How I'm dealing with my Python indentation problem in GNU Emacs

August 30, 2019

The current (cultural) standard for indentation in Python is four space indent levels and indenting only with spaces, never tabs; this is what GNU Emacs' python mode defaults to and what YAPF and other code formatters use. Our new and updated Python 3 code is written in this official standard, as is some relatively recent Python 2 code. However, I spent a very long time writing Python code using 8-space indent levels and tab-based indentation, which means that I have a great deal of existing Python code in this style, including almost all of our existing Python 2 code at work and all of DWiki. For various reasons I don't want to reformat or reindent all of this code, so I want to work on existing code in its current style, whatever that is. Because Python 3 doesn't like it when you mix spaces and tabs, this should include the use of tabs in indentation.

My existing .emacs settings for this across various different systems were basically an inconsistent mess. On my desktop I was reflexively clinging to my old indentation style with various python mode settings; on our Ubuntu login servers, I'd stopped overriding the python mode defaults due to shifting toward the standard style, but that left me with the tab problem. Today, as part of dealing with my .emacs in general, I decided that I wanted to have the same .emacs everywhere, and that drove me to actively work out a solution.

First, I realized that if I was willing to really commit to shifting my indentation style to the standard on, the only real problem I had was with tabs. GNU Emacs's python mode will automatically detect the current indentation level for existing Python code, and for new files I'll use 4-space indents with spaces no matter what the other files existed in the project. For tabs, I want to continue using tabs if and only if the file is already in my old 8-space tab based indentation style, so the only problem is detecting this.

As far as I can tell there are no existing GNU Emacs features or functions to do this, so I wrote some ELisp to be run as a python-mode hook (which means it happens on a per-file basis). I won't claim it's very good ELisp, but here it is:

(defun cks/leading-tabs-p ()
  "Detect if the current buffer has a line with leading tab(s)."
      (goto-char (point-min))
      (if (re-search-forward "^\t+" nil t)

(add-hook 'python-mode-hook
          (lambda ()
            (if (and (= python-indent-offset 8) (cks/leading-tabs-p))
                (setq indent-tabs-mode t))))

The detection of tab-indented lines here is highly imperfect and can be fooled by all sorts of things, but for my purposes it's good enough; misfires are unlikely in practice. I'm not sure I even have any Python code that uses 8-space indentation but without tabs.

(The start of cks/leading-tabs-p is copied directly from the python-mode function that scans the buffer to determine the indentation level it currently uses. The function naming is superstition based on what I've seen around the Internet.)

I also decided to write some ELisp functions to toggle back and forth between the modern style and my old style and to report the indentation state of a buffer:

(defun cks/python-toggle ()
  "Toggle between old-style Python 2 and modern Python 3 settings."
  (if (= python-indent-offset 8)
      (progn (setq indent-tabs-mode nil) (setq python-indent-offset 4)
             (message "Set to modern Python 3 (4-level spaces)"))
    (progn (setq indent-tabs-mode t) (setq python-indent-offset 8)
           (message "Set to ancient Python 2 with tabs"))))

(defun cks/rep-python ()
  "Report the Python indentation status of the current buffer."
  (message "Python indentation is %d-space indents with %s %s" python-indent-offset
           (if (eq indent-tabs-mode t) "tabs" "spaces only")
           (cond ((and (= python-indent-offset 4) (eq indent-tabs-mode nil))
                  "(Python 3 standard)")
                 ((and (= python-indent-offset 8) (eq indent-tabs-mode t))
                  "(my Python 2 style)")
                 (t "(something weird)"))))

It's deliberate that after cks/python-toggle, I'm in one or the other of my standard indentation styles, even if the buffer started out in some weird style.

PS: Both python-indent-offset and indent-tabs-mode are buffer-local variables by the time I get my hands on them, so I can just directly use setq and so on. There may be a better way to do this these days, but my ELisp knowledge is old and rusty.

Comments on this page:

I use "dtrt-indent" for this, it supports pretty much any language, not just python.

Written on 30 August 2019.
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Last modified: Fri Aug 30 00:13:43 2019
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