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To: python-list@python.org
From: "Frank Millman" <frank@chagford.com>
Subject: Re: Argument Presence Checking via Identity or Boolean Operation?
Date: Thu, 4 Jun 2015 10:34:28 +0200
References: <CAAKgPaGY_4yB7z=R-UO=s5TEWwEXZsE1nSEXw00sQPMfH_Xt2g@mail.gmail.com> <mkp10p$n0l$1@ger.gmane.org>
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"Peter Otten" <__peter__@web.de> wrote in message 
news:mkp10p$n0l$1@ger.gmane.org...
> Russell Brennan wrote:
>
>> I'm going to x-post this to stackoverflow but...
>>
>> When checking a method's arguments to see whether they were set, is it
>> pythonic to do an identity check:
>>
>> def doThis(arg1, arg2=None):
>>   if arg2 is None:
>>     arg2 = myClass()
>>
>>
>> Or is it proper form to use a short-circuiting boolean:
>>
>> def doThis(arg1, arg2=None):
>>     arg2 = arg2 or myClass()
>
> When I read this I always have to stop and consider whether there are 
> valid
> falsey arguments that could be passed as arg2.
>
> An obvious example is
>
> def do_this(arg=None):
>    arg = arg or []
>    arg.append(42)
>
> a = []
> do_this(a)
> do_this(a)
> print(a) # expected: [42, 42] actual output: []
>
> Choosing the first approach is not just a matter of style, it avoids a
> source of subtle bugs.
>

I have a slight variation in that I want to keep a reference to the 
argument -

def __init__(self, arg=None):
    self.arg = arg or []

Based on your comment, I have changed it to -

def __init__(self, arg=None):
    self.arg = [] if arg is None else arg

Does this pass your 'stop and consider' test?

Frank Millman