[Python-Dev] Re: Update PEP 292 (original) (raw)

Tim Peters tim.peters at gmail.com
Fri Aug 20 19:01:29 CEST 2004

[Brett]

That is coming down to a question of convenience compared to explicitness. Do we want "Give me $1,000,000 now!" to raise ValueError, or do we want to let it pass through?

"Explicit is better than implicit" is good since it will cut down on errors (which is what Tim is arguing for along with "Special cases aren't special enough to break the rules"). But Raymond is pushing for "practicality beats purity" which is more or less true,

It's far less true for non-Americans. I know it took me several decades to learn that "$ means currency" isn't an international standard enforced by the death penalty -- although it should be .

but I am starting to agree with Tim that dollar amounts in an interpolated string probably won't hapeen that often. If anything the number will be what is interpolated in.

And that's where "handy shortcuts" bite too. "OK, I want to interpolate the value of variable 'dollars'. A $ sign on its own means a $ sign, and to interpolate the value of 'dollars' I do $dollars, so putting them those together I must need

"Total: <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow></mrow><annotation encoding="application/x-tex"></annotation></semantics></math></span><span class="katex-html" aria-hidden="true"></span></span>dollars"

". Which doesn't do what they want, although

"Total: $ $dollars"

might be close enough. A consistent (if you want $ in the output, you must have in the input) rule may be surprising at first, but it doesn't remain surprising.

So going for a more explicit version is seeming to be the better solution. Perk of the current implementation is that if someone wants a looser definition they can just change the regex themselves (you guys could even go as far as to break up the regex into individual class attributes and just have a property that does a '|'.join on them as a read method so it is even easier to just change the one group).

We could indeed make this so complicated nobody will understand it . The use of named (as opposed to the current numbered) capturing groups could make it a lot less dependent on details of how the regexp happens to be written, though.

So I guess it's your call, Mr PEP Champion, on whether you want all individual, non-interpolating $ signs to lead to an error or just random orphaned ones at the end of the line.

I still don't grasp why the end of the line is somehow special here. The real thrust of what I'm after is catching what will certainly be common typos, like

$(barry)
${barry)
$[barry]
$(barry}
${barry{
${barry

The specific gimmick I suggested flags those as errors, and flags an odd number of trailing $;s too, but the latter isn't a kind of typo I expect occurs in real life.

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