Python function attributes - uses and abuses

eliben_8206 0
Viewed 160645

Not many are aware of this feature, but Python's functions (and methods) can have attributes. Behold:

>>> def foo(x):
...     pass
...     
>>> foo.score = 10
>>> 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', 'score']
>>> foo.score
10
>>> foo.score += 1
>>> foo.score
11

What are the possible uses and abuses of this feature in Python ? One good use I'm aware of is PLY's usage of the docstring to associate a syntax rule with a method. But what about custom attributes ? Are there good reasons to use them ?

function attributes python
8 Answers

I typically use function attributes as storage for annotations. Suppose I want to write, in the style of C# (indicating that a certain method should be part of the web service interface)

class Foo(WebService):
    @webmethod
    def bar(self, arg1, arg2):
         ...

then I can define

def webmethod(func):
    func.is_webmethod = True
    return func

Then, when a webservice call arrives, I look up the method, check whether the underlying function has the is_webmethod attribute (the actual value is irrelevant), and refuse the service if the method is absent or not meant to be called over the web.

user_330060

I've used them as static variables for a function. For example, given the following C code:

int fn(int i)
{
    static f = 1;
    f += i;
    return f;
}

I can implement the function similarly in Python:

def fn(i):
    fn.f += i
    return fn.f
fn.f = 1

This would definitely fall into the "abuses" end of the spectrum.

user_288040

You can do objects the JavaScript way... It makes no sense but it works ;)

>>> def FakeObject():
...   def test():
...     print "foo"
...   FakeObject.test = test
...   return FakeObject
>>> x = FakeObject()
>>> x.test()
foo
Marc_4078800

Function attributes can be used to write light-weight closures that wrap code and associated data together:

#!/usr/bin/env python

SW_DELTA = 0
SW_MARK  = 1
SW_BASE  = 2

def stopwatch():
   import time

   def _sw( action = SW_DELTA ):

      if action == SW_DELTA:
         return time.time() - _sw._time

      elif action == SW_MARK:
         _sw._time = time.time()
         return _sw._time

      elif action == SW_BASE:
         return _sw._time

      else:
         raise NotImplementedError

   _sw._time = time.time() # time of creation

   return _sw

# test code
sw=stopwatch()
sw2=stopwatch()
import os
os.system("sleep 1")
print sw() # defaults to "SW_DELTA"
sw( SW_MARK )
os.system("sleep 2")
print sw()
print sw2()

1.00934004784

2.00644397736

3.01593494415

Kevin_Little_140280

Sometimes I use an attribute of a function for caching already computed values. You can also have a generic decorator that generalizes this approach. Be aware of concurrency issues and side effects of such functions!

Admin1

I was always of the assumption that the only reason this was possible was so there was a logical place to put a doc-string or other such stuff. I know if I used it for any production code it'd confuse most who read it.

user_507460
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