Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Dennis Lee Bieber <wlfraed@ix.netcom.com>
Newsgroups: comp.lang.python
Subject: Re: What is a function parameter =[] for?
Date: Thu, 19 Nov 2015 09:05:29 -0500
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On Thu, 19 Nov 2015 13:19:25 +0000, BartC <bc@freeuk.com> declaimed the
following:

>You presumably think differently because you have some inside knowledge 
>of how Python works, and know that that [] undergoes a one-time 
>assignment to a local, persistent 'default' variable where it's value 
>can indeed by changed. (Thanks to another Python feature where an 
>assignment is a very shallow copy of an object.) And it is that volatile 
>variable that is the actual default.
>
>But not everyone is going to know that.

	If one has not taken time to read the language reference manual and/or
tutorials, one should expect to be taken by surprise.

	This behavior has existed in Python from the beginning... That's over
20 years now. It is DOCUMENTED -- in documents that come with every
standard installation of the language, not something one needs to hunt
down. It should not be a surprise to anyone who has taken the time to read
and understand the documentation -- or, at least, has searched the
documentation once they have been surprised (I'll concede that one
read-through at the start may not be sufficient <G>; one may need to
encounter a behavior before the documentation for it makes sense).

Python 2.7 LRM section 7.6 Function Definitions:

"""
Default parameter values are evaluated when the function definition is
executed. This means that the expression is evaluated once, when the
function is defined, and that the same “pre-computed” value is used for
each call. This is especially important to understand when a default
parameter is a mutable object, such as a list or a dictionary: if the
function modifies the object (e.g. by appending an item to a list), the
default value is in effect modified. This is generally not what was
intended. A way around this is to use None as the default, and explicitly
test for it in the body of the function, e.g.:
"""
 (and the first sentence is even BOLDED)


	But in no way is this "inside knowledge". There is no cabal of folks
hoarding arcana from the rest of the public.
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
    wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/