Re: self modifying code
First of all, the test can be optimized by adding a boolean flag which indicates if the data has been initialized or not, and then just testing a boolean value instead of doing an is comparison, at the cost of an extra global variable. But this is still ugly (actually uglier IMO). I think this is a more Pythonic way to do it. This class implements a function which initializes itself upon the first call: class InitializingFunction(object): def __init__(self, init): def initializer(*args, **kw): self.func = init() return self(*args, **kw) self.func = initializer def __call__(self, *args, **kw): return self.func(*args, **kw) Now you can write your function almost exactly like you did before: def init(): data = somethingcomplexandcostly() def foo(a): return simple(data, a) return foo func = InitializingFunction(init) What have we gained from this? Two major advantages: * no ugly 'global' statement * no reliance on the function's name And now you can easily create such functions forever using this class to abstract away the ugly implementation ;) Notice that since Function Decorators were introduced in Python2.4, you can use InitializingFunction as a Decorator to achieve the same effect, this time even without the need for a temporary name for a function: @InitializingFunction def func(): data = somethingcomplexandcostly() def foo(a): return simple(data, a) return foo And finally I must note that no matter which way you turn this around, it will still be hard to read! -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
[EMAIL PROTECTED] wrote: What have we gained from this? Two major advantages: * no ugly 'global' statement * no reliance on the function's name I don't dispute either of the above, however, the actual overhead of your approach appears to be much higher (see below) probably because it has two function calls instead on one to get the answer. And now you can easily create such functions forever using this class to abstract away the ugly implementation ;) ... yes indeed ##file dingo.py class InitializingFunction(object): def __init__(self, init): def initializer(*args, **kw): self.func = init() return self(*args, **kw) self.func = initializer def __call__(self, *args, **kw): return self.func(*args, **kw) def init(): data = 42 def foo(arg): return arg+data return foo a = InitializingFunction(init) def b(arg): global b data = 42 def b(arg): return arg+data return b(arg) ## Testing with timeit C:\Tmp\Python\lib\timeit.py -sfrom dingo import a;a(0) a(1) 10 loops, best of 3: 2.25 usec per loop C:\Tmp\Python\lib\timeit.py -sfrom dingo import b;b(0) b(1) 100 loops, best of 3: 0.52 usec per loop so since the simple function is fairly trivial the overhead seems to be around 4 times that of the weird approach. The global naming stuff is pretty flaky and relies on the way names are looked up; in particular it seems as though references to the original global will be held at least throughout a single statement. If the first call is print b(0),b(1) then b is initialised twice. This 'masterpiece of obfuscation' ;) gets round that problem, but is pretty odd to say the least and still relies on knowing the class name. class Weird(object): @staticmethod def __call__(arg): data = 42 def func(arg): return arg+data Weird.__call__ = staticmethod(func) return func(arg) c = Weird() it is still more expensive than b, but not by much C:\Tmp\Python\lib\timeit.py -sfrom dingo import c;c(1) c(1) 100 loops, best of 3: 0.709 usec per loop -- Robin Becker -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
Yes, my implementation was less efficient because of the extra function call. class Weird(object): @staticmethod def __call__(arg): data = 42 def func(arg): return arg+data Weird.__call__ = staticmethod(func) return func(arg) c = Weird() Ugh... you've used a class just like a function. You can't have two different objects of this class, since you are overriding a static method of the class! And you've hard-coded the data into the class definition. Yes, it works, but I would never, ever trust such code to someone else to maintain. And you'll have to manually define such a class for every such function. That's not very Pythonic. Here's a reusable function that will define such a class for you, as well as hide most of the ugliness (in fact, it supports -exactly- the same interface as my previous implementation): def InitializingFunction(func): class temp: @staticmethod def __call__(*args, **kw): temp.__call__ = staticmethod(func()) return temp.__call__(*args, **kw) return temp() @InitializingFunction def func(): data = somethingcomplexandcostly() def foo(a): return simple(data, a) return foo -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
[EMAIL PROTECTED] wrote: Yes, my implementation was less efficient because of the extra function call. class Weird(object): @staticmethod def __call__(arg): data = 42 def func(arg): return arg+data Weird.__call__ = staticmethod(func) return func(arg) c = Weird() ugh indeed Ugh... you've used a class just like a function. You can't have two different objects of this class, since you are overriding a static method of the class! And you've hard-coded the data into the class definition. Yes, it works, but I would never, ever trust such code to someone else to maintain. And you'll have to manually define such a class for every such function. That's not very Pythonic. no arguments here Here's a reusable function that will define such a class for you, as well as hide most of the ugliness (in fact, it supports -exactly- the same interface as my previous implementation): def InitializingFunction(func): class temp: @staticmethod def __call__(*args, **kw): temp.__call__ = staticmethod(func()) return temp.__call__(*args, **kw) return temp() @InitializingFunction def func(): data = somethingcomplexandcostly() def foo(a): return simple(data, a) return foo I already tried this kind of factory function, but in fact I think the original global test version outperforms even the statically coded Weird class. ie data = None def d(arg): global data if data is None: data = 42 return arg+data @InitializingFunction def e(): data = 43 def foo(a): return data+a return foo are both better than any except the global function replacement nonsense C:\Tmp\Python\lib\timeit.py -sfrom dingo import d;d(0) d(1) 100 loops, best of 3: 0.556 usec per loop C:\Tmp\Python\lib\timeit.py -sfrom dingo import e;e(0) e(1) 100 loops, best of 3: 1.09 usec per loop but the structured approach is still twice as slow as the simplistic one :( -- Robin Becker -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
Personally, I would almost always pay the x2 efficiency price in order to use a class. But then I don't know what you're writing. Of course, if you really need it to be efficient, you can write it as a C extension, or use Pyrex, etc. and get -much- better results. -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
Ben C wrote: ... Why not just: data = None def func(a): global data if not data: data = somethingcomplexandcostly() return simple(data, a) well in the original instance the above reduced to something like data=None def func(arg): global data if data: data = .. return ''.join(map(data.__getitem__,arg)) so the actual function is pretty low cost, but the extra cost of the test is actually not very significant, but if the actual function had been cheaper eg def func(arg): global data if data is None: data = return data+arg then the test is a few percent of the total cost; why keep it? All the other more complex solutions involving namespaces, singletons etc seem to add even more overhead. Or nicer to use a singleton perhaps than a global, perhaps something like this: class Func(object): exists = False def __init__(self): assert not Func.exists Func.exists = True self.data = None def simple(self, a): assert self.data is not None # ... do something with self.data presumably return something def __call__(self, a): if self.data is None: self.data = somethingcomplexandcostly() return self.simple(a) func = Func() func(a) -- Robin Becker -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
Robin Becker schrieb: When young I was warned repeatedly by more knowledgeable folk that self modifying code was dangerous. Is the following idiom dangerous or unpythonic? def func(a): global func, data data = somethingcomplexandcostly() def func(a): return simple(data,a) return func(a) It took me quite a while to figure out how it works, so, yes, I'd say it's unpythonic ;-). It's not really dangerous, but it can get nasty if someone tries to rename the function, or put it into a class. But that's probably not the kind of self-modifying code you've been warned against anyway: I've only ever seen self-modifying code in assembly language or in lisp, the idea is that you really change the code (i.e. single opcodes in the function that's currently running), so you can e.g. make an infinite loop, and eventually overwrite the loop statement to do something else so the loop ends. I'm not sure if you can do the same thing in Python, maybe by changing the bytecode of a running function. It could be replaced by data = somethingcomplexandcostly() def func(a): return simple(data,a) but this always calculates data. You could of course initialize data with None and calculate it only on demand. Or you could use: http://www.phyast.pitt.edu/~micheles/python/documentation.html#memoize This has the advantage of encapsulating the memoization logic so it can be tested (and understood) separately from your code. -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
Robin Becker wrote: When young I was warned repeatedly by more knowledgeable folk that self modifying code was dangerous. Is the following idiom dangerous or unpythonic? def func(a): global func, data data = somethingcomplexandcostly() def func(a): return simple(data,a) return func(a) It could be replaced by data = somethingcomplexandcostly() def func(a): return simple(data,a) but this always calculates data. Consider data = None def func(a): global data if data is None: data = costly() return simple(data, a) if you want lazy evaluation. Not only is it easier to understand, it also works with from lazymodule import func at the cost of just one object identity test whereas your func() implementation will do the heavy-lifting every time func() is called in the client (unless func() has by chance been invoked as lazymodule.func() before the import). Peter -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
Peter Otten wrote: def func(a): global func, data data = somethingcomplexandcostly() def func(a): return simple(data,a) return func(a) at the cost of just one object identity test whereas your func() implementation will do the heavy-lifting every time func() is called in the client (unless func() has by chance been invoked as lazymodule.func() before the import). in the example code the heavy lifting, costly(), is done only once as the function that does it is overwritten. As pointed out it won't work as simply in a class. Memoisation could improve the performance of the normal case form of the function ie def func(a): return simple(data,a) but I guess that would depend on whether simple(data,a) is relatively expensive compared to the costs the memo lookup. -- Robin Becker -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
Robin Becker [EMAIL PROTECTED] writes: When young I was warned repeatedly by more knowledgeable folk that self modifying code was dangerous. Is the following idiom dangerous or unpythonic? def func(a): global func, data data = somethingcomplexandcostly() def func(a): return simple(data,a) return func(a) 1. I don't think most people would call that self-modifying code. I won't try defining that term precisely because I know you'll just pick holes in my definition ;-) 2. The use of global func is just plain weird :-) 3. Peter Otten's version is OK, but how about this, using a closure instead of globals (UNTESTED) def make_func(): namespace = object() namespace.data = None def func(a): if namespace.data is None: namespace.data = somethingcomplexandcostly() return simple(namespace.data, a) return func func = make_func() John -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
John J. Lee wrote: 1. I don't think most people would call that self-modifying code. I won't try defining that term precisely because I know you'll just pick holes in my definition ;-) Don't really disagree about the rewriting code, but the function does re-define itself. 2. The use of global func is just plain weird :-) 3. Peter Otten's version is OK, but how about this, using a closure instead of globals (UNTESTED) def make_func(): namespace = object() namespace.data = None def func(a): if namespace.data is None: namespace.data = somethingcomplexandcostly() return simple(namespace.data, a) return func func = make_func() ... the inner function is almost precisely what I started with, except I used the global namespace. However, it keeps the test in side the function which costs about 1%. -- Robin Becker -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
On 2006-04-29, Robin Becker [EMAIL PROTECTED] wrote: When young I was warned repeatedly by more knowledgeable folk that self modifying code was dangerous. Is the following idiom dangerous or unpythonic? def func(a): global func, data data = somethingcomplexandcostly() def func(a): return simple(data,a) return func(a) It looks quite clever (a bit too clever ... :) It could be replaced by data = somethingcomplexandcostly() def func(a): return simple(data,a) but this always calculates data. Why not just: data = None def func(a): global data if not data: data = somethingcomplexandcostly() return simple(data, a) Or nicer to use a singleton perhaps than a global, perhaps something like this: class Func(object): exists = False def __init__(self): assert not Func.exists Func.exists = True self.data = None def simple(self, a): assert self.data is not None # ... do something with self.data presumably return something def __call__(self, a): if self.data is None: self.data = somethingcomplexandcostly() return self.simple(a) func = Func() func(a) -- http://mail.python.org/mailman/listinfo/python-list
Re: self modifying code
John J. Lee wrote: Robin Becker [EMAIL PROTECTED] writes: When young I was warned repeatedly by more knowledgeable folk that self modifying code was dangerous. Is the following idiom dangerous or unpythonic? def func(a): global func, data data = somethingcomplexandcostly() def func(a): return simple(data,a) return func(a) 1. I don't think most people would call that self-modifying code. I won't try defining that term precisely because I know you'll just pick holes in my definition ;-) 2. The use of global func is just plain weird :-) 3. Peter Otten's version is OK, but how about this, using a closure instead of globals (UNTESTED) def make_func(): namespace = object() namespace.data = None def func(a): if namespace.data is None: namespace.data = somethingcomplexandcostly() return simple(namespace.data, a) return func func = make_func() Unfortunately, this doesn't work because you can add attributes to plain object instances: namespace = object() namespace.data = None Traceback (most recent call last): File interactive input, line 1, in ? AttributeError: 'object' object has no attribute 'data' Maybe you want something like: def make_func(): ... def func(a): ... if func.data is None: ... func.data = somethingcomplexandcostly() ... return simple(func.data, a) ... func.data = None ... return func ... func = make_func() def somethingcomplexandcostly(): ... print 'executing somethingcomplexandcostly' ... return 42 ... def simple(data, a): ... return data, a ... func(1) executing somethingcomplexandcostly (42, 1) func(2) (42, 2) STeVe -- http://mail.python.org/mailman/listinfo/python-list
Re: Self-modifying Code
Steven D'Aprano a écrit : (snip) Having said that, here is a good example of self-modifying code: http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/68429 The RingBuffer class dynamically modifies itself once it is full so that its behaviour changes. This is nothing more than a pythonic implementation of the state pattern. I would not call this self-modifying code, since the code itself is not modified. -- http://mail.python.org/mailman/listinfo/python-list
Re: Self-modifying Code
On Thu, 19 May 2005 21:49:46 +0200, Do Re Mi chel La Si Do wrote: Hi, you, also ! A view, with a little difference : def titi(par): if par222: return par*2 else: return par*10 print titi(123) print titi(1234) #now, change the function, on instant txt=def titi(par): if par222: return str(par)*2 else: return str(par)*5 exec(txt,globals(),globals()) print titi(123) print titi(1234) Self-modifying code is almost always a TERRIBLE idea. Using exec is dangerous unless you can control what is being executed. Using exec on code a user supplies is a huge security hole. Self-modifying code is one of those ideas that seems cute when you first learn about it, but in practice is a bad idea. It makes debugging your code much more difficult. It makes comprehending how your code works harder. Generally, you should avoid it, and shun code that modifies itself. Having said that, here is a good example of self-modifying code: http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/68429 The RingBuffer class dynamically modifies itself once it is full so that its behaviour changes. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Self-modifying Code
Hi ! I often use the auto-modification of code, to allow the users to adapt software to the evolution of their needs. When this technique is controlled, and framed well, it presents only few problems. AMHA, to speak about danger, it is the result of a lack of practice and tests. It is a little the same debate as: dynamic language versus static language. @-salutations Michel Claveau -- http://mail.python.org/mailman/listinfo/python-list
Re: Self-modifying Code
[EMAIL PROTECTED] schrieb: [...] Also Python can (writing and running a module, in-line): fNew =open(newModule.py,'w') lNew=['print 123\n','print 454\n','print 789\n'] fNew.writelines(lNew) fNew.close() from newModule import * [...] You don't even need a file for this. Try: exec(print 123; print 456; print 789) Bye, Dennis -- http://mail.python.org/mailman/listinfo/python-list
Re: Self-modifying Code
Hi, you, also ! A view, with a little difference : def titi(par): if par222: return par*2 else: return par*10 print titi(123) print titi(1234) #now, change the function, on instant txt=def titi(par): if par222: return str(par)*2 else: return str(par)*5 exec(txt,globals(),globals()) print titi(123) print titi(1234) Michel Claveau -- http://mail.python.org/mailman/listinfo/python-list
