pointed at a possible cause: use of libedit instead of readline on OS X. A possible solution was found at libedit/readline conflict with ipython 3.2.2 on OS X 10.7 Lion.

I installed readline from MacPorts.

~ $ sudo port install py34-readline

These were the packages installed.

~ $ port installed
The following ports are currently installed:
bzip2 @1.0.6_0 (active)
expat @2.1.0_0 (active)
gettext @0.19.6_0 (active)
libedit @20150325-3.1_0 (active)
libiconv @1.14_0 (active)
ncurses @6.0_0 (active)
openssl @1.0.2d_0 (active)
py34-readline @6.2.4.1_1 (active)
py34-setuptools @18.2_0 (active)
python3_select @0.0_1 (active)
python34 @3.4.3_5 (active)
python_select @0.3_5 (active)
sqlite3 @3.8.11.1_0 (active)
xz @5.2.1_0 (active)
zlib @1.2.8_0 (active)

My repro steps showed the symptom was gone.

~ $ which -a python3.4
/opt/local/bin/python3.4
~ $ python3.4
Python 3.4.3 (default, Aug 26 2015, 18:29:14)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.56)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> abs(1-0)
1
>>>
>>>
~ $
~ $

Conclusion

Folks, install readline when installing Python on Mac OS X.