On 16Feb2015 19:10, Devin Jeanpierre <jeanpierr...@gmail.com> wrote:
On Mon, Feb 16, 2015 at 6:15 PM, Steven D'Aprano <st...@pearwood.info> wrote:
Here is a fork of that recipe. It uses an inner class for the new
namedtuple class. The only thing which needs exec is the __new__ method.
http://code.activestate.com/recipes/578918-yet-another-namedtuple/
This demonstrates a powerful truth about Python: *most of the time* you
don't need to use exec or eval because the standard language features
are powerful enough to solve the problem for you. Using a dynamically
created inner class is *almost* enough to solve this problem, only the
__new__ method defeats it. If our requirements where just a little less
demanding, we could avoid exec completely.
No, exec is not necessary at all. If they had to the author could have
reimplemented the argument assignment logic by hand. [... example...]
I see your counter counter example and raise you another counter.
One might use exec() to use code that is valid in one python version but not
another, when you need your program to run in both i.e. to get code that is
syntacticly invalid in one version, but to use it (conditionally) in another
version.
I only have one use case for this presently: I have a use of exec() in my
cs.py3 python2/3 compatability module:
def raise3(exc_type, exc_value, exc_traceback):
if sys.hexversion >= 0x03000000:
raise exc_type(exc_value).with_traceback(exc_traceback)
else:
# subterfuge to let this pass a python3 parser; ugly
exec('raise exc_type, exc_value, exc_traceback')
I'm using exec() here because a Python 3 interpreter will reject the 3 argument
form of raise. Elsewhere in my code I just call cs.py3.raise3() with the
requisite arguments. Note that the string passed to exec() is hardwired, not
obtained from elsewhere in any form.
Like all sane people, I consider using exec() a code smell: if you're using it
you should consider heavily alternatives to it.
Cheers,
Cameron Simpson <c...@zip.com.au>
I think... Therefore I ride. I ride... Therefore I am.
- Mark Pope <erec...@yarrow.wt.uwa.edu.au>
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