hashable and mutable functions

[cybb20]

BPO	1297986
Nosy	@mwhudson, @birkenfeld, @rhettinger

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2005-09-22.09:20:10.000>
created_at = <Date 2005-09-21.17:55:31.000>
labels = ['type-feature', 'library']
title = 'hashable and mutable functions'
updated_at = <Date 2005-09-22.09:20:10.000>
user = 'https://bugs.python.org/cybb20'

bugs.python.org fields:

activity = <Date 2005-09-22.09:20:10.000>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2005-09-21.17:55:31.000>
creator = 'cybb20'
dependencies = []
files = []
hgrepos = []
issue_num = 1297986
keywords = []
message_count = 4.0
messages = ['54619', '54620', '54621', '54622']
nosy_count = 4.0
nosy_names = ['mwh', 'georg.brandl', 'rhettinger', 'cybb20']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue1297986'
versions = []

[cybb20]

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.

[birkenfeld]

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.

[mwhudson]

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.

[rhettinger]

Logged In: YES
user_id=80475

Adding a third -1 from me and closing the RFE.