Possibly dumb question about dicts and __hash__()

[Joel Hedlund]

Hi!

There's one thing about dictionaries and __hash__() methods that puzzle me. I
have a class with several data members, one of which is 'name' (a str). I would
like to store several of these objects in a dict for quick access
({namebject} style). Now, I was thinking that given a list of objects I might
do something like

d = {}
for o in objects:
d[o] = o

and still be able to retrieve the data like so:

d[name]

if I just defined a __hash__ method like so:

def __hash__(self):
return self.name.__hash__()

but this fails miserably. Feel free to laugh if you feel like it. I cooked up a
little example with sample output below if you care to take the time.

Code:
---------------------------------------------------------------
class NamedThing(object):
def __init__(self, name):
self.name = name
def __hash__(self):
return self.name.__hash__()
def __repr__(self):
return '<foo>'
name = 'moo'
o = NamedThing(name)
print "This output puzzles me:"
d = {}
d[o] = o
d[name] = o
print d
print
print "If I wrap all keys in hash() calls I'm fine:"
d = {}
d[hash(o)] = o
d[hash(name)] = o
print d
print
print "But how come the first method didn't work?"
---------------------------------------------------------------

Output:
---------------------------------------------------------------
This output puzzles me:
{'moo': <foo>, <foo>: <foo>}

If I wrap all keys in hash() calls I'm fine:
{2038943316: <foo>}

But how come the first method didn't work?
---------------------------------------------------------------

I'd be grateful if anyone can shed a litte light on this, or point me to some
docs I might have missed.

Also:
Am I in fact abusing the __hash__() method? If so - what's the intended use of
the __hash__() method?

Is there a better way of implementing this?

I realise I could just write

d[o.name] = o

but this problem seems to pop up every now and then and I'm curious if there's
some neat syntactic trick that I could legally apply here.

Thanks for your time!
/Joel Hedlund

 

[johnzenger]

Use __repr__. Behold:
def __init__(self, name):
self.name = name
def __repr__(self):
return self.name

a = NamedThing("Delaware")
b = NamedThing("Hawaii")
d = {}
d[a] = 1
d = 50
print d {Delaware: 1, Hawaii: 50}
d[a] 1
d

50

Although this is a bit illegal, because repr is not supposed to be used
this way.

Hi!

There's one thing about dictionaries and __hash__() methods that puzzle me. I
have a class with several data members, one of which is 'name' (a str). I would
like to store several of these objects in a dict for quick access
({namebject} style). Now, I was thinking that given a list of objects I might
do something like

d = {}
for o in objects:
d[o] = o

and still be able to retrieve the data like so:

d[name]

if I just defined a __hash__ method like so:

def __hash__(self):
return self.name.__hash__()

but this fails miserably. Feel free to laugh if you feel like it. I cooked up a
little example with sample output below if you care to take the time.

Code:
---------------------------------------------------------------
class NamedThing(object):
def __init__(self, name):
self.name = name
def __hash__(self):
return self.name.__hash__()
def __repr__(self):
return '<foo>'
name = 'moo'
o = NamedThing(name)
print "This output puzzles me:"
d = {}
d[o] = o
d[name] = o
print d
print
print "If I wrap all keys in hash() calls I'm fine:"
d = {}
d[hash(o)] = o
d[hash(name)] = o
print d
print
print "But how come the first method didn't work?"
---------------------------------------------------------------

Output:
---------------------------------------------------------------
This output puzzles me:
{'moo': <foo>, <foo>: <foo>}

If I wrap all keys in hash() calls I'm fine:
{2038943316: <foo>}

But how come the first method didn't work?
---------------------------------------------------------------

I'd be grateful if anyone can shed a litte light on this, or point me to some
docs I might have missed.

Also:
Am I in fact abusing the __hash__() method? If so - what's the intended use of
the __hash__() method?

Is there a better way of implementing this?

I realise I could just write

d[o.name] = o

but this problem seems to pop up every now and then and I'm curious if there's
some neat syntactic trick that I could legally apply here.

Thanks for your time!
/Joel Hedlund

 

[Joel Hedlund]

Hi!

Thanks for the quick response!

> Although this is a bit illegal, because repr is not supposed to be used
> this way.

How illegal is it? If I document it and put it in an opensource project, will
people throw tomatoes?

/Joel

Use __repr__. Behold:


def __init__(self, name):
self.name = name
def __repr__(self):
return self.name

a = NamedThing("Delaware")
b = NamedThing("Hawaii")
d = {}
d[a] = 1
d = 50
print d

{Delaware: 1, Hawaii: 50}

d[a]

1

d

50

Although this is a bit illegal, because repr is not supposed to be used
this way.

Hi!

There's one thing about dictionaries and __hash__() methods that puzzle me. I
have a class with several data members, one of which is 'name' (a str). I would
like to store several of these objects in a dict for quick access
({namebject} style). Now, I was thinking that given a list of objects I might
do something like

d = {}
for o in objects:
d[o] = o

and still be able to retrieve the data like so:

d[name]

if I just defined a __hash__ method like so:

def __hash__(self):
return self.name.__hash__()

but this fails miserably. Feel free to laugh if you feel like it. I cooked up a
little example with sample output below if you care to take the time.

Code:
---------------------------------------------------------------
class NamedThing(object):
def __init__(self, name):
self.name = name
def __hash__(self):
return self.name.__hash__()
def __repr__(self):
return '<foo>'
name = 'moo'
o = NamedThing(name)
print "This output puzzles me:"
d = {}
d[o] = o
d[name] = o
print d
print
print "If I wrap all keys in hash() calls I'm fine:"
d = {}
d[hash(o)] = o
d[hash(name)] = o
print d
print
print "But how come the first method didn't work?"
---------------------------------------------------------------

Output:
---------------------------------------------------------------
This output puzzles me:
{'moo': <foo>, <foo>: <foo>}

If I wrap all keys in hash() calls I'm fine:
{2038943316: <foo>}

But how come the first method didn't work?
---------------------------------------------------------------

I'd be grateful if anyone can shed a litte light on this, or point me to some
docs I might have missed.

Also:
Am I in fact abusing the __hash__() method? If so - what's the intended use of
the __hash__() method?

Is there a better way of implementing this?

I realise I could just write

d[o.name] = o

but this problem seems to pop up every now and then and I'm curious if there's
some neat syntactic trick that I could legally apply here.

Thanks for your time!
/Joel Hedlund

 

[johnzenger]

Actually, come to think of it, __str__ works just as well.
def __init__(self, name):
self.name = name
def __str__(self):
return self.name

d = {}
d[a] = 1
d = 50
d

d[a] 1
d

50

This is what you should use, instead of my first answer. From the docs
for __repr__: "If at all possible, this should look like a valid Python
expression that could be used to recreate an object with the same value
(given an appropriate environment). If this is not possible, a string
of the form "<...some useful description...>" should be returned. ...
This is typically used for debugging, so it is important that the
representation is information-rich and unambiguous."

Use __repr__. Behold:
def __init__(self, name):
self.name = name
def __repr__(self):
return self.name

a = NamedThing("Delaware")
b = NamedThing("Hawaii")
d = {}
d[a] = 1
d = 50
print d {Delaware: 1, Hawaii: 50}
d[a] 1
d

50

Although this is a bit illegal, because repr is not supposed to be used
this way.

Hi!

There's one thing about dictionaries and __hash__() methods that puzzle me. I
have a class with several data members, one of which is 'name' (a str). I would
like to store several of these objects in a dict for quick access
({namebject} style). Now, I was thinking that given a list of objects I might
do something like

d = {}
for o in objects:
d[o] = o

and still be able to retrieve the data like so:

d[name]

if I just defined a __hash__ method like so:

def __hash__(self):
return self.name.__hash__()

but this fails miserably. Feel free to laugh if you feel like it. I cooked up a
little example with sample output below if you care to take the time.

Code:
---------------------------------------------------------------
class NamedThing(object):
def __init__(self, name):
self.name = name
def __hash__(self):
return self.name.__hash__()
def __repr__(self):
return '<foo>'
name = 'moo'
o = NamedThing(name)
print "This output puzzles me:"
d = {}
d[o] = o
d[name] = o
print d
print
print "If I wrap all keys in hash() calls I'm fine:"
d = {}
d[hash(o)] = o
d[hash(name)] = o
print d
print
print "But how come the first method didn't work?"
---------------------------------------------------------------

Output:
---------------------------------------------------------------
This output puzzles me:
{'moo': <foo>, <foo>: <foo>}

If I wrap all keys in hash() calls I'm fine:
{2038943316: <foo>}

But how come the first method didn't work?
---------------------------------------------------------------

I'd be grateful if anyone can shed a litte light on this, or point me to some
docs I might have missed.

Also:
Am I in fact abusing the __hash__() method? If so - what's the intended use of
the __hash__() method?

Is there a better way of implementing this?

I realise I could just write

d[o.name] = o

but this problem seems to pop up every now and then and I'm curious if there's
some neat syntactic trick that I could legally apply here.

Thanks for your time!
/Joel Hedlund

 

[Joel Hedlund]

Beautiful!

But how come my attempt didn't work? I've seen docs that explain how __hash__()
methods are used to put objects in dict buckets:

http://docs.python.org/ref/customization.html#l2h-195

But if it's really hash(str(o)) that's used for dict keys, what good are
__hash__() methods? Or am I reading the docs wrong?

/Joel

 

[Bruno Desthuilliers]

Joel Hedlund a écrit :
(snip)

> How illegal is it? If I document it and put it in an opensource project,
> will people throw tomatoes?

Don't know, but they'll sure do if you insist on top-posting !-)

 

[Peter Otten]

There's one thing about dictionaries and __hash__() methods that puzzle
me. I have a class with several data members, one of which is 'name' (a
str). I would like to store several of these objects in a dict for quick
access ({namebject} style). Now, I was thinking that given a list of
objects I might do something like

d = {}
for o in objects:
d[o] = o

and still be able to retrieve the data like so:

d[name]

if I just defined a __hash__ method like so:

def __hash__(self):
return self.name.__hash__()

Just the hash is not enough. You need to define equality, too:
.... def __init__(self, name):
.... self.name = name
.... def __hash__(self):
.... return hash(self.name)
.... def __eq__(self, other):
.... try:
.... other_name = other.name
.... except AttributeError:
.... return self.name == other
.... return self.name == other_name
.... def __repr__(self):
.... return "Named(name=%r)" % self.name
....

items = [Named(n) for n in "alpha beta gamma".split()]
d = dict(zip(items, items))
d["alpha"]

Named(name='alpha')

Peter

 

[Joel Hedlund]

Hi!

> Just the hash is not enough. You need to define equality, too:

Thanks a million for clearing that up.

Cheers!
/Joel