Aaron Brady wrote in news:582ef883-0176-4984-9521-6c1894636...@a26g2000prf.googlegroups.com in comp.lang.python:
> On Jan 18, 10:44 am, Rob Williscroft <r...@freenet.co.uk> wrote: >> Aaron Brady wrote >> innews:6a10378f-addb-4d56-bc1b-0c382b3cb...@t26g2000prh > .googlegroups.com >> in comp.lang.python: >> >> > It is too bad that it is so much work to detect whether 'y' was >> > passed in the function call directly. However, sentinel is just as >> > good (or nearly); at worst, you need one sentinel per argument per >> > function, >> >> One per Module should be good enough. The only reason None doesen't >> suffice is that it has other legitimate uses. Though to be honest >> I would always use None as the sentinel if it wasn't a legitimate >> argument. >> >> > which is possible to create, which has a specific meaning. If you >> > are >> > making systematic function calls, e.g. with a dictionary or list, >> > you can just use the sentinel in the dictionary. >> >> IIUYC then, one sentinel is still only needed as the missing argument >> is indicated by *both* position and value or by name and value (in >> the case of a keyword-dictionary), so seperate distinct sentinel >> objects aren't required, for example: >> >> SENTINEL = object() >> >> def f( a, b, c = SENTINEL, d = SENTINEL ): >> print( "values: %r" % ( ( a, b, c, d ), ) ) >> if c is SENTINEL: >> print( "c is missing" ) >> if d is SENTINEL: >> print( "d is missing" ) >> >> f( *( 1, 2, SENTINEL, SENTINEL ) ) >> >> f( **dict( a = 1 , b = 2, d = 4 ) ) >> >> f( **dict( a = 1 , b = 2, d = 4, c = SENTINEL ) ) > I don't have a concrete example, so you may prove to be right, but I'm > not convinced. I'm afraid I can't think of a use case for passing default values around eiither, and I suspect if we were to come up with one, a better solution that didn't involve passing default values around could be found. > If you have one function with an argument that defaults to an empty > list, and calls another with an argument that defaults to an empty > dict, then what is the meaning of passing sentinel to the first one? > Whereas, if each had their own, then passing the first one's default > would mean the empty list, and passing the second one's default would > mean the dict. If you *mean* to pass an "empty list" or "empty dict"'s you should do it like: function_taking_list( [] ) function_taking_dict( {} ) Its when you don't (have reason to) care that you need default arguments. > (Or, even if that evaluates correctly, perhaps there is no such useful > program.) I agree, though I think some confusion arises here as there are two (at least) distinct meanings for default arguments in python: 1) provide a value for when the caller doesn't, eg: def f( a = 1 ): ... 2) provide a cache for the functions /private/ use, eg: def f( cache = {} ): ... If the two are mixed up, then I can imagine a situation where somebody might want to start passing default caches around. It could only end in tears. Rob. -- http://www.victim-prime.dsl.pipex.com/
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