reference vs. name space question
steve at REMOVE-THIS-cybersource.com.au
Sun Oct 10 09:06:12 CEST 2010
On Sat, 09 Oct 2010 21:00:45 -0700, chad wrote:
> Maybe I'm being a bit dense, but how something like
> [cdalten at localhost oakland]$ python
> Python 2.6.2 (r262:71600, May 3 2009, 17:04:44) [GCC 4.1.1 20061011
> (Red Hat 4.1.1-30)] on linux2 Type "help", "copyright", "credits" or
> "license" for more information.
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> NameError: name 'spam' is not defined
> Generate an error, but something like
In this case, you have asked Python to look up the name 'spam'. Since
spam has never been bound to any object, (no assignment spam=something
has occurred) the lookup fails and Python raises an exception.
>>>> def foo(x, y):
> ... pass
Why should it fail? You haven't attempted to look up names x or y. You
have executed a function definition statement, which creates a new
function object taking two arguments named x and y. The names only exist
in the function's local namespace, they can only be referenced from
inside the function's local namespace, and since you haven't yet called
the function, Python doesn't make any attempt to look up the names, and
so there is no error.
If you do call the function, one of two things will happen:
(1) You do supply arguments for x and y, in which case the names will be
bound and looking them up will succeed; or
(2) You don't supply arguments for x and/or y, and Python will raise
TypeError before the function code is executed.
> I mean, in the first case, 'spam' isn't bound to anything.
> Likewise, in the second case, both 'x' and 'y' aren't bound to anything.
> I don't see why the interpreter doesn't complain about 'x' and 'y' not
> being defined.
If it did, it would make it a bit hard to write functions if you couldn't
refer to formal parameters.
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