Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!1.eu.feeder.erje.net!border1.nntp.ams1.giganews.com!nntp.giganews.com!newsfeed.xs4all.nl!newsfeed3a.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail Return-Path: X-Original-To: python-list@python.org Delivered-To: python-list@mail.python.org X-Spam-Status: OK 0.000 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'else:': 0.03; 'elif': 0.05; 'true,': 0.05; 'definitions.': 0.07; 'transform': 0.07; 'variables': 0.07; 'attributes': 0.09; 'happen.': 0.09; 'logic': 0.09; 'subject:instance': 0.09; 'subject:method': 0.09; 'def': 0.12; 'wrote': 0.14; '"called"': 0.16; '(0,': 0.16; '1):': 0.16; 'accepts': 0.16; 'attribute.': 0.16; 'branch:': 0.16; 'clause.': 0.16; 'cleaner': 0.16; 'csv': 0.16; 'first:': 0.16; 'func': 0.16; 'func_globals': 0.16; "function's": 0.16; 'globals': 0.16; 'globals.': 0.16; 'length,': 0.16; 'loop.': 0.16; 'preserve': 0.16; 'script.)': 0.16; 'subject:class': 0.16; 'subject:which': 0.16; 'user-defined': 0.16; 'applies': 0.16; 'appropriate': 0.16; '(not': 0.18; 'variable': 0.18; '8bit%:5': 0.22; '>>>': 0.22; 'input': 0.22; 'tests': 0.22; 'print': 0.22; '>>>': 0.24; 'instance,': 0.24; 'skip': 0.24; 'skip:% 10': 0.24; 'question': 0.24; 'sort': 0.25; 'script': 0.25; 'this:': 0.26; 'pass': 0.26; 'certain': 0.27; 'function': 0.29; 'raise': 0.29; 'thus': 0.29; 'message-id:@mail.gmail.com': 0.30; "i'm": 0.30; 'code': 0.31; 'about.': 0.31; 'long.': 0.31; 'post.': 0.31; 'produces': 0.31; 'file': 0.32; 'class': 0.32; 'skip:c 30': 0.32; 'stuff': 0.32; 'run': 0.32; 'another': 0.32; 'text': 0.33; 'plain': 0.33; 'skip:_ 10': 0.34; 'could': 0.34; "can't": 0.35; 'classes': 0.35; 'skip:s 30': 0.35; 'something': 0.35; 'definition': 0.35; 'but': 0.35; 'received:google.com': 0.35; 'add': 0.35; "didn't": 0.36; 'method': 0.36; 'useful': 0.36; 'subject:?': 0.36; 'example,': 0.37; 'skip:& 10': 0.38; 'requiring': 0.38; 'rich': 0.38; 'needed': 0.38; 'to:addr:python-list': 0.38; 'rather': 0.38; 'short': 0.38; 'skip:& 20': 0.39; 'subject:" ': 0.39; 'to:addr:python.org': 0.39; 'system.': 0.39; 'called': 0.40; '8bit%:6': 0.40; 'how': 0.40; 'even': 0.60; 'chain': 0.60; 'break': 0.61; 'challenge': 0.61; 'new': 0.61; 'simple': 0.61; 'first': 0.61; 'here:': 0.62; 'real': 0.63; 'skip:n 10': 0.64; 'decided': 0.64; 'more': 0.64; 'occur': 0.65; 'situation': 0.65; 'world': 0.66; 'gathering': 0.68; 'repeat': 0.74; 'potentially': 0.81; 'discovered': 0.83; 'end.': 0.84; 'nowadays,': 0.84; 'esteemed': 0.91; 'before?': 0.93 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:date:message-id:subject:from:to:content-type; bh=YbOyFxYwCVHQMQPqe8tub6dZ2pMGTToc2XzXKTUzOLo=; b=cJHKHgiv8BaJGuW1RImKhM7pAGS0h+XfblODCiH6OAguBkk2NAXEo4DfYDLfTvessx s+2Vt+jzz9opmolZu4fohof77e/92HplNvYDno3CSTNvBlM8sKp/vm69s7E86fGIx5Kg XPekeYcHXYRZ0KXiZqqM+vmJLaSnIGyiKwIIkgIE2Lga0TBTdN0viL4yiwbtLbywB3Gl pmUMlXUdWzdUMgT1PdcEzuJesJkQlUklf2vP1Ez2znw+odt83EkN/FN2mAuuXidwjG6n b9nvfKj6zEFbNRvscqCbnwC5sefRJfUcYg6rVfGeFUXVWbl2r957ylEEQYMcYuKH3rfP BOJA== MIME-Version: 1.0 X-Received: by 10.202.193.8 with SMTP id r8mr2725773oif.27.1431087863260; Fri, 08 May 2015 05:24:23 -0700 (PDT) Date: Fri, 8 May 2015 07:24:23 -0500 Subject: A __call__ "method" which is itself another callable class instance? From: Skip Montanaro To: Python Content-Type: multipart/alternative; boundary=001a113cd73cbb6d880515911b43 X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.20+ Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Newsgroups: comp.lang.python Message-ID: Lines: 202 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1431087871 news.xs4all.nl 2896 [2001:888:2000:d::a6]:37203 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:90266 --001a113cd73cbb6d880515911b43 Content-Type: text/plain; charset=UTF-8 This is just a pedantic post. It has no real useful application, just something to think about. I apologize that the setup is rather long. The stuff to think about is at the end. (I also apologize for using rich text/html. In this HTML world in which we live nowadays, I'm never certain that mail systems preserve leading whitespace, even in plain text mail.) I have a script which accepts a CSV file as input and the definition of a transform function. It applies the transform function to each row of the CSV file and produces a new CSV file on standard out. One of the things I discovered I needed in certain circumstances was to inject user-defined variables into the system. Since it's a single pass sort of thing (not long-lived), I decided to inject those variables into the function's globals. (I could have added them to builtins, I suppose, but it didn't occur to me when I first wrote the script.) The challenge was to find the appropriate globlas dictionary into which to inject these variable definitions. My code wound up looking like this: if inspect.ismethod(func): func_globals = func.im_func.func_globals elif inspect.isfunction(func): func_globals = func.func_globals elif inspect.isbuiltin(func): func_globals = sys.modules[func.__module__].__dict__ elif inspect.ismethoddescriptor(func): raise TypeError("Can't get globals of a method descriptor") What if func is actually an instance of a class with a __call__ method? So I added another branch: elif hasattr(func, "__call__"): func = func.__call__ But now what? This required me to to start the process over again. I also needed an else clause. Let's add that first: else: raise TypeError("Don't know how to find globals from %s objects" % type(func)) I need to repeat the tests in the case where I found a non-function-like object with a __call__ attribute. I decided to wrap this logic in a for loop which could only run twice: for _i in (0, 1): if inspect.ismethod(func): func_globals = func.im_func.func_globals elif inspect.isfunction(func): func_globals = func.func_globals elif inspect.isbuiltin(func): func_globals = sys.modules[func.__module__].__dict__ elif inspect.ismethoddescriptor(func): raise TypeError("Can't get globals of a method descriptor") elif hasattr(func, "__call__"): func = func.__call__ # Try again... continue else: raise TypeError("Don't know how to find globals from %s objects" % type(func)) break else: raise TypeError("Don't know how to find globals from %s objects" % type(func)) I thought about using while True, but thought, "nah, that's never going to happen." Then I wondered if it *could* happen. I constructed a short chain of __call__ attributes which might require the loop to repeat more than twice, but I can't think of a situation where it would actually be useful: class C1(object): def __call__(self): print "called" class C2(object): __call__ = C1() If you instantiate C2 and call the instance, it works as you would expect: >>> o = C2() >>> o() called >From that simple example, it's straightforward to create a chain of classes of any length, thus requiring my for loop to potentially be a while True loop. The question for the esteemed gathering here: have you ever encountered the need for this class-instance-as-a-dunder-call-method before? Perhaps in some sort of code generation situation? Or cleaner functions-with-attributes? Skip --001a113cd73cbb6d880515911b43 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
This is just a pedantic post. It has no real useful applic= ation, just something to think about. I apologize that the setup is rather = long. The stuff to think about is at the end. (I also apologize for using r= ich text/html. In this HTML world in which we live nowadays, I'm never = certain that mail systems preserve leading whitespace, even in plain text m= ail.)

I have a script which accepts a CSV file as input = and the definition of a transform function. It applies the transform functi= on to each row of the CSV file and produces a new CSV file on standard out.=

One of the things I discovered I needed in certain circ= umstances was to inject user-defined variables into the system. Since it= 9;s a single pass sort of thing (not long-lived), I decided to inject those= variables into the function's globals. (I could have added them to bui= ltins, I suppose, but it didn't occur to me when I first wrote the scri= pt.)

The challenge was to find the appropriate glo= blas dictionary into which to inject these variable definitions. My code wo= und up looking like this:
=C2=A0 =C2=A0 i= f inspect.ismethod(func):
func_globals =3D fun= c.im_func.func_globals
=C2=A0 =C2=A0 elif inspect.isfunction(func):
func_globals =3D func.func_globals
=C2=A0 =C2=A0 elif inspect.isbuiltin(func):
func_globals =3D sys.modules[func.__module__].__dict__<= /div>
=C2=A0 =C2=A0 elif inspect.is= methoddescriptor(func):
raise TypeError("= Can't get globals of a method descriptor")
=
What if func is a= ctually an instance of a class with a __call__ method? So I added another b= ranch:

=
=C2=A0 =C2=A0 elif hasattr(func, &= quot;__call__"):
= func =3D func.__call__

But now what? This required me to to s= tart the process over again. I also needed an else clause. Let's add th= at first:

=C2=A0 =C2=A0 else:
raise TypeError("Don't know how to find globals = from %s objects" %=C2=A0t= ype(func))

I need to repeat the tests= in the case where I found a non-function-like object with a __call__ attri= bute. I decided to wrap this logic in a for loop which could only run twice= :

=C2=A0 = =C2=A0 for _i in (0, 1):
=C2=A0 =C2=A0 =C2=A0 =C2=A0 if inspect.ismethod(func):
= =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 func_globals =3D func.im_func.func_globals
=C2=A0 =C2=A0 =C2=A0 =C2=A0 elif inspect.isfuncti= on(func):
=C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 func_globals =3D func.func_globals
=C2=A0 =C2=A0 =C2=A0 =C2=A0 el= if inspect.isbuiltin(func):
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 func_globals =3D sys.module= s[func.__module__].__dict__
=C2=A0 =C2=A0 =C2=A0 =C2=A0 elif inspect.ismethoddescriptor(func):
=C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 raise TypeError("Can't get globals of a method d= escriptor")
=C2= =A0 =C2=A0 =C2=A0 =C2=A0 elif hasattr(func, "__call__"):
=C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 func =3D func.__call__
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 # Try again...
=C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 continue
=C2=A0 =C2=A0 =C2=A0 =C2=A0 else:
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 raise TypeError(= "Don't know how to find globals from %s objects" % type(func)= )
=C2=A0 =C2=A0 =C2=A0= =C2=A0 break
=C2=A0 = =C2=A0 else:
=C2=A0 = =C2=A0 =C2=A0 =C2=A0 raise TypeError("Don't know how to find globa= ls from %s objects" % type(func))

I thought about using while True, but thought, "nah, that's neve= r going to happen." Then I wondered if it *could* happen. I constructe= d a short chain of __call__ attributes which might require the loop to repe= at more than twice, but I can't think of a situation where it would act= ually be useful:

=C2=A0 =C2=A0 class C1(object):
=C2=A0 =C2=A0 =C2=A0 =C2=A0def __call__(self):
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0print "called"

=C2= =A0 =C2=A0 class C2(object):
=C2=A0 =C2=A0 =C2=A0 =C2=A0__call__ =3D C1()
<= div>
If you instantiate C2 and call the instance, it works as= you would expect:

=C2=A0 =C2=A0 >>> o =3D C2()
=C2=A0 =C2=A0 >>> o()
<= font face=3D"monospace, monospace">=C2=A0 =C2=A0 called
<= div>
From that simple example, it's straightforward to cr= eate a chain of classes of any length, thus requiring my for loop to potent= ially be a while True loop. The question for the esteemed gathering here: h= ave you ever encountered the need for this class-instance-as-a-dunder-call-= method before? Perhaps in some sort of code generation situation? Or cleane= r functions-with-attributes?

Skip
=
--001a113cd73cbb6d880515911b43--