A gotcha with Python tuples
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 TEMPLATE_DIRS = ( "/some/project/directory" )
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 ',
'.
|
|