Emacs setup for following coding guidelines

The Astropy coding guidelines are listed in Coding Guidelines. This document will describe some configuration options for Emacs, that will help in ensuring that Python code satisfies the guidelines. Emacs can be configured in several different ways. So instead of providing a drop in configuration file, only the individual configurations are presented below.

For this setup we will need flymake, pyflakes and the pep8 Python script, in addition to python-mode.

Flymake comes with Emacs 23. The rest can be obtained from their websites, or can be installed using easy_install or pip.

Global settings

No tabs

This setting will cause all tabs to be replaced with spaces. The number of spaces to use is set in the Basic settings section below.

;; Don't use TABS for indentations.
(setq-default indent-tabs-mode nil)

Maximum number of characters in a line

Emacs will automatically insert a new line after “fill-column” number of columns. PEP8 specifies a maximum of 79, but this can be set to a smaller value also, for example 72.

;; Set the number to the number of columns to use.
(setq-default fill-column 79)

;; Add Autofill mode to mode hooks.
(add-hook 'text-mode-hook 'turn-on-auto-fill)

;; Show line number in the mode line.
(line-number-mode 1)

;; Show column number in the mode line.
(column-number-mode 1)

Syntax highlighting

Enable syntax highlighting. This will also highlight lines that form a region.

(global-font-lock-mode 1)

Python specific settings

Basic settings

Indentation is automatically added. When a tab is pressed it is replaced with 4 spaces. When backspace is pressed on an empty line, the cursor will jump to the previous indentation level.

(load-library "python")

(autoload 'python-mode "python-mode" "Python Mode." t)
(add-to-list 'auto-mode-alist '("\\.py\\'" . python-mode))
(add-to-list 'interpreter-mode-alist '("python" . python-mode))

(setq interpreter-mode-alist
      (cons '("python" . python-mode)
      '(lambda () (progn
                    (set-variable 'py-indent-offset 4)
                    (set-variable 'indent-tabs-mode nil))))

Highlight the column where a line must stop

The “fill-column” column is highlighted in red. For this to work, download column-marker.el and place it in the Emacs configuration directory.

;; Highlight character at "fill-column" position.
(require 'column-marker)
(set-face-background 'column-marker-1 "red")
(add-hook 'python-mode-hook
          (lambda () (interactive)
            (column-marker-1 fill-column)))


Flymake will mark lines that do not satisfy syntax requirements in red. When cursor is on such a line a message is displayed in the mini-buffer. When mouse pointer is on such a line a “tool tip” message is also shown.

For flymake to work with pep8 and pyflakes, create an executable file named pychecker with the following contents. This file must be in the system path.


pyflakes "$1"
pep8 --ignore=E221,E701,E202 --repeat "$1"

Add the following code to Emacs configurations.

;; Setup for Flymake code checking.
(require 'flymake)
(load-library "flymake-cursor")

;; Script that flymake uses to check code. This script must be
;; present in the system path.
(setq pycodechecker "pychecker")

(when (load "flymake" t)
  (defun flymake-pycodecheck-init ()
    (let* ((temp-file (flymake-init-create-temp-buffer-copy
           (local-file (file-relative-name
                        (file-name-directory buffer-file-name))))
      (list pycodechecker (list local-file))))
  (add-to-list 'flymake-allowed-file-name-masks
               '("\\.py\\'" flymake-pycodecheck-init)))

(add-hook 'python-mode-hook 'flymake-mode)


Flymake will save files with suffix _flymake in the current directory. If it crashes for some reason, then these files will not get deleted.

Sometimes there is a delay in refreshing the results.

Delete trailing white spaces and blank lines

To manually delete trailing whitespaces, press C-t C-w, which will run the command “delete-whitespaces`. This command is also run when a file is saved, and hence all trailing whitespaces will be deleted on saving a Python file.

To make sure that all “words” are separated by only one space, type M-SPC (use the ALT key since M-SPC sometimes brings up a context menu.).

To collapse a set of blank lines to one blank line, place the cursor on one of these and press C-x C-o. This is useful for deleting multiple black lines at the end of a file.

;; Remove trailing whitespace manually by typing C-t C-w.
(add-hook 'python-mode-hook
          (lambda ()
            (local-set-key (kbd "C-t C-w")

;; Automatically remove trailing whitespace when file is saved.
(add-hook 'python-mode-hook
        (add-hook 'local-write-file-hooks

;; Use M-SPC (use ALT key) to make sure that words are separated by
;; just one space. Use C-x C-o to collapse a set of empty lines
;; around the cursor to one empty line. Useful for deleting all but
;; one blank line at end of file. To do this go to end of file (M->)
;; and type C-x C-o.