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Argument of the bool function

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  Argument of the bool function candide <candide@free.invalid> - 2011-04-08 18:26 +0200
    Re: Argument of the bool function Benjamin Kaplan <benjamin.kaplan@case.edu> - 2011-04-08 12:41 -0400
      Re: Argument of the bool function candide <candide@free.invalid> - 2011-04-10 14:54 +0200
        Re: Argument of the bool function Chris Angelico <rosuav@gmail.com> - 2011-04-10 23:02 +1000
          Re: Argument of the bool function Mel <mwilson@the-wire.com> - 2011-04-10 12:21 -0400
            Re: Argument of the bool function "Colin J. Williams" <cjw@ncf.ca> - 2011-04-10 13:51 -0400
            Re: Argument of the bool function Ben Finney <ben+python@benfinney.id.au> - 2011-04-11 09:46 +1000
            Re: Argument of the bool function Ethan Furman <ethan@stoneleaf.us> - 2011-04-11 16:00 -0700
            Re: Argument of the bool function Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-04-25 16:29 +0200
              Re: Argument of the bool function Chris Angelico <rosuav@gmail.com> - 2011-04-26 05:33 +1000
                Re: Argument of the bool function Paul Rubin <no.email@nospam.invalid> - 2011-04-25 16:26 -0700
                  Re: Argument of the bool function Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-04-26 02:38 +0000
              Re: Argument of the bool function Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-04-25 23:28 +0200
                Re: Argument of the bool function Ian Kelly <ian.g.kelly@gmail.com> - 2011-04-25 17:44 -0600
    Re: Argument of the bool function Mel <mwilson@the-wire.com> - 2011-04-08 16:42 +0000
    Re: Argument of the bool function Ian Kelly <ian.g.kelly@gmail.com> - 2011-04-08 10:43 -0600
      Re: Argument of the bool function candide <candide@free.invalid> - 2011-04-08 23:32 +0200
        Re: Argument of the bool function Ethan Furman <ethan@stoneleaf.us> - 2011-04-08 15:03 -0700
          Re: Argument of the bool function candide <candide@free.invalid> - 2011-04-09 00:59 +0200
            Re: Argument of the bool function Lie Ryan <lie.1296@gmail.com> - 2011-04-09 15:45 +1000
              Re: Argument of the bool function Grant Edwards <invalid@invalid.invalid> - 2011-04-10 03:35 +0000
                Re: Argument of the bool function rusi <rustompmody@gmail.com> - 2011-04-09 21:15 -0700
                  Re: Argument of the bool function Robert Kern <robert.kern@gmail.com> - 2011-04-10 01:22 -0500
            Re: Argument of the bool function Robert Kern <robert.kern@gmail.com> - 2011-04-09 18:22 -0500
              Re: Argument of the bool function candide <candide@free.invalid> - 2011-04-10 02:33 +0200
        Re: Argument of the bool function Ben Finney <ben+python@benfinney.id.au> - 2011-04-09 07:57 +1000

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#2855 — Argument of the bool function

About the standard function bool(), Python's official documentation 
tells us the following :

bool([x])
Convert a value to a Boolean, using the standard truth testing procedure.


In this context, what exactly a "value" is referring to ?


For instance,


 >>> x=42
 >>> bool(x=5)
True
 >>>


but _expression_ :

x=42


has no value.

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#2857

On Fri, Apr 8, 2011 at 12:26 PM, candide <candide@free.invalid> wrote:
> About the standard function bool(), Python's official documentation tells us
> the following :
>
> bool([x])
> Convert a value to a Boolean, using the standard truth testing procedure.
>
>
> In this context, what exactly a "value" is referring to ?
>
>
> For instance,
>
>
>>>> x=42
>>>> bool(x=5)
> True
>>>>
>
>
> but _expression_ :
>
> x=42
>
>
> has no value.
>

That's because bool(x=5) isn't doing what you think.

>>> bool(y=5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'y' is an invalid keyword argument for this function

bool(x=5) is just passing the value 5 as the argument "x" to the function.

"value" means just what you'd think- any constant or any value that's
been assigned to.

>
>
>
>
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>

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#2941

Le 08/04/2011 18:41, Benjamin Kaplan a écrit :

> bool(x=5) is just passing the value 5 as the argument "x" to the function.
>


Anyway, passing x as a keyword argument to the bool function appears to 
be very rare : i did a regexp search for about 30000 source-code Python 
files (among them official Python source-code, Django, Sphinx, Eric 
source-code and many more sources of valuable Python code) and I didn't 
find even one.

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#2942

On Sun, Apr 10, 2011 at 10:54 PM, candide <candide@free.invalid> wrote:
> Anyway, passing x as a keyword argument to the bool function appears to be
> very rare : i did a regexp search for about 30000 source-code Python files
> (among them official Python source-code, Django, Sphinx, Eric source-code
> and many more sources of valuable Python code) and I didn't find even one.

Who would use keyword arguments with a function that takes only one arg anyway?

ChrisA

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#2950

Chris Angelico wrote:

> Who would use keyword arguments with a function that takes only one arg
> anyway?

It's hard to imagine.  Maybe somebody trying to generalize function calls 
(trying to interpret some other language using a python program?)

# e.g. input winds up having the effect of ..
function = bool
name = 'x'
value = 'the well at the end of the world'
## ...
actions.append ((function, {name:value}))
## ...
for function, args in actions:
    results.append (function (**args))

Not something I, for one, do every day.  But regularity in a language is 
good when you can get it, especially for abstract things like that.

I can sort of guess that `dir` was perhaps coded in C for speed and doesn't 
spend time looking for complicated argument lists.

Python is a pragmatic language, so all the rules come pre-broken.


	Mel.

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#2955

On 10-Apr-11 12:21 PM, Mel wrote:
> Chris Angelico wrote:
>
>> Who would use keyword arguments with a function that takes only one arg
>> anyway?
>
> It's hard to imagine.  Maybe somebody trying to generalize function calls
> (trying to interpret some other language using a python program?)
>
> # e.g. input winds up having the effect of ..
> function = bool
> name = 'x'
> value = 'the well at the end of the world'
> ## ...
> actions.append ((function, {name:value}))
> ## ...
> for function, args in actions:
>      results.append (function (**args))
>
> Not something I, for one, do every day.  But regularity in a language is
> good when you can get it, especially for abstract things like that.
>
> I can sort of guess that `dir` was perhaps coded in C for speed and doesn't
> spend time looking for complicated argument lists.
>
> Python is a pragmatic language, so all the rules come pre-broken.
>
>
> 	Mel.
This thread has lasted 3 days so far.

I presume that it is agreed they the following is a satisfactory outcome:

*** Python 2.7.1 (r271:86832, Nov 27 2010, 18:30:46) [MSC v.1500 32 bit 
(Intel)] on win32. ***
 >>> bool(x=0)
False
 >>> bool(x=1)
True
 >>>

Colin W.

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#2961

Mel <mwilson@the-wire.com> writes:

> Python is a pragmatic language, so all the rules come pre-broken.

+1 QOTW

-- 
 \       “Science shows that belief in God is not only obsolete. It is |
  `\                        also incoherent.” —Victor J. Stenger, 2001 |
_o__)                                                                  |
Ben Finney

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#2998

Mel wrote:
> Python is a pragmatic language, so all the rules come pre-broken.

+1 QOTW

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#3982

Am 10.04.2011 18:21, schrieb Mel:
> Chris Angelico wrote:
>
>> Who would use keyword arguments with a function that takes only one arg
>> anyway?
>
> It's hard to imagine.  Maybe somebody trying to generalize function calls
> (trying to interpret some other language using a python program?)
>
> # e.g. input winds up having the effect of ..
> function = bool
> name = 'x'
> value = 'the well at the end of the world'
> ## ...
> actions.append ((function, {name:value}))
> ## ...
> for function, args in actions:
>      results.append (function (**args))

Wrong structure.

Better do

function = bool
value = 'the well at the end of the world'
## ...
actions.append((function, (value,), {}))
## ...
for function, args, kwargs in actions:
      results.append(function(*args, **kwargs))

or maybe even better (taking care for closures):

function = bool
value = 'the well at the end of the world'
## ...
actions.append(lambda val=value: function(val))
## ...
for function in actions:
      results.append(function())


>
> Not something I, for one, do every day.  But regularity in a language is
> good when you can get it, especially for abstract things like that.
>
> I can sort of guess that `dir` was perhaps coded in C for speed and doesn't
> spend time looking for complicated argument lists.
>
> Python is a pragmatic language, so all the rules come pre-broken.
>
>
> 	Mel.

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#3988

On Tue, Apr 26, 2011 at 12:29 AM, Thomas Rachel
<nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de>
wrote:
> for function in actions:
>     results.append(function())

Can this become:

results = [function() for function in actions]

Chris Angelico

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#4007

Chris Angelico <rosuav@gmail.com> writes:
> results = [function() for function in actions]

results = map(apply, actions)

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#4012

On Mon, 25 Apr 2011 16:26:37 -0700, Paul Rubin wrote:

> Chris Angelico <rosuav@gmail.com> writes:
>> results = [function() for function in actions]
> 
> results = map(apply, actions)

Sadly not in Python 3, where map is lazy and you need to add a call to 
list to make it equivalent to the list comp.



-- 
Steven

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#4002

Am 25.04.2011 16:29, schrieb Thomas Rachel:

> or maybe even better (taking care for closures):
>
> function = bool
> value = 'the well at the end of the world'
> ## ...
> actions.append(lambda val=value: function(val))
> ## ...
> for function in actions:
> results.append(function())

Or yet even better:

class Job(object):
     def __init__(self, target, *args, **kwargs):
         self.target = lambda: target(*args, **kwargs)
     def __call__(self):
         return self.target()

in order to do

actions.append(Job(function, val))
actions.append(Job(function, x=val))

and then (thanks, Chris...)

results = [function() for function in actions]


or maybe (additionally or alternatively)

class ActionQueue(list):
     def addJob(self, target, *args, **kwargs):
         self.append(lambda: target(*args, **kwargs))
     def __call__(self):
         if 0: # first thought
             for job in self:
                 yield job()
         else: # second thought - clean up...
             while self:
                 job = self.pop(0)
                 yield job()

with

actions = ActionQueue()

actions.addJob(function, val)
actions.addJob(function, x=val)

results = list(actions()) # for collecting them and having them at once
# with generator, all call results can as well be emitted as soon as 
they are available - depending what shall be done with the results


mmm... too much imagination I think... ;-)


Thomas

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#4008

On Mon, Apr 25, 2011 at 3:28 PM, Thomas Rachel
<nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de>
wrote:
> Am 25.04.2011 16:29, schrieb Thomas Rachel:
>
>> or maybe even better (taking care for closures):
>>
>> function = bool
>> value = 'the well at the end of the world'
>> ## ...
>> actions.append(lambda val=value: function(val))
>> ## ...
>> for function in actions:
>> results.append(function())
>
> Or yet even better:
>
> class Job(object):
>    def __init__(self, target, *args, **kwargs):
>        self.target = lambda: target(*args, **kwargs)
>    def __call__(self):
>        return self.target()
>
> in order to do
>
> actions.append(Job(function, val))
> actions.append(Job(function, x=val))

from functools import partial
actions.append(partial(function, val))
actions.append(partial(function, x=val))

Cheers,
Ian

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#2858

candide wrote:
> About the standard function bool(), Python's official documentation 
> tells us the following :
>
> bool([x])
> Convert a value to a Boolean, using the standard truth testing procedure.
>
> In this context, what exactly a "value" is referring to ?
>
> For instance,
>  >>> x=42
>  >>> bool(x=5)
> True
>  >>>

Cute.  What's happening here is that `x=5` isn't really an expression. 
It's passing a value to the named parameter `x`, specified in the
definition of `bool`.  Try it with something else:

Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> bool(y=5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'y' is an invalid keyword argument for this function



Mel.

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#2859

On Fri, Apr 8, 2011 at 10:26 AM, candide <candide@free.invalid> wrote:
>>>> x=42
>>>> bool(x=5)
> True
>>>>
>
>
> but _expression_ :
>
> x=42
>
>
> has no value.

"x=42" is an assignment statement, not an expression.
In "bool(x=5)", "x=5" is also not an expression.  It's passing the
expression "5" in as the parameter x, using a keyword argument.

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#2877

Le 08/04/2011 18:43, Ian Kelly a écrit :

> "x=42" is an assignment statement, not an expression.

Right, I was confounding with C ;)

In fact, respect to this question, the documentation makes things 
unambiguous :


-----------------
In contrast to many other languages, not all language constructs are 
expressions. There are also statements which cannot be used as 
expressions, such as print or if. Assignments are also statements, not 
expressions.
-----------------






> In "bool(x=5)", "x=5" is also not an expression.  It's passing the
> expression "5" in as the parameter x, using a keyword argument.


You are probably right but how do you deduce this brilliant 
interpretation from the wording given in the documentation ?

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#2879

candide wrote:
> Le 08/04/2011 18:43, Ian Kelly a écrit :
>> In "bool(x=5)", "x=5" is also not an expression.  It's passing the
>> expression "5" in as the parameter x, using a keyword argument. 
 >>
> You are probably right but how do you deduce this brilliant 
> interpretation from the wording given in the documentation ?

Look at your original post, which contains the excerpt from the docs 
that you put there:
 >
 > bool([x])
 > Convert a value to a Boolean, using the standard truth testing
 > procedure.
 >

As you can see, the parameter name is 'x'.

~Ethan~

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#2885

Le 09/04/2011 00:03, Ethan Furman a écrit :

>  > bool([x])
>  > Convert a value to a Boolean, using the standard truth testing
>  > procedure.
>  >
>
> As you can see, the parameter name is 'x'.


OK, your response is clarifying my point ;)


I didn't realize that in the bool([x]) syntax, identifier x refers to a 
"genuine" argument [I was considering x as referring to a "generic" 
object having a boolean value].


Nevertheless, compare with the definition the doc provides for the 
builtin function dir():

dir([object])
[definition omited, just observe the declaration syntax]

Now, lets make a try

 >>> dir(object="Explicit is better than implicit")
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: dir() takes no keyword arguments
 >>>

Not very meaningful, isn't it ?

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#2891

On 04/09/11 08:59, candide wrote:
> Le 09/04/2011 00:03, Ethan Furman a écrit :
> 
>>  > bool([x])
>>  > Convert a value to a Boolean, using the standard truth testing
>>  > procedure.
>>  >
>>
>> As you can see, the parameter name is 'x'.
> 
> 
> OK, your response is clarifying my point ;)
> 
> 
> I didn't realize that in the bool([x]) syntax, identifier x refers to a
> "genuine" argument [I was considering x as referring to a "generic"
> object having a boolean value].
> 
> 
> Nevertheless, compare with the definition the doc provides for the
> builtin function dir():
> 
> dir([object])
> [definition omited, just observe the declaration syntax]
> 
> Now, lets make a try
> 
>>>> dir(object="Explicit is better than implicit")
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: dir() takes no keyword arguments
>>>>
> 
> Not very meaningful, isn't it ?

The error says it unambiguously, dir() does not take *keyword*
arguments; instead dir() takes *positional* argument:

    dir("Explicit is better than implicit")

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