Make staticmethod objects callable?

[Nicolas Fleury]

Hi everyone,
I was wondering if it would make sense to make staticmethod objects
callable, so that the following code would work:

class A:
@staticmethod
def foo(): pass
bar = foo()

I understand staticmethod objects don't need to implement __call__ for
their other use cases, but would it still make sense to implement
__call__ for that specific use case? Would it be error-prone in any way?

Thx and regards,
Nicolas

 

[Steven Bethard]

I was wondering if it would make sense to make staticmethod objects
callable, so that the following code would work:

class A:
@staticmethod
def foo(): pass
bar = foo()

Do you have a real-world use case? I pretty much never use
staticmethods (preferring instead to use module-level functions) so I'm
having a hard time coming up with some code where this would be really
useful. I'm also a little wary of breaking the parallel between
classmethod and staticmethod which are currently *just* descriptors
(without any other special functionality).

Maybe instead of making just staticmethods callable, both staticmethods
and classmethods could gain an 'im_func' attribute like bound and
unbound methods have?

STeVe

 

[Felipe Almeida Lessa]

Em Ter, 2006-02-28 às 15:17 -0500, Nicolas Fleury escreveu:

class A:
@staticmethod
def foo(): pass
bar = foo()

# Why not:

def foo(): pass

class A:
bar = foo()
foo = staticmethod(foo)

--
"Quem excele em empregar a força militar subjulga os exércitos dos
outros povos sem travar batalha, toma cidades fortificadas dos outros
povos sem as atacar e destrói os estados dos outros povos sem lutas
prolongadas. Deve lutar sob o Céu com o propósito primordial da
'preservação'. Desse modo suas armas não se embotarão, e os ganhos
poderão ser preservados. Essa é a estratégia para planejar ofensivas."

-- Sun Tzu, em "A arte da guerra"

 

[Nicolas Fleury]

Em Ter, 2006-02-28 às 15:17 -0500, Nicolas Fleury escreveu:



# Why not:

def foo(): pass

class A:
bar = foo()
foo = staticmethod(foo)

Well, you could even do:

class A:
def foo(): pass
bar = foo()
staticmethod(foo)

But it's a bit sad to have a special syntax for decorators then. It's a
usability problem, nothing to do with functionality. There's obvious
workarounds, but IMHO any user saying "it should just work" is at least
partly right.

Regards,
Nicolas

 

[Nicolas Fleury]

Do you have a real-world use case? I pretty much never use
staticmethods (preferring instead to use module-level functions) so I'm
having a hard time coming up with some code where this would be really
useful. I'm also a little wary of breaking the parallel between
classmethod and staticmethod which are currently *just* descriptors
(without any other special functionality).

Well, IMHO, if staticmethod is worth being standard, than calling these
static methods inside the class definition is worth being supported. To
be honest I don't use static methods like you, but users come see me
asking "Why?" and I find their code logically structured so I conclude
there's a usability problem here.

Maybe instead of making just staticmethods callable, both staticmethods
and classmethods could gain an 'im_func' attribute like bound and
unbound methods have?

I don't like it either (__get__ can be called anyway). My problem is
the expectation of the user. I wonder if it should just work. Adding a
few lines of code in staticmethod is worth it if it avoids all these
questions over and over again, no?

Regards,
Nicolas

 

[Murali]

You have a text-database, each record has some "header info" and some
data (text-blob). e.g.

--------------------
<HEADER>
name = "Tom"
phone = "312-996-XXXX"
</HEADER>
I last met tom in 1998. He was still single then.
blah
blah
<HEADER>
name = "John"
birthday = "1976-Mar-12"
</HEADER>
I need to ask him his email when I see him next.
------------------

I use this format for a file to keep track of tit bits of information.
Lets say the file has several hundred records. I know want to say
generate a birthday list of people and their birthdays. Ofcourse along
with that I also need the "text-blob" (because I dont want to send a
birthday card to a person I dont like). In order to do this I execute a
script

../filter --input=database.txt --condn='similar(name,"tom")'.

The way it is implemented is simple. Have a class which has dict as its
base class. For each record the text between <hEADER> and </HEADER> is
executed with the class instance as "locals()". Now that I have a list
of class instances, I just exec the condition and those instances where
it evaluates True comprise the output text file.

To make the user, not have to know too much python, one would like to
define "functions" which can be used. For eg. similar would have the
following definition

@staticmethod
def similar(regexp,str):
return re.match("(?i)^.*%s.*$" % regexp, str) != None

This way the "locals()" dictionary in the exec'ed environment has
access to the function similar (if similar was callable). At the same
time, I can enclose all these functions in their own name space (as
static methods of a class).

Right now, I declare all these "helper" functions in a different
module, and "attach" the "helper" functions as keys into the
dictionary. If only staticmethods were callable. For some reason (I
dont recall why) the idea of converting the staticmethod into a
callable still did not work, e.g.

class Callable:
def __init__(self,method):
self.__call__ = method

class Record(dict):

@staticmethod
def similar(regexp,string):
....

self['similar'] = Callable(similar)

The above did not work. The error message was still related to a
staticmethod not being a callable.

- Murali

 

[Steven D'Aprano]

Hi everyone,
I was wondering if it would make sense to make staticmethod objects
callable, so that the following code would work:

This is one of the more unintuitive areas of Python,
with side effects and different behaviour depending on
whether code is called inside or outside a class:
.... def speak():
.... return "dead parrot"
.... speak = staticmethod(speak)
.... def playdead():
.... return "still dead"
....<type 'instancemethod'>

So, based on this evidence, staticmethod() converts an
instance method into an ordinary function. Parrot.speak
certainly behaves like an ordinary function.
'dead parrot'


But now try to do the same outside of a class:
<type 'staticmethod'>

f is not a function?
<type 'instancemethod'>

So, based on this evidence, staticmethod() inside a
class definition converts instance methods to
functions. Outside a class definition, staticmethod()
does one of two things: it either converts an instance
method to a static method, or if the output is assigned
to a class attribute, it leaves it as an instance method.

Hmmm.

class A:
@staticmethod
def foo(): pass
bar = foo()

Here is a work around:
.... def foo(): return "foo foo foo"
.... foo = staticmethod(foo)
.... def __init__(self):
.... if not hasattr(self.__class__, "bar"):
.... self.__class__.bar = self.foo()
....

>>> dir(A) ['__doc__', '__init__', '__module__', 'foo']
>>> a = A()
>>> dir(A) ['__doc__', '__init__', '__module__', 'bar', 'foo']
>>> a.foo() 'foo foo foo'
>>> a.bar

'foo foo foo'


Here is a more interesting example:
.... def foo():
.... return lambda x: x+1
.... foo = staticmethod(foo)
.... def __init__(self):
.... if not hasattr(self.__class__, "bar"):
.... self.__class__.bar = \
.... staticmethod(self.foo())
....

>>> dir(B) ['__doc__', '__init__', '__module__', 'foo']
>>> b = B()
>>> dir(B) ['__doc__', '__init__', '__module__', 'bar', 'foo']
>>> b.foo()

>>> b.bar

>>> b.bar(3)

4


Hope this helps.

 

[Steven Bethard]

So, based on this evidence, staticmethod() inside a class definition
converts instance methods to functions. Outside a class definition,
staticmethod() does one of two things: it either converts an instance
method to a static method, or if the output is assigned to a class
attribute, it leaves it as an instance method.

This is exactly why I'm concerned about augmenting staticmethod's
behavior. When people run into this, it should be the opportunity to
explain to them how descriptors work. Descriptors are hugely important
in the new object system, and even if we hide them by giving
staticmethod a __call__, we'll still run into problems when people try
to do:

class C(object):
@classmethod
def foo(cls):
print cls
bar = foo(None)

Then, not only do we have to explain how descriptors work, but we also
have to explain why staticmethod has a __call__, and classmethod doesn't.

(For anyone else out there reading who doesn't already know this, Steven
D'Aprano's comments are easily explained by noting that the __get__
method of staticmethod objects returns functions, and classes always
call the __get__ methods of descriptors when those descriptors are class
attributes:
.... @staticmethod
.... def foo():
.... pass
.... print foo
....
.... def foo():
.... pass
....<function foo at 0x00E80130>

Yes, you have to explain descriptors, but at the point that you start
trying to do funny things with staticmethods and classmethods, I think
you need to start learning about them anyway.)

All that said, you should probably just submit a patch and see what
happens. I'll make a brief comment on it along the above lines, but
since I'm not a committer, it's not really worth your time to try to
convince me.

STeVe

 

[Nicolas Fleury]

...
Yes, you have to explain descriptors, but at the point that you start
trying to do funny things with staticmethods and classmethods, I think
you need to start learning about them anyway.)

That's all good points, but IMHO, descriptors are a much more advanced
Python feature than static methods, especially for programmers from
other backgrounds, like Java/C#/C++. We basically have the choice
between hiding something unnecessarily complex or force to understand a
useful feature

All that said, you should probably just submit a patch and see what
happens. I'll make a brief comment on it along the above lines, but
since I'm not a committer, it's not really worth your time to try to
convince me.

I might do it, but even if I'm not a commiter, I'll continue trying to
convince myself

Nicolas

 

[Terry Reedy]

... def speak():
... return "dead parrot"
... speak = staticmethod(speak)
... def playdead():
... return "still dead"
...
<type 'instancemethod'>

So, based on this evidence, staticmethod() converts an
instance method into an ordinary function. Parrot.speak
certainly behaves like an ordinary function.

Actually, staticmethod() prevents an ordinary function from being converted
to (wrapped as) a method upon access via the class.
def f(s): pass

type(C.__dict__['f'])

type(C.f)

<type 'instancemethod'>

As to the general topic: my impression is that staticmethod was a rather
optional addon with limited usage and that beginners hardly need to know
about it.

Terry Jan Reedy

 

[Steven D'Aprano]

(For anyone else out there reading who doesn't already know this, Steven
D'Aprano's comments are easily explained by noting that the __get__
method of staticmethod objects returns functions, and classes always
call the __get__ methods of descriptors when those descriptors are class
attributes:

....

A usage of the word "easily" that I am unfamiliar with.

*wink*

Why all the indirection to implement something which is, conceptually,
the same as an ordinary function?