[Python-Dev] Re: Alternative Implementation forPEP292:SimpleString Substitutions (original) (raw)

Raymond Hettinger raymond.hettinger at verizon.net
Fri Sep 10 07:50:40 CEST 2004

[Barry]

And to make effbot and Raymond happy, it won't auto-promote to unicode if everything's an 8bit string.

Glad to see that my happiness now ranks as a development objective ;-)

There will be updated unit tests, and I will update the documentation and the PEP as appropriate -- if we've reached agreement on it.

+1 Beautiful job.

Barry asked me to bring up one remaining implementation issue for discussion on python-dev.

The docs clearly state that only python identifiers are allowed as placeholders:

[_A-Za-z][_A-Za-z0-9]*

The challenge is that templates can be exposed to non-programmer end-users with no reason to suspect that one letter of their alphabet is different from another. So, as it stands right now, there is a usability issue with placeholder errors passing silently:

>>> fechas = {u'hoy':u'lunes', u'mañana':u'martes'}
>>> t = Template(u'¿Puede volver <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>h</mi><mi>o</mi><mi>y</mi><mi>o</mi></mrow><annotation encoding="application/x-tex">hoy o </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8889em;vertical-align:-0.1944em;"></span><span class="mord mathnormal">h</span><span class="mord mathnormal">oyo</span></span></span></span>mañana?')
>>> t.safe_substitute(fechas)
u'¿Puede volver lunes o $mañana?'

The substitution failed silently (no ValueError as would have occurred with @oradangling@ or a dangling @oradangling). It may be especially baffling for the user because one placeholder succeeded and the other failed without a hint of why (he can see the key in the mapping, it just won't substitute). No clue is offered that the Template was looking for $ma, a partial token, and didn't find it (the situation is even worse if it does find $ma and substitutes an unintended value).

I suggest that the above should raise an error:

ValueError:  Invalid token $mañana on line 1, column 24

It is easily possible to detect and report such errors (see an example in nondist/sandbox/string/curry292.py).

The arguments against such reporting are:

In addition to the above usability issue, there is one other nit. The new invocation syntax offers us the opportunity for to also accept keyword arguments as mapping alternatives:

def substitute(self, mapping=None, **kwds):
    if mapping is None:
       mapping == kwds
 . . .

When applicable, this makes for beautiful, readable calls:

t.substitute(who="Barry", what="mailmeister", when=now())

This would be a simple and nice enchancement to Barry's excellent implementation. I recommend that keyword arguments be adopted.

Raymond

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