[issue4331] Can't use _functools.partial() created function as method

[R. David Murray]

R. David Murray added the comment:

See also issue 11470.

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[issue4331] Can't use _functools.partial() created function as method

[Ulrich Eckhardt]

Ulrich Eckhardt added the comment:

There is at least one thing that is missing in the patch, it lacks the 
necessary tests. The partialbug.py demonstrates the issue, it could be used as 
a base. However, even then, there is still one thing that is problematic: The 
fact that partial() returns something that behaves like a static method is 
documented and changing that is not backward compatible.

I still think that something like this should become part of Python though. 
Jack Diederich argues that you can use lambda to achieve the same, but that is 
not always true. If you want to bind an argument to the current value of a 
variable instead of a constant, lambda fails. You need the closure created by a 
function call to bind those variables inside a local function. Having a 
dedicated function for that is IMHO preferable to people copying the 
Python-only equivalent of partial() to achieve the same effect or even 
inventing their own.

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[issue4331] Can't use _functools.partial() created function as method

[Ramchandra Apte]

Changes by Ramchandra Apte :


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versions: +Python 3.3, Python 3.4 -Python 3.0

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[issue4331] Can't use _functools.partial() created function as method

[Matt Joiner]

Matt Joiner added the comment:

What's preventing this from being committed and closed?

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[issue4331] Can't use _functools.partial() created function as method

[Ulrich Eckhardt]

Ulrich Eckhardt added the comment:

Just for the record, the behaviour is documented, unfortunately in the very 
last line of the functools documentation: "Also, partial objects defined in 
classes behave like static methods and do not transform into bound methods 
during instance attribute look-up."

Concerning how exactly they should behave during that lookup, I'd use the least 
surprising variant, namely that they are not treated differently from other 
functions: The first parameter is implicitly "self".

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nosy: +eckhardt

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[issue4331] Can't use _functools.partial() created function as method

[Matt Joiner]

Matt Joiner  added the comment:

I've attached a patch that implements the descriptor protocol for 
functools.partial with minimum changes.

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Added file: http://bugs.python.org/file25016/functools.partial-descrget.patch

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[issue4331] Can't use _functools.partial() created function as method

[Matt Joiner]

Changes by Matt Joiner :


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nosy: +anacrolix

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[issue4331] Can't use _functools.partial() created function as method

[Jesús Cea Avión]

Changes by Jesús Cea Avión :


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[issue4331] Can't use _functools.partial() created function as method

[Jack Diederich]

Jack Diederich  added the comment:

correction:
  run = partial(1)
should have been
  run = partial(show_funcs, 1)

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[issue4331] Can't use _functools.partial() created function as method

[Jack Diederich]

Jack Diederich  added the comment:

We talked about it at sprints and the semantics are ambiguous and there are 
alternatives.

Ambiguous:
  def show_funcs(*args): print(args)
  class A():
run = partial(1)
  ob = A()
  ob.run(2,3)
Should this print (self, 1, 2, 3) or (1, self, 2, 3)?  And what about
  partial(ob.run, 2)(3)

Alternatives: partial is a convenience function not an optimization (it doesn't 
offer a speedup.  So you can write a lambda or named function that has the 
exact semantics you want without suffering a speed penalty.

So unless there are a lot of good use cases with obvious behavior, we should 
refuse the temptation to guess and leave partial as-is.

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[issue4331] Can't use _functools.partial() created function as method

[R. David Murray]

R. David Murray  added the comment:

I would expect the second and would view the first as a bug.

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nosy: +r.david.murray

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[issue4331] Can't use _functools.partial() created function as method

[Jack Diederich]

Jack Diederich  added the comment:

I'm having some trouble wrapping my head around this one.  It isn't obvious to 
me that
my_method(*args):
  print(args)
class A():
  meth = partial(my_method, 'argA')
ob = A()
ob.meth('argB')

should print (, 'argA', 'argB') and not
('argA', , 'argB')

The patch seems to prefer the first form but if you are using a partial 
shouldn't you expect 'argA' to always be the first argument to the partial-ized 
function?

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[issue4331] Can't use _functools.partial() created function as method

[Alexander Belopolsky]

Alexander Belopolsky  added the comment:

Please see issue7830 for a related patch.

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[issue4331] Can't use _functools.partial() created function as method

[Alexander Belopolsky]

Alexander Belopolsky  added the comment:

Christophe,

It looks like your patch goes out of its way to avoid creating nested partials. 
 This is a worthwhile goal and I think it should be done in partial_new so that 
partial(partial(f, x), y) returns partial(f, x, y).

If fact, I was surprised to learn that current partial implementation does not 
behave this way:

>>> partial(partial(f, 1), 2).func


Does anyone know the reason for the current behavior?  It is possible that I am 
missing some subtlety related to keyword arguments.

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nosy: +Alexander.Belopolsky

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[issue4331] Can't use _functools.partial() created function as method

[Christophe Simonis]

Christophe Simonis  added the comment:

I followed the advice of Raymond and implement a descriptor on partial.

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keywords: +patch
Added file: http://bugs.python.org/file15800/issue4331.patch

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[issue4331] Can't use _functools.partial() created function as method

[Christophe Simonis]

Changes by Christophe Simonis :


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nosy: +Christophe Simonis

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[issue4331] Can't use _functools.partial() created function as method

[Alexander Belopolsky]

Changes by Alexander Belopolsky <[EMAIL PROTECTED]>:


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nosy: +belopolsky

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[issue4331] Can't use _functools.partial() created function as method

[Raymond Hettinger]

Raymond Hettinger <[EMAIL PROTECTED]> added the comment:

Reclassifying as a feature request.
A descriptor could be added to partial()
so that it too would have automatic
method binding just like pure python functions.

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nosy: +rhettinger
type: behavior -> feature request
versions: +Python 2.7, Python 3.0 -Python 2.5, Python 2.6

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[issue4331] Can't use _functools.partial() created function as method

[Calvin Spealman]

Calvin Spealman <[EMAIL PROTECTED]> added the comment:

I don't think this is any kind of bug, it is simply a product of only 
function objects being decorated automatically as methods. Your python 
version works because it is, in fact, a function. _functools.partial 
objects are not functions, but simply callable objects.

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nosy: +ironfroggy

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[issue4331] Can't use _functools.partial() created function as method

[scott sadler]

scott sadler <[EMAIL PROTECTED]> added the comment:

A short update, I believe that the reason that it was working in one
instance was because of some abstractions by a base class (Django model,
get_absolute_url).

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[issue4331] Can't use _functools.partial() created function as method

[scott sadler]

New submission from scott sadler <[EMAIL PROTECTED]>:

Calling a function created by _functools.partial as a method raises an
exception:

"TypeError: method_new() takes exactly n non-keyword arguments (0 given)"

Where method_new is the function passed into partial() and n is the
number of arguments it expects.

This does not happen when using a python version of partial().

Strangely, in the circumstance that I originally encountered the bug,
there was one instance that I was doing this and it _DID WORK_. The
function being passed into partial() was the same as in the place where
it was failing. The only significant difference that I could see was
that the input function to partial() was being imported, rather than
being defined in the same namespace as it was used I was unable to
reproduce it in my test case (attatched).

Tested on 2.6 and 2.5.2

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components: Extension Modules
files: partialbug.py
messages: 75928
nosy: ssadler
severity: normal
status: open
title: Can't use _functools.partial() created function as method
type: behavior
versions: Python 2.5, Python 2.6
Added file: http://bugs.python.org/file12017/partialbug.py

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