Potential Exceptions using builtin str() type in Python

Question

When working with built-in types like int and float in Python, it's common to employ exception handling in cases where input might be unreliable:

def friendly_int_convert(val):
    "Convert value to int or return 37 & print an alert if conversion fails"
    try:
        return int(val)
    except ValueError:
        print('Sorry, that value doesn\'t work... I chose 37 for you!')
        return 37

Are there any prominent edge-cases to be aware of when using str()?

def friendly_str_convert(val):
    "Convert value to str or return 'yo!' & print an alert if conversion fails"
    try:
        return str(val)
    except Exception: # Some specific Exception here
        print('Sorry, that value doesn\'t work... I chose \'yo!\' for you!')
        return 'yo!'

I really don't like using a broad Exception since there are cases like NameError that signify a problem with the code and should raise an error. I've considered UnicodeError as a candidate but I'm not sure whether str() causes it (vs. foo.encode() and foo.decode() where it's easier to understand) and would love an example of what input, if any, would trigger it.

In summary: Is it generally safe to use str() without a try / except block even with unreliable input?

Answer 1

In summary: Is it generally safe to use str() without a try / except block even with unreliable input?

That depends on what kind of input we're talking about. You've tagged this question Python 3, so you don't need to worry about the UnicodeEncodeErrors you'd get with Python 2 and Unicode input, but the object you're receiving could do pretty much anything in its __str__ or __repr__, raising pretty much any kind of exception. For example,

In [18]: import weakref

In [19]: class Foo(object): pass

In [20]: str(weakref.proxy(Foo()))
---------------------------------------------------------------------------
ReferenceError                            Traceback (most recent call last)
<ipython-input-20-396b2ab40052> in <module>()
----> 1 str(weakref.proxy(Foo()))

ReferenceError: weakly-referenced object no longer exists

Answer 2

There's a huge difference between str and int in this regard. int can definitely raise TypeError and ValueError.

As far as I can think, the only exception that str can raise for normal objects is UnicodeEncodeError:

>>> s = u"a\xac\u1234\u20ac\U00008000"
>>> str(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode characters in position 1-4: ordinal not in range(128)

And that only happens on python2.x.

Of course, I can easily make a class that fails with just about any exception imaginable:

>>> class MyError(Exception):
...   pass
... 
>>> class Foo(object):
...   def __str__(self):
...     raise MyError
... 
>>> f = Foo()
>>> str(f)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __str__
__main__.MyError

For the most part, I would question some of the implicit assumptions that all exceptions need to be handled at this point. Generally, it's best to only handle exceptions that you know how to handle. In this case, exotic exceptions that happen because the user put junk into the function should probably be handled at the level where the junk is going in -- not within the function itself. Catching the error and returning some value which is likely nonsense isn't going to be super helpful for debugging issues, etc.

Answer 3

Due to concerns you've raised, I'd do except Exception as e:. Exception is generic "catch-all" in Python 3 for "normal" exceptions (other than "system-level" exceptions resulting from process getting signals, KeyboardInterrupt, etc).

If I were you I'd log the actual exception (e in my example above) at the very least, to see what actually happen (your code silently drops actual exception object by doing except Exception:).