.. title: Python readline Bug
.. slug: python-readline-bug
.. date: 2015-10-06 22:23:12 UTC
.. tags: python, mac os x
.. category:
.. link:
.. description:
.. type: text

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

.. TEASER_END: Read more

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 <http://stackoverflow.com/questions/8288789/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.