A gotcha with Python tuples

January 19, 2015

Here's a little somewhat subtle Python syntax issue that I recently got to relearn (or be reminded of) by stubbing my toe on it. Let's start with an example, taken from our Django configuration:

# Tuple of directories to find templates

This looks good (and used to be accepted by Django), but it's wrong. I'm being tripped up by the critical difference in Python between '(A)' and '(A,)'. While I intended to define a one-element tuple, what I've actually done is set TEMPLATE_DIRS to a single string, which I happened to write in parentheses for no good reason (as far as the Python language is concerned, at least). This is still the case even though I've split the parenthesized expression over three lines; Python doesn't care about how many lines I use (or even how I indent them).

(Although it is not defined explicitly in the not a specification, this behavior is embedded in CPython; CPython silently ignores almost all newlines and whitespace inside ('s, ['s, and {'s.)

I used to be very conscious of this difference and very careful about putting a , at the end of my single-element tuples. I think I got into the habit of doing so when I at least thought that the % string formatting operation only took a tuple and would die if given a single element. At some point % started accepting bare single elements (or at least I noticed it did) and after that I got increasing casual about "..." % (a,) versus "..." % (a) (which I soon changed to "..." % a, of course). Somewhere along this the reflexive add-a-comma behavior fell out of my habits and, well, I wound up writing the example above.

(And Django accepted it for years, probably because any number of people wrote it like I did so why not be a bit friendly and magically assume things. Note that I don't blame Django for tightening up their rules here; it's probably a good idea as well as being clearly correct. Django already has enough intrinsic magic without adding more.)

As a side note, I think Python really has to do things this way. Given that () is used for two purposes, '(A)' for a plain A value is at least ambiguous. Adopting a heuristic that people really wanted a single element tuple instead of a uselessly parenthesized expression strikes me as too much magic for a predictable language, especially when you can force the tuple behavior with a ','.

Written on 19 January 2015.
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Last modified: Mon Jan 19 23:19:52 2015
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