Re: [Tutor] Ask a class for it's methods
On Fri, Dec 12, 2008 at 6:06 PM, Shrutarshi Basu technorapt...@gmail.com wrote: Is there a way to ask an object for a list of it's methods (with argument requirements if possible)? Take a look at the inspect module. If it does not directly give you what you need, look at the source - it looks at function attributes that you can directly access. Kent ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Ask a class for it's methods
On Sat, 13 Dec 2008 02:59:34 +0100, Andreas Kostyrka wrote: On Fri, Dec 12, 2008 at 06:06:35PM -0500, Shrutarshi Basu wrote: I have a list containing strings like : func1[] func2[1,2] func3[blah] I want to turn them into method calls (with numeric or string arguments) on a supplied object. I'm trying to figure out the best way to do this. Since these lists could be very big, and the methods could be rather complex (mainly graphics manipulation) I would like to start by getting a list of the object's methods and make sure that all the strings are valid. Is there a way to ask an object for a list of it's methods (with argument requirements if possible)? Well, there are ways, but they are not reliable by design. Objects can return dynamically methods. So use something like this: if callable(getattr(obj, func1)): # func1 exists. Guess nowaday with Python3 released, you should not use callable, but instead test on __call__ if hasattr(getattr(obj, func1), __call__): or the more pythonic version would just call func() and catch exception if it's not callable: try: func1() except TypeError: print func1 is not callable ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Ask a class for it's methods
On Sat, Dec 13, 2008 at 08:03:10AM +, Lie Ryan wrote: On Sat, 13 Dec 2008 02:59:34 +0100, Andreas Kostyrka wrote: On Fri, Dec 12, 2008 at 06:06:35PM -0500, Shrutarshi Basu wrote: I have a list containing strings like : func1[] func2[1,2] func3[blah] I want to turn them into method calls (with numeric or string arguments) on a supplied object. I'm trying to figure out the best way to do this. Since these lists could be very big, and the methods could be rather complex (mainly graphics manipulation) I would like to start by getting a list of the object's methods and make sure that all the strings are valid. Is there a way to ask an object for a list of it's methods (with argument requirements if possible)? Well, there are ways, but they are not reliable by design. Objects can return dynamically methods. So use something like this: if callable(getattr(obj, func1)): # func1 exists. Guess nowaday with Python3 released, you should not use callable, but instead test on __call__ if hasattr(getattr(obj, func1), __call__): or the more pythonic version would just call func() and catch exception if it's not callable: try: func1() except TypeError: print func1 is not callable But it happens to be wrong :) Consider: def func1(): raise TypeError(except does not care where in the callstack the exception happens!!!) Common sources, IMHE, for TypeErrors include int(not_a_string_or_number), which raises a TypeError. Andreas ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Ask a class for it's methods
On Sat, 13 Dec 2008 10:19:34 +0100, Andreas Kostyrka wrote:
On Sat, Dec 13, 2008 at 08:03:10AM +, Lie Ryan wrote:
On Sat, 13 Dec 2008 02:59:34 +0100, Andreas Kostyrka wrote:
On Fri, Dec 12, 2008 at 06:06:35PM -0500, Shrutarshi Basu wrote:
I have a list containing strings like :
func1[]
func2[1,2]
func3[blah]
I want to turn them into method calls (with numeric or string
arguments) on a supplied object. I'm trying to figure out the best
way to do this. Since these lists could be very big, and the methods
could be rather complex (mainly graphics manipulation) I would like
to start by getting a list of the object's methods and make sure
that all the strings are valid. Is there a way to ask an object for
a list of it's methods (with argument requirements if possible)?
Well, there are ways, but they are not reliable by design. Objects
can return dynamically methods.
So use something like this:
if callable(getattr(obj, func1)):
# func1 exists.
Guess nowaday with Python3 released, you should not use callable, but
instead test on __call__
if hasattr(getattr(obj, func1), __call__):
or the more pythonic version would just call func() and catch exception
if it's not callable:
try:
func1()
except TypeError:
print func1 is not callable
But it happens to be wrong :)
Consider:
def func1():
raise TypeError(except does not care where in the callstack the
exception happens!!!)
Common sources, IMHE, for TypeErrors include
int(not_a_string_or_number), which raises a TypeError.
Then it's the lower function's fault (i.e. bug) for not handling that
exception, and if you really need it, it's trivial to check the kind of
TypeError reraising the exception if it's not what we expect.
try:
func1()
except TypeError, e:
if not e.message.endswith('object is not callable'):
print 'There is something deeply wrong in your func1'
raise
print func1 is not callable
___
Tutor maillist - Tutor@python.org
http://mail.python.org/mailman/listinfo/tutor
[Tutor] Ask a class for it's methods
I have a list containing strings like : func1[] func2[1,2] func3[blah] I want to turn them into method calls (with numeric or string arguments) on a supplied object. I'm trying to figure out the best way to do this. Since these lists could be very big, and the methods could be rather complex (mainly graphics manipulation) I would like to start by getting a list of the object's methods and make sure that all the strings are valid. Is there a way to ask an object for a list of it's methods (with argument requirements if possible)? The next question is once I've validated the list, what's the easiest way to turn the list element into a method call? I'll be parsing the string to separate out the method name and arguments. If I store the method name (say func1) in a variable, say var, could I do object.var() and have if call the func1 method in object? Thanks for your help and I'll surely have more questions as I get along, Thanks Basu -- The ByteBaker : http://www.bytebaker.com ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Ask a class for it's methods
Shrutarshi Basu technorapt...@gmail.com wrote I have a list containing strings like : func1[] func2[1,2] func3[blah] I want to turn them into method calls (with numeric or string arguments) on a supplied object. The easiest way is to call getattr() which will return a reference to the method if it exists. be rather complex (mainly graphics manipulation) I would like to start by getting a list of the object's methods and make sure that all the strings are valid. Why not just handle it using try/except? Thats exactly what exceptions were designed for. Is there a way to ask an object for a list of it's methods (with argument requirements if possible)? You can try dir() but that won't give you the parameters. But again try/except can catch an invalid call and you can detect whether the number of parameters is wrong in the except clause. method name (say func1) in a variable, say var, could I do object.var() and have if call the func1 method in object? no, but you can do this: class C: ... def f(self, x): print x ... c = C() dir(c) ['__doc__', '__module__', 'f'] m = getattr(c,'f') m(42) 42 try: m(43,'foo') ... except TypeError: ... # parse the error message here ... print 'Method of', c, 'takes ??? arguments' ... Method of __main__.C instance at 0x01BF8350 takes ??? arguments HTH, -- Alan Gauld Author of the Learn to Program web site http://www.freenetpages.co.uk/hp/alan.gauld ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Ask a class for it's methods
I normally would use exceptions, because I think exceptions are a great idea. But since the functions may be time-consuming graphics functions and the lists could easily be hundreds of such calls, I don't want the user to sit around for something that might fail. Of course, I'm just starting so my assumptions about time might not turn out to be valid, so I could just use exceptions in the end. This is an option I'm exploring. Getattr and dir seem to be the way to go for now, so I'll be trying to apply them over the weekend and see how it turns out. Any more ideas welcome. Thanks, Basu -- The ByteBaker : http://www.bytebaker.com ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Ask a class for it's methods
On Fri, 12 Dec 2008 20:05:23 -0500, Shrutarshi Basu wrote: I normally would use exceptions, because I think exceptions are a great idea. But since the functions may be time-consuming graphics functions and the lists could easily be hundreds of such calls, I don't want the user to sit around for something that might fail. Of course, I'm just starting so my assumptions about time might not turn out to be valid, so I could just use exceptions in the end. This is an option I'm exploring. The general rule of thumb is usually that exception is slow on the exceptional cases arise, while being faster if the try block doesn't fail. On the other hand, using if-block is usually slow all over, but not as slow as exception's exceptional cases. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Ask a class for it's methods
On Fri, Dec 12, 2008 at 06:06:35PM -0500, Shrutarshi Basu wrote: I have a list containing strings like : func1[] func2[1,2] func3[blah] I want to turn them into method calls (with numeric or string arguments) on a supplied object. I'm trying to figure out the best way to do this. Since these lists could be very big, and the methods could be rather complex (mainly graphics manipulation) I would like to start by getting a list of the object's methods and make sure that all the strings are valid. Is there a way to ask an object for a list of it's methods (with argument requirements if possible)? Well, there are ways, but they are not reliable by design. Objects can return dynamically methods. So use something like this: if callable(getattr(obj, func1)): # func1 exists. Guess nowaday with Python3 released, you should not use callable, but instead test on __call__ if hasattr(getattr(obj, func1), __call__): The next question is once I've validated the list, what's the easiest way to turn the list element into a method call? I'll be parsing the string to separate out the method name and arguments. If I store the method name (say func1) in a variable, say var, could I do object.var() and have if call the func1 method in object? getattr(object, var)() # calls the method named in var: class A: def x(self, a, b): return a + b instance = A() name = x args_positional = (1, 2) print getattr(instance, name)(*args_positional) # prints 3 args_by_name = dict(a=1, b=2) print getattr(instance, name)(**args_by_name) # prints 3 too. Andreas ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
