How can a function access its own attributes?
is it possible to access the python function object attributes from within the function scope?
e.g. let's have
def f():
return SOMETHING
f._x = "foo"
f() # -> "foo"
now, what SOMETHING has to be, if we want to have the _x attribute content "foo" returned? if it's even possible (simply)
thanks
UPDATE:
i'd like the following work also:
g = f
del f
g() # -> "foo"
UPDATE 2:
Statement that it is not possible (if it is the case), and why, is more satisfying than providing a way how to fake it e.g. with a different object than a function
Solution
Make one of the function's default arguments be a reference to the function itself.
def f(self):
return self.x
f.func_defaults = (f,)
Example usage:
>>> f.x = 17
>>> b = f
>>> del f
>>> b()
17
Explanation
The original poster wanted a solution that does not require a global name lookup. The simple solution
def f():
return f.x
performs a lookup of the global variable f on each call, which does not meet the requirements. If f is deleted, then the function fails. The more complicated inspect proposal fails in the same way.
What we want is to perform early binding and store the bound reference within the object itself. The following is conceptually what we are doing:
def f(self=f):
return self.x
In the above, self is a local variable, so no global lookup is performed. However, we can't write the code as-is, because f is not yet defined when we try to bind the default value of self to it. Instead, we set the default value after f is defined.
Decorator
Here's a simple decorator to do this for you. Note that the self argument must come last, unlike methods, where self comes first. This also means that you must give a default value if any of your other arguments take a default value.
def self_reference(f):
f.func_defaults = f.func_defaults[:-1] + (f,)
return f
@self_reference
def foo(verb, adverb='swiftly', self=None):
return '%s %s %s' % (self.subject, verb, adverb)
Example:
>>> foo.subject = 'Fred'
>>> bar = foo
>>> del foo
>>> bar('runs')
'Fred runs swiftly'
You could just use a class to do this
>>> class F(object):
... def __call__(self, *args, **kw):
... return self._x
...
>>> f=F()
>>> f._x = "foo"
>>> f()
'foo'
>>> g=f
>>> del f
>>> g()
'foo'
Well, let's look at what function is:
>>> def foo():
... return x
...
>>> foo.x = 777
>>> foo.x
777
>>> foo()
Traceback (most recent call last):
File "<interactive input>", line 1, in <module>
File "<interactive input>", line 2, in foo
NameError: global name 'x' is not defined
>>> dir(foo)
['__call__', '__class__', '__delattr__', '__dict__', '__doc__', '__get__',
'__getattribute__', '__hash__', '__init__', '__module__', '__name__', '__new__',
'__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__',
'func_closure', 'func_code', 'func_defaults', 'func_dict', 'func_doc',
'func_globals', 'func_name', 'x']
>>> getattr(foo, 'x')
777
Aha! So the attribute was added to the function object but it won't see it because it is looking for global x instead.
We can try to grab the frame of the function execution and try to look what's there (essentially what Anthony Kong suggested but w/o inspect module):
>>> def foo():
... import sys
... return sys._getframe()
...
>>> fr = foo()
>>> dir(fr)
['__class__', '__delattr__', '__doc__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__str__', 'f_back', 'f_builtins', 'f_code', 'f_exc_traceback', 'f_exc_type', 'f_exc_value', 'f_globals', 'f_lasti', 'f_lineno', 'f_locals', 'f_restricted', 'f_trace']
>>> fr.f_locals
{'sys': <module 'sys' (built-in)>}
>>> fr.f_code
<code object foo at 01753020, file "<interactive input>", line 1>
>>> fr.f_code.co_code
'd\x01\x00d\x00\x00k\x00\x00}\x00\x00|\x00\x00i\x01\x00\x83\x00\x00S'
>>> fr.f_code.co_name
'foo'
Aha! So maybe we can get the name of the function from the name of the code block and then look in round-about way for the attribute? Sure enough:
>>> getattr(fr.f_globals[fr.f_code.co_name], 'x')
777
>>> fr.f_globals[fr.f_code.co_name].x
777
>>> def foo():
... import sys
... frm = sys._getframe()
... return frm.f_globals[frm.f_code.co_name].x
...
>>> foo.x=777
>>> foo()
777
That's great! But would it stand the renaming and deletion of original function?
>>> g = foo
>>> g.func_name
'foo'
>>> g.func_code.co_name
'foo'
Ah, very doubtful. The function object and its code object still insist they are called foo. Sure enough, here is where it breaks:
>>> g.x
777
>>> g.x=888
>>> foo.x
888
>>> g()
888
>>> del foo
>>> g()
Traceback (most recent call last):
File "<interactive input>", line 1, in <module>
File "<interactive input>", line 4, in foo
KeyError: 'foo'
Dang! So in general it can't be done through introspection via the execution frames. The problems seems to be that there is a difference between function object and code object - code objects are what is executed and is just one attribute func_code of the function-object and as such has no access to the func_dict attribute, where our attribute x is:
>>> g
<function foo at 0x0173AE30>
>>> type(g)
<type 'function'>
>>> g.func_code
<code object foo at 017532F0, file "<interactive input>", line 1>
>>> type(g.func_code)
<type 'code'>
>>> g.func_dict
{'x': 888}
There is of course other chicanery you can do so that it seems as function - in particular the trick with class definition... but that is not a function per se. It all depends on what do you really need to do with that.
As a workaround you could use a factory function to fix your scope:
def factory():
def inner():
print inner.x
return inner
>>> foo=factory()
>>> foo.x=11
>>> foo()
11
>>> bar = foo
>>> del foo
>>> bar()
11
I doubt this is the best way to accomplish this, but you can access the attributes by using the method's name within the method:
>>> def foo():
... print foo.x
...
>>> foo()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in foo
AttributeError: 'function' object has no attribute 'x'
>>> foo.x = 5
>>> foo()
5