[ python-Feature Requests-1297986 ] hashable and mutable functions

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Feature Requests item #1297986, was opened at 2005-09-21 18:55
Message generated for change (Comment added) made by mwh
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Category: Python Library
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: ChristianJ (cybb20)
Assigned to: Nobody/Anonymous (nobody)
Summary: hashable and mutable functions

Initial Comment:
It is not easy to check if an object is hashable, ie
hasattr(list(), '__hash__') -> True 


try: hash(list())
except TypeError: pass

seems to be a possible way to see if an object is
hashable, however it is not satisfiable that this
information needs to be retrieved by using exception
handling.

My proposal:
There should be a hashable(obj) function returning a
bool object and additionally it would be nice to have
something like ismutable(obj) function, possibly as
built-in functions.

Reason:
callable() is a built-in function and returns
information about an object whether it's callable or
not, that is a basic info about this object. 
If an object is hashable or mutable is a state of an
object that is also very important to know.


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>Comment By: Michael Hudson (mwh)
Date: 2005-09-22 09:16

Message:
Logged In: YES 
user_id=6656

def ishashable(ob):
     try:
         hash(ob)
         return True
     except TypeError:
         return False

is the only reasonable way to implement this function (incidentally, do you 
know how hasattr works?).

I don't see the need for a builtin, but am not feeling bolshy enough to just 
close the tracker item.  Maybe someone else does.

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Comment By: Reinhold Birkenfeld (birkenfeld)
Date: 2005-09-22 07:27

Message:
Logged In: YES 
user_id=1188172

> try: hash(list())
> except TypeError: pass
> 
> seems to be a possible way to see if an object is
> hashable, however it is not satisfiable that this
> information needs to be retrieved by using exception
> handling.

Why?

> My proposal:
> There should be a hashable(obj) function returning a
> bool object and additionally it would be nice to have
> something like ismutable(obj) function, possibly as
> built-in functions.

How should "ismutable" be implemented?

> Reason:
> callable() is a built-in function and returns
> information about an object whether it's callable or
> not, that is a basic info about this object.
> If an object is hashable or mutable is a state of an
> object that is also very important to know.

It's easier to ask for forgiveness than permission.

Even callable() has been called a mistake. Just call
it and see where you come.

So, -1.

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