Is there a short-circuiting dictionary "get" method?

[Dave Opstad]

In this snippet:

d = {'x': 1}
value = d.get('x', bigscaryfunctio n())

the bigscaryfunctio n is always called, even though 'x' is a valid key.
Is there a "short-circuit" version of get that doesn't evaluate the
second argument if the first is a valid key? For now I'll code around
it, but this behavior surprised me a bit...

Dave

[Peter Hansen]

Re: Is there a short-circuiting dictionary "get&qu ot; method?

Dave Opstad wrote:[color=blue]
> In this snippet:
>
> d = {'x': 1}
> value = d.get('x', bigscaryfunctio n())
>
> the bigscaryfunctio n is always called, even though 'x' is a valid key.
> Is there a "short-circuit" version of get that doesn't evaluate the
> second argument if the first is a valid key? For now I'll code around
> it, but this behavior surprised me a bit...[/color]

try:
value = d['x']
except KeyError:
value = bigscaryfunctio n()

get() is just a method, and arguments to methods are always
evaluated before being passed to the method, so the short
answer is "no, there is no 'version' of get() that will do
what you want".

-Peter

[Bill Mill]

Re: Is there a short-circuiting dictionary "get&qu ot; method?

Dave,

On Wed, 09 Mar 2005 09:45:41 -0800, Dave Opstad <opstad@batnet. com> wrote:[color=blue]
> In this snippet:
>
> d = {'x': 1}
> value = d.get('x', bigscaryfunctio n())
>
> the bigscaryfunctio n is always called, even though 'x' is a valid key.
> Is there a "short-circuit" version of get that doesn't evaluate the
> second argument if the first is a valid key? For now I'll code around
> it, but this behavior surprised me a bit...[/color]

There is no short-circuit function like you're asking for, because
it's impossible in python. To pass an argument to the 'get' function,
python evaluates the bigscaryfunctio n before calling 'get'.

(I believe this means that python doesn't have "lazy evaluation", but
the language lawyers may shoot me down on that. Wikipedia seems to say
that it means python doesn't have "delayed evaluation").

Here are two ways to do what you want:

if 'x' in d: value = d['x']
else: value = bigscaryfunctio n()

or:

def sget(dict, key, func, *args):
if key in dict: return key
else: return func(*args)

sget(d, 'x', bigscaryfunctio n)

Both methods are untested, but should work with minor modifications.

Peace
Bill Mill
bill.mill at gmail.com

[bearophileHUGS@lycos.com]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?

Maybe this can help:

value = d.get('x', lambda: bigscaryfunctio n())

Bearophile

[F. Petitjean]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?

Le Wed, 09 Mar 2005 09:45:41 -0800, Dave Opstad a écrit :[color=blue]
> In this snippet:
>
> d = {'x': 1}
> value = d.get('x', bigscaryfunctio n())
>
> the bigscaryfunctio n is always called, even though 'x' is a valid key.
> Is there a "short-circuit" version of get that doesn't evaluate the
> second argument if the first is a valid key? For now I'll code around
> it, but this behavior surprised me a bit...[/color]
def scary():
print "scary called"
return 22

d = dict(x=1)
d.get('x', lambda *a : scary())
# print 1
d.get('z', (lambda *a : scary())())
scary called
22

First (wrong) version :
d.get('z', lambda *a : scary())
<function <lambda> at 0x40598e9c>[color=blue]
>
> Dave[/color]

[Bill Mill]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?

On 9 Mar 2005 10:05:21 -0800, bearophileHUGS@ lycos.com
<bearophileHUGS @lycos.com> wrote:[color=blue]
> Maybe this can help:
>
> value = d.get('x', lambda: bigscaryfunctio n())[/color]
[color=blue][color=green][color=darkred]
>>> def test(): print 'gbye'[/color][/color][/color]
....[color=blue][color=green][color=darkred]
>>> d = {}
>>> z = d.get('x', lambda: test())
>>> z[/color][/color][/color]
<function <lambda> at 0x008D6870>

So this seems to be merely an obfuscation of:
[color=blue][color=green][color=darkred]
>>> z = d.get('x', test)
>>> z[/color][/color][/color]
<function test at 0x008D66B0>

I just wanted to ask, am I missing something?

Peace
Bill Mill
bill.mill at gmail.com

[Bill Mill]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?

On 09 Mar 2005 18:13:01 GMT, F. Petitjean <littlejohn.75@ news.proxad.net > wrote:[color=blue]
> Le Wed, 09 Mar 2005 09:45:41 -0800, Dave Opstad a écrit :[color=green]
> > In this snippet:
> >
> > d = {'x': 1}
> > value = d.get('x', bigscaryfunctio n())
> >
> > the bigscaryfunctio n is always called, even though 'x' is a valid key.
> > Is there a "short-circuit" version of get that doesn't evaluate the
> > second argument if the first is a valid key? For now I'll code around
> > it, but this behavior surprised me a bit...[/color]
> def scary():
> print "scary called"
> return 22
>
> d = dict(x=1)
> d.get('x', lambda *a : scary())
>
> # print 1
> d.get('z', (lambda *a : scary())())
> scary called
> 22[/color]

but:
[color=blue][color=green][color=darkred]
>>> d.get('x', (lambda *a: test())())[/color][/color][/color]
test called
1

So how is this different than d.get('x', test()) ?

Peace
Bill Mill
bill.mill at gmail.com

[Kent Johnson]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?

F. Petitjean wrote:[color=blue]
> Le Wed, 09 Mar 2005 09:45:41 -0800, Dave Opstad a écrit :[color=green]
>>Is there a "short-circuit" version of get that doesn't evaluate the
>>second argument if the first is a valid key? For now I'll code around
>>it, but this behavior surprised me a bit...[/color]
>
> def scary():
> print "scary called"
> return 22
>
> d = dict(x=1)
> d.get('x', lambda *a : scary())
> # print 1
> d.get('z', (lambda *a : scary())())
> scary called
> 22[/color]

So you have to change the code at the point of call depending on whether the requested value is in
the dict? ;)

If you can get this to work I'm sure we can find other applications for such 'smart code' :-)

Kent

[Reinhold Birkenfeld]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?

Dave Opstad wrote:[color=blue]
> In this snippet:
>
> d = {'x': 1}
> value = d.get('x', bigscaryfunctio n())
>
> the bigscaryfunctio n is always called, even though 'x' is a valid key.
> Is there a "short-circuit" version of get that doesn't evaluate the
> second argument if the first is a valid key? For now I'll code around
> it, but this behavior surprised me a bit...[/color]

Well, if the dict only contains ints, here is a dirty hack (but don't
use it instead of the try/except approach):

class Littletinyproxy :
def __int__(self):
return bigscaryfunctio n()

d = dict(x=1)
value = int(d.get('x', Littletinyproxy ()))


Reinhold

[Steven Bethard]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?

Bill Mill wrote:[color=blue]
> On 9 Mar 2005 10:05:21 -0800, bearophileHUGS@ lycos.com
> <bearophileHUGS @lycos.com> wrote:
>[color=green]
>>Maybe this can help:
>>
>>value = d.get('x', lambda: bigscaryfunctio n())[/color]
>
>[color=green][color=darkred]
>>>>def test(): print 'gbye'[/color][/color]
> ...[color=green][color=darkred]
>>>>d = {}
>>>>z = d.get('x', lambda: test())
>>>>z[/color][/color]
> <function <lambda> at 0x008D6870>
>
> So this seems to be merely an obfuscation of:
>[color=green][color=darkred]
>>>>z = d.get('x', test)
>>>>z[/color][/color]
> <function test at 0x008D66B0>
>
> I just wanted to ask, am I missing something?[/color]

Nope that looks right. See "Overuse of lambda" in
http://www.python.org/moin/DubiousPython for discussion of exactly this
mistake.

STeVe

[Michael Spencer]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?

Dave Opstad wrote:[color=blue]
> In this snippet:
>
> d = {'x': 1}
> value = d.get('x', bigscaryfunctio n())
>
> the bigscaryfunctio n is always called, even though 'x' is a valid key.
> Is there a "short-circuit" version of get that doesn't evaluate the
> second argument if the first is a valid key? For now I'll code around
> it, but this behavior surprised me a bit...
>
> Dave[/color]
If (and this is a big if) you know that the dictionary contains no values that
evaluate to boolean false, then you can use the short-circuiting 'or' operator:
[color=blue][color=green][color=darkred]
>>> def bigscaryfunctio n():[/color][/color][/color]
... print "scary"
...[color=blue][color=green][color=darkred]
>>> d= globals()
>>> d.get("key") or bigscaryfunctio n()[/color][/color][/color]
scary[color=blue][color=green][color=darkred]
>>> d.get("__name__ ") or bigscaryfunctio n()[/color][/color][/color]
'LazyDictget'[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Alternatively, you can just write your own getter function:[color=blue][color=green][color=darkred]
>>> def lazyget(dict_, key, default):[/color][/color][/color]
... if key in dict_:
... return dict_[key]
... else:
... return default()
...[color=blue][color=green][color=darkred]
>>> lazyget(d,"key" ,bigscaryfuncti on)[/color][/color][/color]
scary[color=blue][color=green][color=darkred]
>>> lazyget(d,"__na me__",bigscaryf unction)[/color][/color][/color]
'LazyDictget'[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

The optimal choice of whether to "look before you leap" i.e., "if key in dict_"
or simply catch KeyError, depends on the ratio of hits to misses. Google will
turn up some experimental data on this, but, I seem to recall that if more than
10% attempts are misses, then LBYL is faster, because raising the exception is slow


Michael

[Jeff Epler]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?

untested

def my_getter(m, i, f):
try:
return m[i]
except (KeyError, IndexError):
return f()

my_getter(d, 'x', bigscaryfunctio n)
my_getter(d, 'y', lambda: scaryinlineexpr esion)

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[Skip Montanaro]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?


Dave> In this snippet:
Dave> d = {'x': 1}
Dave> value = d.get('x', bigscaryfunctio n())

Dave> the bigscaryfunctio n is always called, even though 'x' is a valid
Dave> key.

I sometimes use

value = d.get('x') or bsf()

Of course, this bsf() will get called if d['x'] evaluates to false, not just
None, so it won't work in all situations. It may help often enough to be
useful though.

Skip

[Terry Reedy]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?


"Skip Montanaro" <skip@pobox.com > wrote in message
news:16943.1847 5.568383.983546 @montanaro.dynd ns.org...[color=blue]
> value = d.get('x') or bsf()
>
> Of course, this bsf() will get called if d['x'] evaluates to false, not
> just
> None,[/color]

value = (d.get('x') is not None) or bsf() #??

tjr

[Steve Holden]

Re: Is there a short-circuiting dictionary &quot;get&qu ot; method?

Terry Reedy wrote:[color=blue]
> "Skip Montanaro" <skip@pobox.com > wrote in message
> news:16943.1847 5.568383.983546 @montanaro.dynd ns.org...
>[color=green]
>> value = d.get('x') or bsf()
>>
>>Of course, this bsf() will get called if d['x'] evaluates to false, not
>>just
>>None,[/color]
>
>
> value = (d.get('x') is not None) or bsf() #??
>[/color]
Unfortunately this will set value to True for all non-None values of
d['x']. Suppose d['x'] == 3:
[color=blue][color=green][color=darkred]
>>> 3 is not None[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

regards
Steve