Somthing strange with list as default argument

[Julien Sagnard]

Hi all,

When passing a list as default argument for a function, only one list is
created for all calls.
exemple:

>>> def f(a=[]):

.... a.append("x")
.... print a
....

>>> f() ['x']
>>> f() ['x', 'x']
>>> f() ['x', 'x', 'x']
>>> f() ['x', 'x', 'x', 'x']
>>>

I'm not found any reference of this. Is it a bug ?
I'm working on Windows 2000 with python 2.3.4

 

[Alex Martelli]

Hi all,

When passing a list as default argument for a function, only one list is
created for all calls.
Right.

I'm not found any reference of this. Is it a bug ?

No, you just didn't look in the right places, for example in the
Tutorial which is the recommended first reading for new Python users.

Under 4.7.1 the Tutorial says:

"""
Important warning: The default value is evaluated only once. This makes
a difference when the default is a mutable object such as a list,
dictionary, or instances of most classes.
"""

The 'Important warning' is in bold, too...


Alex

 

[Duncan Booth]

When passing a list as default argument for a function, only one list is
created for all calls.
exemple:

I'm not found any reference of this. Is it a bug ?
I'm working on Windows 2000 with python 2.3.4

You didn't search hard enough (although its not that easy a thing to search
for if you don't know the answer).

Please read the FAQ:

http://www.python.org/doc/faq/general.html#why-are-default-values-shared-between-objects

 

[Sam Holden]

Hi all,

When passing a list as default argument for a function, only one list is
created for all calls.
exemple:

def f(a=[]):

... a.append("x")
... print a
...

f() ['x']
f() ['x', 'x']
f() ['x', 'x', 'x']
f() ['x', 'x', 'x', 'x']

I'm not found any reference of this. Is it a bug ?

http://docs.python.org/tut/node6.html#SECTION006710000000000000000
(see the bold "important warning")

http://docs.python.org/ref/function.html
(see the paragraph with the bold text)

http://www.python.org/doc/faq/general.html
#why-are-default-values-shared-between-objects
(see the whole of 1.4.21 - link should be all one string with no new line...)

The tutorial, the reference docs, and the FAQ all mention this. There
isn't anywhere left to reference it.

 

[Julien Sagnard]

Thank you