Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
On Jan 15, 12:15 am, Paul Rubin http://[EMAIL PROTECTED] wrote:
Steven D'Aprano [EMAIL PROTECTED] writes:
map = {'a': Aclass, 'b': Bclass, 'c': Cclass}
class_ = map.get(astring, default=Zclass)
The result I want is the class, not the result of calling the class
(which would be an instance). If I wanted the other semantics, I'd be
using defaultdict instead.
I used default as a keyward arg name indicating the presence of
a callable. I probably should have called it defaultfunc or something.
x = d.get('a', f) # -- default value is f
x = d.get('a', defaultfunc=f) # -- default value is result of f() .
Nice idea, but if I want args I need to write it like that:
x=d.get('a', defaultfunc=f, funcargs=(1,2,3))
instead of d['a', f(1,2,3)]
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Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
On Jan 14, 8:07 pm, aspineux [EMAIL PROTECTED] wrote:
On Jan 14, 7:49 pm, Chris Mellon [EMAIL PROTECTED] wrote:
On Jan 14, 2008 12:39 PM, aspineux [EMAIL PROTECTED] wrote:
This append in both case
dict(a=1).get('a', f())
dict(a=1).setdefault('a', f())
This should be nice if f() was called only if required.
Think about the change to Python semantics that would be required for
this to be true, and then use collections.defaultdict instead.
Yes, I missed 'get' and 'setdefault' are functions :-)
Then why not some new semantic
d.get('a', f()) -- d['a', f()]
d.setdefault('a', f()) -- d['a'=f()]
Is is a good idea enough to change the python semantic ?
Or simply is it a good idea ?
Thanks for all your answers.
Anyway these notations are very compact,
don't require the definition of a specific function,
and work with old style/or already existing dictionary,
dictionary you didn't created yourself.
While the use of defaultdict require the definition of such a function
and to control the creation of the dictionary.
For me the best alternative that match the requirement above is
the one provided by Paul Rubin.
Regards.
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Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
aspineux [EMAIL PROTECTED] writes:
Nice idea, but if I want args I need to write it like that:
x=d.get('a', defaultfunc=f, funcargs=(1,2,3))
Yeah, that looks good. The default arg to f should be the key being
looked up.
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short path evaluation, why is f() called here: dict(a=1).get('a', f())
This append in both case
dict(a=1).get('a', f())
dict(a=1).setdefault('a', f())
This should be nice if f() was called only if required.
Regards.
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Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
On Jan 14, 2008 1:39 PM, aspineux [EMAIL PROTECTED] wrote:
This append in both case
dict(a=1).get('a', f())
dict(a=1).setdefault('a', f())
This should be nice if f() was called only if required.
Shortcomings of those methods is probably why collections.defaultdict
is so popular.
def f():
...return 7
...
d = defaultdict(f, a=1)
d['a']
1
d['b']
7
get and setdefault aren't needed when using a default dict, and the
default factory is called only when needed.
--
Neil Cerutti [EMAIL PROTECTED]
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Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
On Jan 14, 2008 12:39 PM, aspineux [EMAIL PROTECTED] wrote:
This append in both case
dict(a=1).get('a', f())
dict(a=1).setdefault('a', f())
This should be nice if f() was called only if required.
Think about the change to Python semantics that would be required for
this to be true, and then use collections.defaultdict instead.
--
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Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
On Jan 14, 7:49 pm, Chris Mellon [EMAIL PROTECTED] wrote:
On Jan 14, 2008 12:39 PM, aspineux [EMAIL PROTECTED] wrote:
This append in both case
dict(a=1).get('a', f())
dict(a=1).setdefault('a', f())
This should be nice if f() was called only if required.
Think about the change to Python semantics that would be required for
this to be true, and then use collections.defaultdict instead.
Yes, I missed 'get' and 'setdefault' are functions :-)
Then why not some new semantic
d.get('a', f()) -- d['a', f()]
d.setdefault('a', f()) -- d['a'=f()]
Is is a good idea enough to change the python semantic ?
Or simply is it a good idea ?
--
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Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
aspineux [EMAIL PROTECTED] writes:
Yes, I missed 'get' and 'setdefault' are functions :-)
Then why not some new semantic
d.get('a', f()) -- d['a', f()]
d.setdefault('a', f()) -- d['a'=f()]
Is is a good idea enough to change the python semantic ?
Or simply is it a good idea ?
Changing python semantics for something like this is nuts. Allowing
passing a callable (sort of like re.sub allows) makes a certain
amount of sense:
d.get('a', default=f)
You can also write (python 2.5, untested):
d['a'] if 'a' in d else f()
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Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
On Mon, 14 Jan 2008 12:08:52 -0800, Paul Rubin wrote:
aspineux [EMAIL PROTECTED] writes:
Yes, I missed 'get' and 'setdefault' are functions :-) Then why not
some new semantic
d.get('a', f()) -- d['a', f()]
d.setdefault('a', f()) -- d['a'=f()]
Is is a good idea enough to change the python semantic ? Or simply is
it a good idea ?
Changing python semantics for something like this is nuts. Allowing
passing a callable (sort of like re.sub allows) makes a certain amount
of sense:
d.get('a', default=f)
But how can Python determine when you want the result to be *the
callable* and when you want it to be *the result of calling the
callable*?
Functions and other callables are first-class objects, and it is quite
reasonable to have something like this:
map = {'a': Aclass, 'b': Bclass, 'c': Cclass}
class_ = map.get(astring, default=Zclass)
The result I want is the class, not the result of calling the class
(which would be an instance). If I wanted the other semantics, I'd be
using defaultdict instead.
--
Steven
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Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
But how can Python determine when you want the result to be *the
callable* and when you want it to be *the result of calling the
callable*?
Functions and other callables are first-class objects, and it is quite
reasonable to have something like this:
map = {'a': Aclass, 'b': Bclass, 'c': Cclass}
class_ = map.get(astring, default=Zclass)
The result I want is the class, not the result of calling the class
(which would be an instance). If I wanted the other semantics, I'd be
using defaultdict instead.
For an example of the defaultdict usage without calling it the
first time:
from collections import defaultdict
def f():
print Doing some expensive calculation
return 42
d = defaultdict(f)
d['hello'] = 3.14159
print 'Hello:', d['hello']
print 'World:', d['world']
print 'World (again):', d['world']
This results in the expensive calculation only being executed
once and having the result stored in the defaultdict. This is a
good thing.
If you're doing as Steven suggests, you can pass and store
function objects or class objects with the same ease:
map = {'a': Aclass, 'b': Bclass, 'c': Cclass}
class_ = map.get(astring, default=Zclass)
Other than tromping on the map built-in, one can then
instantiate the given class with
my_instance = map.get(astring, default=Zclass)(params)
Perfect for the factory pattern if you groove on that sort of thing.
-tkc
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Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
Steven D'Aprano [EMAIL PROTECTED] writes:
map = {'a': Aclass, 'b': Bclass, 'c': Cclass}
class_ = map.get(astring, default=Zclass)
The result I want is the class, not the result of calling the class
(which would be an instance). If I wanted the other semantics, I'd be
using defaultdict instead.
I used default as a keyward arg name indicating the presence of
a callable. I probably should have called it defaultfunc or something.
x = d.get('a', f) # -- default value is f
x = d.get('a', defaultfunc=f) # -- default value is result of f() .
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Re: short path evaluation, why is f() called here: dict(a=1).get('a', f())
On Mon, 14 Jan 2008 15:15:28 -0800, Paul Rubin wrote:
Steven D'Aprano [EMAIL PROTECTED] writes:
map = {'a': Aclass, 'b': Bclass, 'c': Cclass} class_ = map.get(astring,
default=Zclass)
The result I want is the class, not the result of calling the class
(which would be an instance). If I wanted the other semantics, I'd be
using defaultdict instead.
I used default as a keyward arg name indicating the presence of a
callable. I probably should have called it defaultfunc or something.
x = d.get('a', f) # -- default value is f x = d.get('a',
defaultfunc=f) # -- default value is result of f() .
So you're talking about proposed *added* behaviour, rather than
*replacing* the current behaviour?
Sorry if I misunderstood you in the first place.
--
Steven
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