Stand-alone GNU readline module

Do I need this package?

Do the following quick check:

python -c "import readline; print(readline.__doc__)"

If the output is:

Importing this module enables command line editing using GNU readline.

then you already have GNU Readline and you probably don't need this package (unless you know what you are doing!). However, if the output is:

Importing this module enables command line editing using libedit readline.

then you've come to the right place.

Still interested?

Some Posix platforms such as macOS do not ship with GNU Readline installed. Readline is licensed under the GPL, which makes it hard to distribute with proprietary software. A popular alternative is NetBSD's Editline (libedit) library which has a less restrictive BSD license. If you install Python on macOS via a popular open-source package manager such as Homebrew or MacPorts, you'll get a readline extension module that calls libedit internally (even though it's confusingly still called "readline"!).

While a lot of effort has gone into making GNU Readline and Editline interchangeable within Python, they are not fully equivalent. If you want proper Readline support, this module provides it by bundling the standard Python readline module with the GNU Readline source code, which is compiled and statically linked to it. The end result is a package which is simple to install and only requires the system-dependent ncurses library.

The module is called gnureadline so as not to clash with the existing readline module in the standard library. It supports two general needs:

Code that explicitly imports readline

A typical use case is to override readline in your code like this:

try:
    import gnureadline as readline
except ImportError:
    import readline

Tab completion in the standard interactive Python shell

The above trick does not fix tab completion in the Python shell because by the time the shell prints its first output to the screen, it's too late... One solution is to put this workaround in one of the customization modules imported by the site module early on during the startup process.

This is conveniently done for you by installing gnureadline and running:

<python> -m override_readline

where <python> is the specific Python interpreter you want to fix (for example python3). The script first tries to add the workaround to usercustomize and then falls back to sitecustomize if the user site is not enabled (for example in virtualenvs). If you want to go straight to sitecustomize, add the standard -s option:

<python> -s -m override_readline

The script explains in detail what it is doing and also refuses to install the workaround twice. Another benefit of override_readline is that the interactive Python interpreter gains a helpful reminder on startup, like:

Python 3.12.2 (main, Apr 17 2024, 20:25:57) [Clang 15.0.0 (clang-1500.0.40.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
Using GNU readline instead of the default readline (see sitecustomize.py)
>>>

You don't have to run the override_readline script if gnureadline was installed as a dependency of another package. It's only there to help you fix tab completion in the standard Python shell.

While usercustomize and sitecustomize are associated with a specific Python version, you can also fix tab completion for all Python versions by adding the workaround to the PYTHONSTARTUP file (e.g. ~/.pythonrc). This requires some extra setup as seen in this example pythonrc, which also shows a way to maintain separate history files for libreadline and libedit. The PYTHONSTARTUP file only affects the interactive shell, while user / site customization affects general scripts using readline as well. The Python Tutorial has a section describing these customization options.

Please take note that IPython does not depend on gnureadline for tab completion anymore. Since version 5.0 it uses prompt_toolkit instead.

Versions

The module can be used with both Python 2.x and 3.x, and has been tested with Python versions 2.6, 2.7, and 3.2 to 3.12. The first three numbers of the module version reflect the version of the underlying GNU Readline library (major, minor and patch level), while any additional fourth number distinguishes different module updates based on the same Readline library.

The latest development version is available from the GitHub repository.

If you are using Windows, which also ships without GNU Readline, you might want to consider using the pyreadline3 module instead, which is a readline replacement written in pure Python that interacts with the Windows clipboard.