[Python-Dev] Conditional For Statements

[Alex Martelli]

On Mon, May 19, 2008 at 5:33 AM, Steven D'Aprano <steve at pearwood.info> wrote:
   ...
>>>> import this
> The Zen of Python, by Tim Peters
> ...
> There should be one-- and preferably only one --obvious way to do it.
>
> There should be ONE OBVIOUS way to do it, not "only one way".

The "only one way" phrasing is in the "Spirit of C" section of the ISO
C Standard `Rationale' (non-normative, but highly informative),
specifically in point four, "provide only one way to do an operation".
I contend that the somewhat different phrasing is somewhat accidental
and not particularly relevant: the *intent* of this *language design
guideline* is exactly the same (Tim was writing on his own, while the
C Standard was written by a committee, so it's hardly surprising that
the Standard's wording is drier and less funny than Tim's;-). IOW,
I've often argued that this is a deep commonality between the design
philosophy of C and Python (together with "trust the programmer" and
"keep the language small and simple", two more points in the "Spirit
of C" section) which makes them conceptually "closer" to each other
than C is to languages sharing more of its syntax, from C++ to
Javascript; it should be particularly easy to see the contrast with
languages deliberately designed to provide multiple equivalent
synonyms (such as foo.size and foo.length meaning the same thing in
Ruby) or going out of their way to provide multiple equivalent syntax
approaches for exactly the same semantics (such as Cobol's "ADD A TO B
GIVING C" and "C = A + B", or SQL's explicit [INNER] JOIN form and the
equivalent form where the inner join is left implicit in the WHERE
clause).

A design guideline is not a strict rule -- e.g., every C programmer
has met situations where a while statement, and a for statement with
some of the three subparts in the control clause omitted, can both
make sense, and of course every time you write within your loop some
"if(condition) continue" you wonder if in this case it might not be
better to instead use "if(!condition) { ... }" around the rest of the
loop's body, etc, etc -- so much for "only one way", as the various
ways to perform elementary operations can click together in a large
number of combinations and permutations; and adding "obvious" would
not help much either, since both alternatives can easily happen to be
"equally obvious". Python is just in the same situation. Nevertheless,
keeping the guideline firmly in mind (e.g., eschewing synonyms in APIs
you design that provide no added value beyond that of catering to
different tastes or to a meta-taste for variety over uniformity;-) is
still a helpful heuristic -- with or without the "obvious" word (i.e.,
whether you're working with C, which doesn't have the "obvious" in its
phrasing of the principle, or with Python, which does).

Alex