> > Date: Tue, 28 Jun 94 15:51 EDT
> > From: firstname.lastname@example.org (Mike Tibbs)
> > Why does the sys module have "argv" but no "argc"?
> > I have to do:
> > if len(sys.argv) == 1:
> > print 'Usage..."
> > instead of:
> > if sys.argc == 1:
> > print 'Usage...'
> I didn't do it, but it makes sense to me, so... here's my answer:
> You should really wonder why C/C++ provides argc *and* argv to main!
> The answer to this C question is (IMHO) that arrays are not "first
> class citizens" in C/C++, and trying to find the length of an array
> varies from difficult to impossible. If an array has a null
> terminator (as does the argv array in C), then you *can* walk the
> length of the array in order to find its length. If the array has no
> built in terminator, then there is no way to deduce its length. The
> bottom line in C was (probably) that so many people needed to know the
> length of this array, that as a convenience, this value was
> precalculated (someone with more historical perspective might even
> tell me that argv wasn't always null terminated!). Folks that live in
> the Fortran based for-loop world probably also *love* using stuff like
> argc to limit iteration over arrays, rather dealing with pointers ;-).
> Actually though, looking at your code fragment, I could ask why you
> would write (in C):
> if (1 == argc)
> rather than:
> if (NULL == argv)
Simply because the former is more like the English statement:
If the argument count is one, then...
The latter, in English, would be:
If the first argument is empty, then...
> I probably wouldn't really have written stuff like your sample code,
> but if I did, I "wouldn't need no stinkin' argc around here." ;-)
> Getting back to Python:
> In general, it is a bad practice to put the same data in two places.
> The value of argc is *correctly* bundled in the sys.argv list, in the
> form of the len(sys.argv), as lists in Python are first class
> citizens. With such an approach, it is impossible to cause the data
> to become "inconsistent." If you *wish* to cache its value, you can,
> but you are tempting fate, and writing with a higher probability of
> error than need be. As an example, I'm sure most C programmers worth
> their salt (translation of "salt": written lots of C code, and had to
> parse args without using some souped up arg parsing library code) have
> had argc go astray (a.k.a., written a bug) in one of their programs
> when they used argc to track remaining arg counts :-(.
> Summary: In C, they almost-sortof-kindof has a motivation for
> providing the "convenience" of an argc value. In Python, with real
> arrays carrying their own length, the correct architecture falls out.
> (or at least that is how it looks with 20-20 hindsight :-) ).
I usually test the argc just as a quick usage check (in C) but now
Python's architecture looks superior. I keep getting this opinion
reinforced the more I use Python.
> Jim Roskind
> voice: 408.982.4469
> fax: 408.986.1889