staticmethod makes my brain hurt

[Roy Smith]

When I run this (python 2.6.1):

class C:
@staticmethod
def foo():
pass

print "inside", foo, callable(foo)

print "outside", C.foo, callable(C.foo)

I get:

inside <staticmethod object at 0x421df0> False
outside <function foo at 0x41e6f0> True

I don't understand. Why is foo not callable inside of the class
definition? Where this comes up is that I'm trying to use a callable
default in mongoengine:

class User(Document):
@staticmethod
def _get_next_id():
[blah, blah, blah]
return id

user_id = IntField(required=True, default=_get_next_id)

The way mongoengine works is if callable(default) is true, it calls
default() to get the real value to use. At the point where the
IntField() call is made, _get_next_id is not callable, and eventually I
end up with:

ValidationError: <staticmethod object at 0x2a3c1a0> could not be
converted to int

 

[alex23]

class C:
    @staticmethod
    def foo():
        pass

    print "inside", foo, callable(foo)

print "outside", C.foo, callable(C.foo)

I don't understand.  Why is foo not callable inside of the class
definition?

Consider this:
False

A staticmethod by itself does not appear to be callable.

Your internal 'foo' is referring to the staticmethod wrapper. Your
external 'C.foo' refers to the result returned by the class
mechanism's __getattr__, which I'm guessing is munged into a callable
at that point.

Where this comes up is that I'm trying to use a callable

default in mongoengine:

class User(Document):
    @staticmethod
    def _get_next_id():
      [blah, blah, blah]
      return id

    user_id = IntField(required=True, default=_get_next_id)

What you're effectively trying to do is use a class before it has been
constructed to help construct itself.

Just define it as a helper function before the class declaration.

 

[Roy Smith]

What you're effectively trying to do is use a class before it has been
constructed to help construct itself.

Just define it as a helper function before the class declaration.

Yes, this is the workaround I ended up with.

 

[Ethan Furman]

class User(Document):
@staticmethod
def _get_next_id():
[blah, blah, blah]
return id

user_id = IntField(required=True, default=_get_next_id)

If you don't call '_get_next_id()' from any class methods (in other
words, if you don't need to ever say 'self._gen_next_id()') then you can
remove the '@staticmethod':

def _get_next_id():
[blah, blah, blah]
return id

user_id = IntField(required=True, default=_get_next_id)

If you do need to sometimes call it from a method then still leave off
the '@staticmethod', and give 'self' a default of 'None':

def _get_next_id(self=None):
[blah, blah, blah]
return id

user_id = IntField(required=True, default=_get_next_id)

~Ethan~

 

[Steven D'Aprano]

When I run this (python 2.6.1):

class C:
@staticmethod
def foo():
pass
print "inside", foo, callable(foo)

print "outside", C.foo, callable(C.foo)

I get:

inside <staticmethod object at 0x421df0> False
outside <function foo at 0x41e6f0> True

I don't understand. Why is foo not callable inside of the class
definition?

This has come up before.

http://bytes.com/topic/python/answers/34396-static-method-object-not-callable

http://bytes.com/topic/python/answers/462734-make-staticmethod-objects-callable


However, the fix is not as simple as merely making staticmethod objects
callable. This was discussed at the 2011 language summit:

http://www.boredomandlaziness.org/2011/03/python-language-summit-rough-notes.html

See also this thread:

http://mail.python.org/pipermail/python-dev/2011-March/109090.html

 

[Roy Smith]

This has come up before.

http://bytes.com/topic/python/answers/34396-static-method-object-not-callable

http://bytes.com/topic/python/answers/462734-make-staticmethod-objects-callabl
e


However, the fix is not as simple as merely making staticmethod objects
callable. This was discussed at the 2011 language summit:

http://www.boredomandlaziness.org/2011/03/python-language-summit-rough-notes.h
tml

See also this thread:

http://mail.python.org/pipermail/python-dev/2011-March/109090.html

Thanks for the links. It always makes me feel good when I get tripped
up by something complex and subtle. It almost makes up for all the
times when I feel like a dolt because I got tripped up by something
obvious and elementary...

 

[Devin Jeanpierre]

However, the fix is not as simple as merely making staticmethod objects

callable. This was discussed at the 2011 language summit:

http://www.boredomandlaziness.org/2011/03/python-language-summit-rough-notes.html

See also this thread:

http://mail.python.org/pipermail/python-dev/2011-March/109090.html

The notes didn't actually mention what was discussed, but re the
second thread, the issue was not changing staticmethod to be callable.
Making staticmethod callable is fine, but __get__ still has to return
the original function, not self.

The context of the second thread was that staticmethod was imagined as
one way to eliminate the difference between C and Python functions,
when added to a class. But this doesn't quite work, unless
staticmethod.__get__ returned self (which broke for related reasons).
If it returns the original function (as it does now) then the issue is
unresolved: C functions still behave differently from Python
functions, so you can't quite just hide them behind a staticmethod and
let everything remain the same. It doesn't eliminate the difference in
behavior in certain circumstances, e.g.:

class MyClass:
foo = staticmethod(foo)

class MyOtherClass:
foo = MyClass.foo

MyOtherClass().foo() # what happens if foo is builtin vs pure-Python?

In the context of this thread, though, just making it callable without
modifying __get__ is fine. Or at least, I don't see the issue.

Devin

 

[Dotan Cohen]

When I run this (python 2.6.1):

class C:
   @staticmethod
   def foo():
       pass

   print "inside", foo, callable(foo)

print "outside", C.foo, callable(C.foo)

I get:

inside <staticmethod object at 0x421df0> False
outside <function foo at 0x41e6f0> True

I don't understand.  Why is foo not callable inside of the class
definition?  Where this comes up is that I'm trying to use a callable
default in mongoengine:

class User(Document):
   @staticmethod
   def _get_next_id():
     [blah, blah, blah]
     return id

   user_id = IntField(required=True, default=_get_next_id)

The way mongoengine works is if callable(default) is true, it calls
default() to get the real value to use.  At the point where the
IntField() call is made, _get_next_id is not callable, and eventually I
end up with:

ValidationError: <staticmethod object at 0x2a3c1a0> could not be
converted to int

Try this (untested):

class C:
@staticmethod
def foo():
pass

print "inside", C.foo, callable(C.foo)

 

[Ian Kelly]

Try this (untested):

class C:
  @staticmethod
  def foo():
      pass

  print "inside", C.foo, callable(C.foo)

If you had tested this, you would have found that you get a NameError,
since C is not yet bound inside the class block where you define it.

 

[Thomas Rachel]

Am 17.11.2011 03:30 schrieb Roy Smith:

When I run this (python 2.6.1):

class C:
@staticmethod
def foo():
pass

print "inside", foo, callable(foo)

print "outside", C.foo, callable(C.foo)

I get:

inside<staticmethod object at 0x421df0> False
outside<function foo at 0x41e6f0> True

Right. The reason is that on an attribute access, you get a __get__ call
of the "real" attribute.

That is, if your class has a "regular" method, it is stored there as a
formal function. But if you access it (C.f), you get
C.__dict__["f"].__get__(None, C) (not sure about the arguments, but you
should get the idea).

A functions __get__ returns a unbound or bound method, depending on its
own arguments.

With a static method, you don't want this to happen. So you wrap your
"regular" function into a staticmethod object, which has a __get__()
method itself just returning the wrapped function object.

Look at this:
<function <lambda> at 0x00B43E30>


That's how things work.

If you want to get back the "real" function from a staticmethod, you
either call its __get__ with an arbitrary argument, or you do it the
clean way and do a

def deref(s):
class C(object): s=s
return s.s

and so do a

class User(Document):
@staticmethod
def _get_next_id():
[blah, blah, blah]
return id

user_id = IntField(required=True, default=deref(_get_next_id))


HTH,


Thomas

 

[Dotan Cohen]

If you had tested this, you would have found that you get a NameError,
since C is not yet bound inside the class block where you define it.

I hadn't tested, I'm at work far from Idle. Just shooting from the hip.

For that matter, though, this does work in Java (I'm pretty sure but
again, untested right now).

 

[Chris Angelico]

I'm at work far from Idle.

Taken out of context, I'm sure your boss is pleased.

ChrisA

 

[alex23]

If you do need to sometimes call it from a method then still leave off
the '@staticmethod', and give 'self' a default of 'None':

     def _get_next_id(self=None):
       [blah, blah, blah]
       return id

     user_id = IntField(required=True, default=_get_next_id)

And if the OP needs it to be a staticmethod as well, he can just wrap
the nested function:

gen_next_id = staticmethod(_gen_next_id)

I think I like this approach best. I'm annoyed that I forgot functions
declared in a class scope were callable within the definition