[Python-Dev] os.path.join failure mode (original) (raw)

Terry Reedy tjreedy at udel.edu
Sat Feb 9 19:57:41 CET 2013

On 2/9/2013 8:31 AM, R. David Murray wrote:

On Sat, 09 Feb 2013 09:59:13 +0000, Thomas Scrace <tom at 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/ in () ----> 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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