Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!weretis.net!feeder1.news.weretis.net!feeder.erje.net!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
MIME-Version: 1.0
In-Reply-To: <1326880375.27881.YahooMailNeo@web29007.mail.ird.yahoo.com>
References: <1326880375.27881.YahooMailNeo@web29007.mail.ird.yahoo.com>
Date: Wed, 18 Jan 2012 17:09:55 +0000
Subject: Re: "+=" does not work correct all alogn
From: Arnaud Delobelle <arnodel@gmail.com>
To: Wilfried Falk <w_h_falk@yahoo.de>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Cc: "python-list@python.org" <python-list@python.org>
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.4843.1326906598.27778.python-list@python.org>
Lines: 80
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:19094

On 18 January 2012 09:52, Wilfried Falk <w_h_falk@yahoo.de> wrote:
> Hello Pythons,
>
> attached to this email is a pdf-file which shows, that=C2=A0 "+=3D" does =
not work
> well all along. Mybe somebody of you is able to explain my observations i=
n
> this respect. I will be glad about an answer.

I think you are more likely to get answers if you post your code in
the body of your message rather than attach it in a pdf, which is
quite an unusual thing to do!

"""
1.) For   identifier +=3D 1   you sometimes get    print(); identifier =3D
identifier + 1
What means that there is a prior  print().  I could not find out a
rule for when this happens --- but it
does. By replaceing  identifier +=3D 1   with   identifier =3D identifier
+ 1    the malfunction (print()) allways
disappears
"""

I can't comment on this as you don't provide any code.

"""
2.)  Another  "mystery"  is shown by the code below. There   _list +=3D
[something]    is not the same
as   _list =3D _list + [something].
def conc1(a, _list =3D []):
    _list =3D _list + [a]
    return _list
def conc2(a, _list =3D []):
    _list +=3D [a]
    return _list
# Main Program
for i in range(4):
    _list =3D conc1(i)
    print(_list)
print()
for i in range(4):
    _list =3D conc2(i)
    print(_list)
In the first case the result of   print(_list)   is:
[0]
[1]
[2]
[3]
In the second case the result of   print(_list)   is:
[0]
[0, 1]
[0, 1, 2]
[0, 1, 2, 3]
"""

This behaviour is not a bug, it is a consequence of two things:

1. The way mutable default arguments work in Python.  Your
misunderstanding is a very common one for Python beginners.  There's a
good explanation of the behaviour here:

    http://effbot.org/zone/default-values.htm

2. The difference between

    lst +=3D [a]

and

    lst =3D lst + [a]

The first one mutates the list object named 'lst' by appending 'a' at
its end, whereas the second one creates a new list made of the items
of lst with 'a' appended at the end.  So your function conc1 does not
mutate _list, whereas your function conc2 does.

HTH

--=20
Arnaud