Tip for debugging SymPy with PuDB

January 28, 2013

Usually, when I debug SymPy code with PuDB, I create a script that calls the code, then I put a

import pudb; pudb.set_trace()

in the SymPy library code where I want to start debugging. But this is annoying, first because I have to create the script, and second, because I have to modify the library code, and there’s always the risk of accidentally commiting that. Also, if I want to start debugging somewhere else, I have to edit the files and change it.

Well, I just figured out a better way. First, if you haven’t already, add an alias like this in your bash config file (~/.profile or ~/.bashrc):alias pudb='python -m pudb.run. As of this pull request, this is no longer necessary. A pudb script is installed automatically with PuDB.

This will let you run pudb script.py to debug script.py. Next, start PuDB. It doesn’t matter with what. You can just run touch test.py, and then pudb test.py. It occured to me that you can just set the breakpoint when starting isympy with PuDB.

Now, press m, and navigate to where in the library code you want to start debugging. It also helps to use / to search the current file and L to jump to a specific line. When you get to the line where you want to start debugging, press b to set a breakpoint. You can do this in multiple places if you want.

Now, you just have to start isympy from within PuDB. Just run pudb bin/isympy, and immediately press c to jump to the interactive prompt. Now, run whatever code you want to debug. When it gets to the breakpoint, PuDB will open, and you can start debugging. If you type c to continue, it will go back to isympy. But the next time you run something that hits the breakpoint, it will open PuDB again.

This trick works because breakpoints are saved to file (at ~/.config/pudb/saved-breakpoints). In fact, if you want, you can just modify that file in the first step. You can edit your saved breakpoints in the bottom right pane of PuDB.

When you are done and you type Ctrl-D PuDB will pop-up again, asking if you want to quit. That’s because it was running the whole time, underneath isympy. Just press q. Note that you should avoid pressing q while debugging, or else PuDB will quit, and you will be left with just normal isympy (it won’t break at your breakpoints any more). Actually, if you do this, but doing Ctrl-D still opens the PuDB prompt, you can just press “Restart”, and it should start working again. Note that “Restart” will not actually reset isympy: all your saved variables will still be the same, and any changes to the library code will not be reloaded. To do that, you have to completely exit and start over again.

Of course, there is nothing SymPy specific about this trick. As long as you have a script that acts as an entry point to an interactive console for your application, you can use it. If you just use IPython, you can use something like pudb /bin/ipython (replace /bin/ipython with the output of which ipython).

Emacs: One year later

January 1, 2013

As readers of this blog may remember, back in 2011, I decided to move to a command-line based editor. For roughly two weeks in December, 2011, I exclusively used Vim, and for the same amount of time in January, 2012, I used exclusively Emacs. I had used a little of each editor in the past, but this was my first time using them to do true editing work. My experiences are chronicled in my blog posts (parts 1, 2, 3, and 7 months later follow up).

To summarize, I decided to use Emacs, as I found it to be much more intuitive, and much more user-friendly. Today, January 1, marks the one-year point of my using Emacs as my sole text editor, with some exceptions (notably, I’m currently writing this blog post in the browser). So I’d like to make some observations:

  • Either one of these editors (Vim or Emacs) is going to really suck unless you are willing to make a serious investment in customizing them and installing nice addons. For the second point, Emacs has an advantage, because the philosophy of Vim is to be barebones whereas the philosophy of Emacs is to be featureful, so that in particular many things that were once addons of Emacs are now included in the standard installation. For customization, on the one hand, Emacs is easier, because it has a nice interface (M-x customize), but on the other hand, Vim’s scripting language is much easier to hack on than Emacs lisp (I still can’t code in Lisp to save my life; it’s a very challenging programming language).

    But my point here is that neither has really great defaults. For example, in Emacs, M-space is bound to just-one-space, which is great for programming. What it does is remove all spaces around the cursor, except for one. But to be really useful, it also should include newlines. It doesn’t do this by default. Rather, you have to call it with a negative argument. So to be really useful, you have to add

    (defun just-one-space-with-newline ()
      "Call just-one-space with a negative argument"
      (just-one-space -1))
    (global-set-key (kbd "M-SPC") 'just-one-space-with-newline)

    to your .emacs file.

  • Emacs has great features, but I always have to look them up. Or rather, I have to look up the keyboard shortcuts for them. I only have the keyboard shortcuts memorized for the things I do every day. I even ended up forgetting really important ones, like M-w (Emacs version of copy). And if a feature involves several keystrokes to access, forget about it (for example, rectangular selection, or any features of special modes). If I use a new mode, e.g., for some file type that I rarely edit (like HTML), I might as well not have any of the features, other than the syntax highlighting, because I either don’t know what they are, or even if I know that they should exist (like automatic tag completion for html), I have no idea how to access them.

    There’s really something to be said about GUI editors, which give these things to users in a way that they don’t have to memorize anything. Perhaps I should try to use the menu more. Or maybe authors of addons should aim to make features require as little cognitive user interaction as possible (such as the excellent auto-complete-mode I mentioned in part 3).

    I mention this because it is one of the things I complained about with Vim, that the keybindings were too hard to memorize. Of course, the difference with Vim is that one has to memorize keybindings to do even the most basic of editing tasks, whereas with Emacs one can always fall back to more natural things like Shift-Arrow Key to select text or Delete to delete the character under the cursor (and yes, I know you can rebind this stuff in Vim; I refer you to the previous bullet point).

  • I mentioned at the end of part 3 that Vim might still be useful to learn, as vi is available literally anywhere that you have POSIX. I honestly don’t think I would be able to use vi or vim if I had to, customization or no, unless I had my keyboard cheat sheet and a decent amount of time. If I’m stuck on a barebones system and I can’t do anything about it, I’ll use nano/pico before I use vi. It’s not that I hate vi. I just can’t do anything with it. It is the same to me now as it was before I used it in-depth. I have forgotten all the keyboard shortcuts, except for ESC and i.
  • I don’t use emacsclient any more. Ever since I got my new retina MacBook Pro, I don’t need it any more, because with the solid state drive starting Emacs from scratch is instantaneous. I’m glad to get rid of it, because it had some seriously annoying glitches.
  • Add alias e=emacs to your Bash config file (.profile or .bashrc). It makes life much easier. “emacs” is not an easy word to type, at least on QWERTY keyboards.
  • I still feel like I am not nearly as efficient in Emacs as I could be. On the one hand, I know there are built-in features (like rectangular selection) that I do not take advantage of enough. I have been a bit lazy with customization: there are a handful of things that I do often that require several keystrokes, but I still haven’t created custom keyboard shortcuts for (off the top of my head: copying and pasting to/from the Mac OS X clipboard and rigidly indenting/dedenting a block of text (C-u 4 C-x TAB, actually C-c u 4 C-x TAB, since I did the sensible thing and rebound C-u to clear to the previous newline, and bound universal-argument to C-c u) come to mind).

    I feel as if I were to watch someone who has used Emacs for a long time that I would learn a lot of tricks.

  • I really should learn Emacs lisp. There are a lot of little customizations that I would like to make, but they are really niche, and can only be done programmatically. But who has the time to learn a completely new programming language (plus a whole library, as just knowing Lisp is useless if you don’t know the proper Emacs funtions and variables and coding styles)?
  • I’ve still not found a good visual browser for jumping to function definitions in a file (mostly Python function definitions, but also other kinds of headers for other kinds of files). The best I’ve found is imenu. If you know of anything, please let me know. One thing I really liked about Vim was the tag list extension, which did this perfectly (thanks to commenter Scott for pointing it out to me). I’ve been told that Cedet has something like this, but every time I try to install it, I run into some issues that just seem like way too much work (I don’t remember what they are, it won’t compile or something, or maybe it just wants to do just way too much and I can’t figure out how to disable everything except for the parts I want).
  • If you ever code in C, add the following to your Makefile
    	$(CC) -o nul $(FLAGS) -S $(CHK_SOURCES)

    (and if you don’t use a Makefile, start using one now). This is assuming you have CC and FLAGS defined at the top (generally to something like cc and -Wall, respectively). Also, add the following to your .emacs

    ;; ===== Turn on flymake-mode ====
    (add-hook 'c-mode-common-hook 'turn-on-flymake)
    (defun turn-on-flymake ()
      "Force flymake-mode on. For use in hooks."
      (flymake-mode 1))
    (add-hook 'c-mode-common-hook 'flymake-keyboard-shortcuts)
    (defun flymake-keyboard-shortcuts ()
      "Add keyboard shortcuts for flymake goto next/prev error."
      (local-set-key "\M-n" 'flymake-goto-next-error)
      (local-set-key "\M-p" 'flymake-goto-prev-error))

    The last part adds the useful keyboard shortcuts M-n and M-p to move between errors. Now, errors in your C code will show up automatically as you type. If you use the command line version of emacs like I do, and not the GUI version, you’ll also need to install the flymake-cursor module, which makes the errors show up in the mode line, since otherwise it tries to use mouse popups. You can change the colors using M-x customize-face (search for “flymake”).

  • I never got flymake to work with LaTeX. Does anyone know how to do it? It seems it is hardcoded to use MikTeX, the Windows version of LaTeX. I found some stuff, but none of it worked.

    Actually, what I really would like is not syntax checking (I rarely make syntax mistakes in LaTeX any more), but rather something that automatically builds the PDF constantly as I type. That way, I can just look over at the PDF as I am writing (I use an external monitor for this. I highly recommend it if you use LaTeX, especially one of those monitors that swivels to portrait mode).

  • If you use Mac OS X, you can use the very excellent KeyRemap4MacBook program to make regular Mac OS X programs act more like Emacs. Mac OS X already has many Emacs shortcuts built in (like C-a, C-e, etc.), but that only works in Cocoa apps, and it doesn’t include any meta key shortcuts. This lets you use additional shortcuts literally everywhere (don’t worry, it automatically doesn’t use them in the Terminal), including an emulator for C-space and some C-x commands (like C-x C-s to Command-s). It doesn’t work on context sensitive shortcuts, unfortunately, unless the operating system already supports it with another keyboard shortcut (e.g., it can map M-f to Option-right arrow). For example, it can’t enable moving between paragraphs with C-S-{ and C-S-}. If anyone knows how to do that, let me know.
  • For about a month this summer, I had to use a Linux laptop, because my Mac broke and my new Mac took a month to arrive (the downside to ordering a new computer immediately after it is announced by Apple). At this point, my saving of all my customizations to GitHub really helped a lot. I created a new branch for the Linux computer (because several things in my customizations were Mac specific), and just symlinked the files I wanted. A hint I can give to people using Linux is to use Konsole. The Gnome terminal sucks. One thing I never figured out is how to make Konsole (or any other Terminal for that matter) to send Control-Shift shortcuts to Emacs (see http://superuser.com/q/439961/39697). I don’t use Linux any more at the moment, but if anyone knows what was going on there, add an answer to that question.
  • In part 3 mentioned that predictive mode was cool, but not very useful. What it does is basically add tab completion for every word in the English language. Actually, I’ve found using auto-complete-mode even when editing text (or LaTeX) to be very useful. Unlike predictive mode, it only guesses words that you’ve already typed (it turns out that you tend to type the same words over and over, and doubly so if those words are LaTeX math commands). Also, predictive mode has a set order of words, which supposedly helps to use it with muscle memory, whereas auto-complete-mode tries to learn what words you are more likely to use based on some basic statistical machine-learning. Also, auto-complete-mode has a much better visual UI and smarter defaults than predictive mode. The result is that it’s actually quite useful and makes typing plain text, as well as LaTeX (actually, pretty much anything, as long as you tend to use the same words repeatedly) much faster. I recommend enabling auto-complete-mode almost everywhere using hooks, like
    (add-hook 'latex-mode-hook 'auto-complete-mode)
    (add-hook 'LaTeX-mode-hook 'auto-complete-mode)
    (add-hook 'prog-mode-hook 'auto-complete-mode)
    ;; etc.
  • At the end of the day, I’m pretty happy with Emacs. I’ve managed to fix most of the things that make it annoying, and it is orders of magnitude more powerful than any GUI editor or IDE I’ve ever seen, especially at just basic text editing, which is the most important thing (I can always use another program for other things, like debugging or whatever). The editor uses the basic shortcuts that I am used to, and is quite efficient to write in. Extensions like auto-complete-mode make using it much faster, though I could use some more extensions to make it even better (namely, a better isearch and a better imenu). Regarding Vim vs. Emacs, I’d like to quote something I said back in my first blog post about Vim over a year ago:

    Vim is great for text editing, but not so hot for text writing (unless you always write text perfectly, so that you never need to leave insert mode until you are done typing). Just the simple act of deleting a mistyped word (yes, word, that happens a lot when you are decently fast touch typist) takes several keystrokes, when it should in my opinion only take one (two if you count the meta-key).

    Needless to say, I find Emacs to be great for both text editing and text writing.

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