a python pitfall

Date	2015-05-14 13:06 -0500
Subject	a python pitfall
From	Billy Earney <billy.earney@gmail.com>
Newsgroups	comp.lang.python
Message-ID	<mailman.11.1431626772.17265.python-list@python.org> (permalink)

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Hello friends:

I saw the following example at
http://nafiulis.me/potential-pythonic-pitfalls.html#using-mutable-default-arguments
and
did not believe the output produced and had to try it for myself....

def foo(a,b,c=[]):
    c.append(a)
    c.append(b)
    print(c)

foo(1,1)
foo(1,1)
foo(1,1)

produces:
[1, 1]
[1, 1, 1, 1]
[1, 1, 1, 1, 1, 1]

One would expect the following output:
[1, 1]
[1, 1]
[1, 1]

Doesn't this valid the zen of python: "Explicit is better than implicit."  ?

Thanks!
Billy

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a python pitfall Billy Earney <billy.earney@gmail.com> - 2015-05-14 13:06 -0500
  Re: a python pitfall Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-05-15 12:03 +1000

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