Re: Know if a object member is a method

[Manuel Ebert]

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Hi Luca,

use type(something).__name__ , e.g.

>>def x():
pass
class C:
pass
c = C()
type(x).__name__ == 'function'

True

>type(C).__name__ == 'classobj'

True

>type(c).__name__ == 'instance'

True

On Sep 1, 2008, at 10:43 AM, Luca wrote:

Hi all.

I think this is a newbie question... what is the best method to know
if a property of an object is a function?

I'm thinking something as

if type(obj.methodName)==???

Can someone help me?

--
-- luca
--
http://mail.python.org/mailman/listinfo/python-list

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[Steven D'Aprano]

On Mon, 01 Sep 2008 10:52:10 +0200, Manuel Ebert wrote:

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Hi Luca,

use type(something).__name__ , e.g.

>>def x():
>>> pass
>>class C:
>>> pass
>>c = C()
>>type(x).__name__ == 'function'

True

>type(C).__name__ == 'classobj'

True

>type(c).__name__ == 'instance'

True

That's relatively fragile, since such names aren't reserved in any way.
It's easy to fool a name comparison check with an accidental name
collision:

>>class function(object): # not a reserved name

.... pass
....

>>x = function()
type(x).__name__

'function'

>>x() # must be a function then...

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'function' object is not callable
But not so easy to fool a type check:

>>type(x) == new.function

False

Of course that's not bullet-proof either. I leave it as an exercise to
discover how you might break that piece of code.

--
Steven

[Luca]

On Mon, Sep 1, 2008 at 11:35 AM, Steven D'Aprano
<st****@remove.this.cybersource.com.auwrote:

That's relatively fragile, since such names aren't reserved in any way.
It's easy to fool a name comparison check with an accidental name
collision:

>>>class function(object): # not a reserved name

... pass
...

>>>x = function()
type(x).__name__

'function'

>>>x() # must be a function then...

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'function' object is not callable
But not so easy to fool a type check:

>>>type(x) == new.function

False

Of course that's not bullet-proof either. I leave it as an exercise to
discover how you might break that piece of code.

Ok, so...

What is the best way to do this? The "most pythonic"?
--
-- luca

[Marc 'BlackJack' Rintsch]

On Mon, 01 Sep 2008 11:45:36 +0200, Luca wrote:

>But not so easy to fool a type check:

>>>>type(x) == new.function

False

Of course that's not bullet-proof either. I leave it as an exercise to
discover how you might break that piece of code.

Ok, so...

What is the best way to do this? The "most pythonic"?

The "most pythonic" might be not checking at all but simply *call* the
object and deal with possible failures.

What's your use case?

Ciao,
Marc 'BlackJack' Rintsch

[Steven D'Aprano]

On Mon, 01 Sep 2008 11:45:36 +0200, Luca asked about recognizing methods:

What is the best way to do this? The "most pythonic"?

That depends on why you are doing it, and what you want to do with the
information once you've found it.

If you are experimenting in the interactive interpreter, the easiest way
is simply call the method and see what happens:

obj.methodName() # call the method

If it succeeds, then it is some sort of callable, a method or a function
or something more unusual.

If you fear side-effects, then use:

callable(obj.methodName)

Alternatively, you can read the docs:

help(obj)
help(obj.methodName)

If your aim is to write something like a debugger, profiler, or some
other application that needs to inspect arbitrary objects and work out
what they do, then you probably should be using:

isinstance(obj.methodName, new.instancemethod)

But remember that not all callable attributes are instancemethods!

There is no one right way of doing this.

Hope this helps.

--
Steven

[Luca]

On Mon, Sep 1, 2008 at 2:18 PM, Steven D'Aprano
<st***@remove-this-cybersource.com.auwrote:

>
If your aim is to write something like a debugger, profiler, or some
other application that needs to inspect arbitrary objects and work out
what they do, then you probably should be using:

isinstance(obj.methodName, new.instancemethod)

But remember that not all callable attributes are instancemethods!

There is no one right way of doing this.

Hope this helps.

Yes, this helps a lot. In facts I need to do something like a language parser.

Thanks all.

--
-- luca

[Fredrik Lundh]

Luca wrote:

Yes, this helps a lot. In facts I need to do something like a language parser.

a *parser* that works on object structures created by executing a Python
program?

</F>