On 2/9/2013 8:31 AM, R. David Murray wrote:
On Sat, 09 Feb 2013 09:59:13 +0000, Thomas Scrace <t...@scrace.org> wrote:
If a function (or other non-string object) is accidentally passed as an
argument to os.path.join() the result is an AttributeError:
In [3]: os.path.join(fn, "path")
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
/home/tom/<ipython-input-3-44b097ceab04> in <module>()
----> 1 os.path.join(fn, "path")
/usr/lib/python2.7/posixpath.pyc in join(a, *p)
66 if b.startswith('/'):
67 path = b
---> 68 elif path == '' or path.endswith('/'):
69 path += b
70 else:
AttributeError: 'function' object has no attribute 'endswith'
It's relatively easy to run into this if you mean to pass the return value
of a function (fn()) as the argument but accidentally forget to append
parens (()) to the callable, thus passing the function itself instead.
Would it not be more helpful to raise a TypeError("Argument must be a
string") than the slightly more mysterious AttributeError?
It's not the most difficult error in the world to figure out, I admit, but
a TypeError just seems like the more correct thing to do here.
I agree. Since the exception type is not documented and since no one
should intentionally pass anything but strings, and therefore should not
be writing
try:
os.path.join(a,b)
except AttributeError:
barf()
I think it would be acceptable to make a change in 3.4.
We don't generally do that kind of type checking in Python. Sometimes the
error that results is obscure enough that adding an early check of some
sort is worth it, but this one is very clear, so I don't see that an
additional check would add much value here.
Changing AttributeError to TypeError only requires try-except, which is
cheap if there is no error, not an early type check.
The reason we avoid such type checks is that we prefer to operate via
"duck typing", which means that if an object behaves like the expected
input, it is accepted.
And catching the existing AttributeError allows work-alikes. I agree
that an isinstance check is bad as well as unnecessary.
As it is, the core code for path, given above, is *already* wrapped in
try: except TypeError, (to give a more friendly error message!, as
TypeError is most like from a unicode-bytes mixing). So there would be
no extra cost for correct calls and all that is needed is another except
clause.
except AttributeError as e:
bad = e.args[0].split()[0]
msg = "all components must be strings; one is a {}".format(bad)
raise TypeError("all components must be strings")
Thomas, unless David or someone else shoots down this idea here, I say
go ahead and open an enhancement issue and add terry.reedy as nosy.
--
Terry Jan Reedy
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