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Re: Possible bug with TypeError in Python



On 30Jan2008 16:22, Aaron Konstam <akonstam sbcglobal net> wrote:
| On Thu, 2008-01-31 at 07:16 +1100, Cameron Simpson wrote:
| > On 30Jan2008 07:40, Aaron Konstam <akonstam sbcglobal net> wrote:
| > | The following seems like an bug in python in both f7 and f8 but I would
| > | like input before I post a bugzilla. It seems the exception handler
| > | cannot trap the TypeError in python.
| > | For example:
| > | def plus(a,b):
| > | 	try:
| > | 		return(a+b)
| > | 	except TypeError:
| > | 		return None
| > | 
| > | If we define plus as above and call it with: plus(3,) we should get
| > | nothing returned. Instead we get:
| > | Traceback (most recent call last):
| > |   File "./calculator", line 47, in <module>
| > |     exec("register=op[tokens[0]](register)")
| > |   File "<string>", line 1, in <module>
| > | TypeError: plus() takes exactly 2 arguments (1 given)
| > | 
| > | This seems like a bug. An ideas out there?
| > 
| > The 'b' argument to plus() is not optional. This isn't Perl.  plus()
| > is not being called, and your try/except is not being reached.
| > 
| > Not a bug.
| I don't agree. from the point of view of the programmer the statement:
| result(a+b)
| has generated an error and that error should be caught.
| I have taught courses in writing compilers and interpreters and I know
| how this process is implemented in machine code but the response of the
| error trapping system should relate to the statement in python and that
| statement "threw" an error.

Shrug. You're still wrong.

The language specification says that your function definition:

  def plus(a,b):

requires two arguments. You supplied ONLY ONE ("plus(3,)"). Unless you fix
that, the interpreter will never enter the function. If you want to think of
the function like a lambda you're applying the tuple (3,) to the "plus"
function. (3,) is a tuple with only one element. Python checks argument
lists.

Others have already suggested you pass two arguments or allow plus()
to accept one argument.

The thing that threw the error was your function call ("plus(3,)"),
and _not_ the function internal, which were never reached. So the
exception is correct.

Maybe you should post your "calculator" file, since I'm sure more is
going on than you've described. Using exec() is a lot like using eval()
in perl - exceptions will be caught by the wrapper; that may be your
trouble. In perl they land in $@, in python they're probably caught and
rethrown, hence the slightly surprising error line.

But without "calculator" we're all in the dark.

I've returned this discussion to the list where it belongs.

Cheers,
-- 
Cameron Simpson <cs zip com au> DoD#743
http://www.cskk.ezoshosting.com/cs/


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