Jp Calderone <[EMAIL PROTECTED]> wrote in news:[EMAIL PROTECTED]:
> On Thu, 23 Dec 2004 13:36:08 GMT, rzed <[EMAIL PROTECTED]> wrote: >>Stephen Thorne <[EMAIL PROTECTED]> wrote in >> news:[EMAIL PROTECTED]: >> > [snip] >> > >> > { >> > 'one': lambda x:x.blat(), >> > 'two': lambda x:x.blah(), >> > }.get(someValue, lambda x:0)(someOtherValue) >> > >> > The alternatives to this, reletively simple pattern, which is >> > a rough parallel to the 'switch' statement in C, involve >> > creating named functions, and remove the code from the >> > context it is to be called from (my major gripe). >> > >> > [snip] >> >> Not addressing lambdas per se, but something similar to your >> pseudo- switch statement can be done without using them at all. >> One way might be to use a function dictionary wrapped in an >> accessor function, such as this: >> >> def dofns(key): >> fd2 = { >> 0:'print "Key 0"', >> 1:'print "one"', >> 4:"\n".join(['for ix in range(15):', >> ' print "%s%d" % (" "*ix,ix)']), >> } >> try: >> exec(fd2[key]) >> except KeyError: >> print 'Key',key,'not found' >> > > I hate to be nasty, but *ugh* what a horrible way to start the > day. > > There are so many things wrong with this. > > By exec'ing code strings you lose compile-time _syntax_ > checking. I'm all for dynamicism, but it really is nice to let > the compiler do _something_ things for you. > > By exec'ing code strings you slow down the entire function > by forcing a slower name-lookup code path in the interpreter. > > By exec'ing code strings you lose the ability to pass values > back to your caller (oh, you could "return" in one of those, but > *ugg* that'd be even more terrible, and probably not what you > want in most cases, since it doesn't let you get at the value > except from your caller. you could also set names in the local > namespace but that's pretty messed up too, python already has a > function return convention, making up your own is no fun). Thanks for your comments. I'll not argue that I would actually put exactly this code into anything I intended for public consumption; it was to illustrate that there were in fact alternatives to lambda. In point of fact, though, there might be reasons to do something similar (addressing at least a couple of your objections). I don't think speed of execution is going to be a factor unless the main processing in the program centers on this snippet. It's a way to dummy up a switch statement, and while that could mean it was central to the program, it seems to me more likely to be relatively peripheral. In a switching application, the dynamic code shown would more likely be function invocations, but it could on occasion be useful to implement a script processor (I've used them for various reasons). Generally they seem most useful for interacting with humans (and hence with human speed limitations). One class of applications where such a technique could be useful would be in allowing various groups to access a database and perform operations on the data specific to their function. Dynamic searches, for instance, ad hoc reporting and the like. Not all groups would have the same needs or privileges, and using separate function tables is one way to limit activities to a specified range. It's fairly trivial to allow passing and returning data; the main difficulty (apart from security concerns) is choosing the convention to use. > > There are probably more reasons this is bad too. > > Even using `def' to define a function for each of these would > be > preferable. > > Aside from that, exec() isn't a function. You "exec foo", not > "exec(foo)". The latter works simply because parens act as to > set precedent instead of as part of function call syntax when > used this way. > > Also, you should do the dictionary lookup first, in a > try/except, > and then execute it later, _outside_ the try/except, otherwise > you risk masking KeyErrors from exec'd code. > > Jp > Good points, which I'll take to heart when I'm putting this stuff into production code. -- rzed -- http://mail.python.org/mailman/listinfo/python-list
