Re: Specifing arguments type for a function
Paolo Pantaleo wrote: I have a function def f(the_arg): ... and I want to state that the_arg must be only of a certain type (actually a list). Is there a way to do that? You can state that in your documentation. You're very likely to get a reasonable runtime error from this when you start using the_arg in your code. The pythonic way of programming is not with overly strict assumptions about typing, but rather by: a) Assuming as little as possible about the data you get from a caller or from a called function. b) Use a comprehensive suite of automated tests to make sure that you find programming errors. From a hypothetical point of view, tests can only find bugs, and never prove that programs are correct, but in practice, automated tests is one of the best ways to keep bugs out of your code. Compile time type checks can only find a small fraction of the bugs programmers make, and the tests that find the harder bugs will also find all the type mismatches that are problematic in any way. There are those who speculate that we will eventually have methods to formally prove correctness of programs through some kind of analysis, but static type checks and other compile time tests are very far from doing that. I think you will find that this approach of assuming as little as possible about parameters and return values will make your code both more robust and more flexible than a traditional approach with static typing as in C++, Java or ADA etc. -- http://mail.python.org/mailman/listinfo/python-list
Re: Specifing arguments type for a function
Dennis Lee Bieber wrote: On 22 Jun 2006 16:48:47 -0700, George Sakkis [EMAIL PROTECTED] declaimed the following in comp.lang.python: What does __setitem__ have to do with iterability ? It confirms that the object is indexable, and mutable -- ie; a list, not a tuple or a string. Ok, I'll try once more: What does __setitem__ have to do with **iterability**, not mutability or indexability ? I was commenting on Maric's post that although some objects are typically iterable, they are often treated as atomic by some/many/most applications (e.g. strings). It's not rocket science. George -- http://mail.python.org/mailman/listinfo/python-list
Re: Specifing arguments type for a function
Dennis Lee Bieber wrote: On 22 Jun 2006 22:55:00 -0700, George Sakkis [EMAIL PROTECTED] declaimed the following in comp.lang.python: Ok, I'll try once more: What does __setitem__ have to do with **iterability**, not mutability or indexability ? I was commenting on Maric's post that although some objects are typically iterable, they are often treated as atomic by some/many/most applications (e.g. strings). It's not rocket science. And the absence of the setitem would indicate such an object -- it may be iterable in terms of retrieving subparts, but atomic WRT modification. That, at least, is how I interpreted the introduction of the test... Applications that don't need to treat strings as iterables of characters wouldn't do so even if strings were mutable. Atomicity has to do with whether something is considered to be composite or not, not whether it can be modified. George -- http://mail.python.org/mailman/listinfo/python-list
Re: Specifing arguments type for a function
George Sakkis wrote:
Dennis Lee Bieber wrote:
On 22 Jun 2006 16:48:47 -0700, George Sakkis [EMAIL PROTECTED]
declaimed the following in comp.lang.python:
What does __setitem__ have to do with iterability ?
It confirms that the object is indexable, and mutable -- ie; a list,
not a tuple or a string.
Ok, I'll try once more: What does __setitem__ have to do with
**iterability**, not mutability or indexability ?
Nothing.
I was commenting on
Maric's post that although some objects are typically iterable, they
are often treated as atomic by some/many/most applications (e.g.
strings). It's not rocket science.
No, it's not rocket science.
It's not rocket science neither to understand that, for the concrete
examples you used (ie strings and tuples), it's quite easy to detect'em
without testing the concrete type.
As you said, what is to be considered as scalar and what's to be
considered as sequence highly depends on the problem at hand. But doing
the distinction does not always implies testing concrete type or mro.
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Re: Specifing arguments type for a function
George Sakkis wrote:
Dennis Lee Bieber wrote:
On 22 Jun 2006 22:55:00 -0700, George Sakkis [EMAIL PROTECTED]
declaimed the following in comp.lang.python:
Ok, I'll try once more: What does __setitem__ have to do with
**iterability**, not mutability or indexability ? I was commenting on
Maric's post that although some objects are typically iterable, they
are often treated as atomic by some/many/most applications (e.g.
strings). It's not rocket science.
And the absence of the setitem would indicate such an object -- it
may be iterable in terms of retrieving subparts, but atomic WRT
modification.
That, at least, is how I interpreted the introduction of the test...
Applications that don't need to treat strings as iterables of
characters wouldn't do so even if strings were mutable. Atomicity has
to do with whether something is considered to be composite or not, not
whether it can be modified.
Sure. Do you have any generic solution for this ?
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Re: Specifing arguments type for a function
George Sakkis a écrit :
Maric Michaud wrote:
Le Mardi 20 Juin 2006 13:28, Maric Michaud a écrit :
if not getattr(arg, '__iter__') and not getattr(arg, '__getitem__') :
raise ValueError('Function accepts only iterables') # or error handling
code
oops, hasattr of course :
if not hasattr(arg, '__iter__') and not hasattr(arg, '__getitem__') :
raise ValueError('Function accepts only iterables') # or error handling
This is ok - in theory. In practice I've found that e.g. strings are
more often than not handled as scalars although they are typically
iterables.
hasattr('', '__iter__')
False
Also tuples may or may not be considered as iterables,
depending on what they are used for.
hasattr((), '__setitem__')
False
hasattr('', '__setitem__')
False
The definition of scalar is
application-dependent, that's why there is not an isscalar() builtin.
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Re: Specifing arguments type for a function
Paolo Pantaleo: and I want to state that the_arg must be only of a certain type (actually a list). Is there a way to do that? I suggest this very good library, typecheck: http://oakwinter.com/code/typecheck/ Bye, bearophile -- http://mail.python.org/mailman/listinfo/python-list
Re: Specifing arguments type for a function
Bruno Desthuilliers wrote:
George Sakkis a écrit :
This is ok - in theory. In practice I've found that e.g. strings are
more often than not handled as scalars although they are typically
iterables.
hasattr('', '__iter__')
False
hasattr('', '__iter__') or hasattr('', '__getitem__')
True
Also tuples may or may not be considered as iterables,
depending on what they are used for.
hasattr((), '__setitem__')
False
hasattr('', '__setitem__')
False
What does __setitem__ have to do with iterability ?
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Re: Specifing arguments type for a function
Paolo Pantaleo wrote: I have a function def f(the_arg): ... and I want to state that the_arg must be only of a certain type (actually a list). Is there a way to do that? I wrote a cool function decorator just for that purpose. It's posted on the Python Decorator Library at: http://wiki.python.org/moin/PythonDecoratorLibrary#head-308f2b3507ca91800def19d813348f78db34303e If you use it, you will be able to simply write: @accepts(list) def f(the_arg): ... and you will get a warning message if the_arg is not a list. Or, if you want an exception raised, then just: @accepts(list, debug=2) def f(the_arg): ... Let me know what you think, if you do end up using it. I'm eager for feedback. -- http://mail.python.org/mailman/listinfo/python-list
Specifing arguments type for a function
I have a function def f(the_arg): ... and I want to state that the_arg must be only of a certain type (actually a list). Is there a way to do that? Thnx PAolo -- if you have a minute to spend please visit my photogrphy site: http://mypic.co.nr -- http://mail.python.org/mailman/listinfo/python-list
Re: Specifing arguments type for a function
Paolo Pantaleo wrote: I have a function def f(the_arg): ... and I want to state that the_arg must be only of a certain type (actually a list). Is there a way to do that? Yes and no. You can ensure that the passed object is a list, by calling e.g. def f(arg): if not isinstance(arg, list): raise Not a list! Alternatively, you can just use it as an iterable - and the exception will come from arg not being iterable. But what you can't do is make python complain about this: def f(arg): for e in arg: print e f(100) before actually calling f. It will always fail at runtime. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: Specifing arguments type for a function
Paolo Pantaleo wrote: I have a function def f(the_arg): ... and I want to state that the_arg must be only of a certain type (actually a list). Is there a way to do that? Yes and no. You can ensure that the passed object is a list, by calling e.g. def f(arg): if not isinstance(arg, list): raise Not a list! Alternatively, you can just use it as an iterable - and the exception will come from arg not being iterable. But what you can't do is make python complain about this: def f(arg): for e in arg: print e f(100) before actually calling f. It will always fail at runtime. Diez What about def f(arg): if type(arg)=='list': #do something -- --- Rony Steelandt BuCodi rony dot steelandt (at) bucodi dot com Visit the python blog at http://360.yahoo.com/bucodi -- http://mail.python.org/mailman/listinfo/python-list
Re: Specifing arguments type for a function
What about def f(arg): if type(arg)=='list': #do something Several things: - list is a string in your code - which wont work: type([]) == 'list' False It needs to be list w/o quotes. - you won't get sublclasses of list: class Foo(list): ...pass ... type(Foo()) == list False So better use isinstance, as I already mentioned. Diez -- http://mail.python.org/mailman/listinfo/python-list
Re: Specifing arguments type for a function
Rony Steelandt wrote:
Paolo Pantaleo wrote:
I have a function
def f(the_arg):
...
and I want to state that the_arg must be only of a certain type
(actually a list). Is there a way to do that?
Yes and no. You can ensure that the passed object is a list, by
calling e.g.
def f(arg):
if not isinstance(arg, list):
raise Not a list!
Alternatively, you can just use it as an iterable - and the exception
will
come from arg not being iterable.
But what you can't do is make python complain about this:
def f(arg):
for e in arg:
print e
f(100)
before actually calling f. It will always fail at runtime.
Diez
What about
def f(arg):
if type(arg)=='list':
FWIW, type(some_type) returns a type object, not a string. So your
test will always fail. A right way to write this is:
if type(arg) is type([]):
...
#do something
Usually a very bad idea. It defeats the whole point of duck-typing. In
most cases, you don't care about the concrete class - all you want is an
object that implements an (implied) interface.
NB : I say 'usually' because there are a very few cases where testing
the concrete class can make sens - but there again, better to use
isinstance() than type().
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Re: Specifing arguments type for a function
Le Mardi 20 Juin 2006 12:29, Rony Steelandt a écrit :
What about
def f(arg):
if type(arg)=='list':
#do something
And if arg's type is subclass of list ?
The problem with isinstance is : and if arg is not of type list but is a
sequence (a instance of UserList for example) ?
The general case in python is duck typing, that means you define apis that
accept file-like object, iterables, dict-like (mapping) objects, string like
objects...
The problem with iterables is that they don't all implement the __iter__
attribute, for compatibility purpose the right test should be :
if not getattr(arg, '__iter__') and not getattr(arg, '__getitem__') :
raise ValueError('Function accepts only iterables') # or error handling
code
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_
Aristote - www.aristote.info
3 place des tapis
69004 Lyon
Tel: +33 426 880 097
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Re: Specifing arguments type for a function
bruno at modulix wrote: if type(arg) is type([]): Just a tiny nitpick You can just use 'list' instead of 'type([])' if type(arg) is list : # blah blah Regards Sreeram signature.asc Description: OpenPGP digital signature -- http://mail.python.org/mailman/listinfo/python-list
Re: Specifing arguments type for a function
Le Mardi 20 Juin 2006 13:28, Maric Michaud a écrit :
if not getattr(arg, '__iter__') and not getattr(arg, '__getitem__') :
raise ValueError('Function accepts only iterables') # or error handling
code
oops, hasattr of course :
if not hasattr(arg, '__iter__') and not hasattr(arg, '__getitem__') :
raise ValueError('Function accepts only iterables') # or error handling
--
_
Maric Michaud
_
Aristote - www.aristote.info
3 place des tapis
69004 Lyon
Tel: +33 426 880 097
--
http://mail.python.org/mailman/listinfo/python-list
Re: Specifing arguments type for a function
Maric Michaud wrote:
Le Mardi 20 Juin 2006 13:28, Maric Michaud a écrit :
if not getattr(arg, '__iter__') and not getattr(arg, '__getitem__') :
raise ValueError('Function accepts only iterables') # or error handling
code
oops, hasattr of course :
if not hasattr(arg, '__iter__') and not hasattr(arg, '__getitem__') :
raise ValueError('Function accepts only iterables') # or error handling
This is ok - in theory. In practice I've found that e.g. strings are
more often than not handled as scalars although they are typically
iterables. Also tuples may or may not be considered as iterables,
depending on what they are used for. The definition of scalar is
application-dependent, that's why there is not an isscalar() builtin.
George
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Re: Specifing arguments type for a function
K.S.Sreeram wrote:
bruno at modulix wrote:
if type(arg) is type([]):
Just a tiny nitpick
You can just use 'list' instead of 'type([])'
I know. Note that I wrote *A* right way to write this, not *The*
right way...
And FWIW, you could also use arg.__class__ instead of type(arg).
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p in '[EMAIL PROTECTED]'.split('@')])
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