How can I verify that a passed argument is an interible collection?

[Charles Krug]

List:

I'm working on some methods that operate on (mathematical) vectors as
in:

def Convolution(x, y)
"""Returns a list containing the convolution of vectors x and y"""

Is there any way to determine at runtime that x and y are iterible
collections?

Do I *coughs* simply *coughs* trap the exception created by:

for v in x:

when v is a scaler quantity?

Thanks


Charles

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Re: How can I verify that a passed argument is an interible collection?

[Fredrik Lundh]

Charles Krug wrote:
I'm working on some methods that operate on (mathematical) vectors as
in:
def Convolution(x, y)
"""Returns a list containing the convolution of vectors x and y"""
Is there any way to determine at runtime that x and y are iterible
collections?
Do I *coughs* simply *coughs* trap the exception created by:
   for v in x:
when v is a scaler quantity?
why not leave trapping exceptions to whoever uses your function?

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Re: How can I verify that a passed argument is an interible collection?

[Larry Bates]

Others may know better ways but the 2 I know of are:

1) If you know that the arguments will be lists or tuples
you can use isinstance().

if not isinsance(arg, (list, tuple):
print "arg must be list or tuple)

2) Or if you not you could see if the argument has next and
__iter__ methods (more general solution)

if  hasattr(arg, 'next') and not hasattr(arg, '__iter__'):
# perform work on iterable

else:
print "arg must be an iterable"


Larry Bates


Charles Krug wrote:
> List:
> 
> I'm working on some methods that operate on (mathematical) vectors as
> in:
> 
> def Convolution(x, y)
> """Returns a list containing the convolution of vectors x and y"""
> 
> Is there any way to determine at runtime that x and y are iterible
> collections?
> 
> Do I *coughs* simply *coughs* trap the exception created by:
> 
> for v in x:
> 
> when v is a scaler quantity?
> 
> Thanks
> 
> 
> Charles
> 
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Re: How can I verify that a passed argument is an interible collection?

[Roy Smith]

Charles Krug  <[EMAIL PROTECTED]> wrote:
> Do I *coughs* simply *coughs* trap the exception created by:
>
>for v in x:
>
>when v is a scaler quantity?

Sure.  Why not?  If you're coming from a type-bondage language like
C++ or Java, it probabliy feels strange, but it's the Python way.  It
looks like what you want to catch is TypeError.
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Re: How can I verify that a passed argument is an interible collection?

[Diez B. Roggisch]

Charles Krug wrote:

> List:
> 
> I'm working on some methods that operate on (mathematical) vectors as
> in:
> 
> def Convolution(x, y)
> """Returns a list containing the convolution of vectors x and y"""
> 
> Is there any way to determine at runtime that x and y are iterible
> collections?
> 
> Do I *coughs* simply *coughs* trap the exception created by:
> 
> for v in x:
> 
> when v is a scaler quantity?

Yes, thats considered good practice. 
-- 
Regards,

Diez B. Roggisch
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Re: How can I verify that a passed argument is an interible collection?

[Steven Bethard]

Charles Krug wrote:
List:
I'm working on some methods that operate on (mathematical) vectors as
in:
def Convolution(x, y)
"""Returns a list containing the convolution of vectors x and y"""
Is there any way to determine at runtime that x and y are iterible
collections?
Do I *coughs* simply *coughs* trap the exception created by:
for v in x:
when v is a scaler quantity?
Sure, or if you want to do it earlier, you could try something like:
def convolution(x, y):
xiter = iter(x)
yiter = iter(y)
And then use xiter and yiter in your for loops.  This will make sure 
that the TypeErrors get raised as soon as the function is called.

STeVe
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Re: How can I verify that a passed argument is an interible collection?

[John Machin]

On Thu, 21 Apr 2005 09:59:54 -0500, Larry Bates
<[EMAIL PROTECTED]> wrote:

>2) Or if you not you could see if the argument has next and
>__iter__ methods (more general solution)
>
>if  hasattr(arg, 'next') and not hasattr(arg, '__iter__'):
># perform work on iterable
>
>else:
>print "arg must be an iterable"
>


>>> alist = ['Think', 'before', 'posting!']
>>> hasattr(alist, 'next')
False

===>>> Fails the first leg of your test for iterability.

>>> for wd in alist:
... print wd
...
Think
before
posting!

===>>> Looks iterable to me!!

In any case, trying to define "isiterable(x)" in terms of x having or
not having particularly-named attributes doesn't seem to me to be a
very good idea. It's what the method *does* that matters. Other ways
of doing it may be invented. Such a definition has about the same
level of future-proof correlation between cause and effect as "We
waved a dead chicken at the volcano and it stopped erupting".

Cheers,
John


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