NEWBIE: lists as function arguments

[Joe Poniatowski]

From what I've read in the tutorial and FAQ, I should be able to pass a
mutable object like a list as a function argument, and have the caller see
changes made to the list by the function. I must be doing something wrong -
the function in this case assigns items to the list, but in the main module
it's still empty:

Python 2.3.2 (#1, Oct 23 2003, 11:10:37)
[GCC 2.96 20000731 (Red Hat Linux 7.1 2.96-81)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

ls = []
def f1(l2=[]):

.... l2 = ['word1','word2']
.... print l2
....

f1(ls) ['word1', 'word2']
ls []

However, I can return the list and it works OK:

ls = []
def f1(l2=[]):

.... l2 = ['word1','word2']
.... print l2
.... return l2
....

ls = f1(ls) ['word1', 'word2']
ls

['word1', 'word2']

Am I missing something or just interpreting the documentation wrong?
Jp

 

[Peter Otten]

From what I've read in the tutorial and FAQ, I should be able to pass a
mutable object like a list as a function argument, and have the caller see
changes made to the list by the function. I must be doing something wrong
- the function in this case assigns items to the list, but in the main
module it's still empty:

Python 2.3.2 (#1, Oct 23 2003, 11:10:37)
[GCC 2.96 20000731 (Red Hat Linux 7.1 2.96-81)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

ls = []
def f1(l2=[]):

... l2 = ['word1','word2']

You are rebinding the local variable l2 to another list here.
To change the list given as the argument, do l2.append("word3")
or l2[:] = ["word1", "word2"], e. g.:
.... alist[:] = ['word1', 'word2']
....

l1 = ['so', 'what']
f1(l1)
print l1 ['word1', 'word2']

By the way, it is usually a bad idea to use mutable objects as default
arguments:

def add(words, alist=[]):

.... alist.extend(words.split())
.... return alist
....

for w in ["x y", "a b"]:

.... print add(w)
....
['x', 'y']
['x', 'y', 'a', 'b']
I doubt this is what you expected. To default to an empty list, use

def add(words, alist=None):
if alist is None: alist = []
# ...

instead.

Peter

 

[Michael Geary]

Joe Poniatowski:

From what I've read in the tutorial and FAQ, I should be able to
pass a mutable object like a list as a function argument, and have
the caller see changes made to the list by the function. I must be
doing something wrong - the function in this case assigns items to
the list, but in the main module it's still empty:

Python 2.3.2 (#1, Oct 23 2003, 11:10:37)
[GCC 2.96 20000731 (Red Hat Linux 7.1 2.96-81)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

ls = []
def f1(l2=[]):

... l2 = ['word1','word2']
... print l2
...

f1(ls) ['word1', 'word2']
ls

[]

You may be thinking of a language that has variables and assignment
statements that change those variables. Python doesn't actually have either
of these. Instead, it has names that are bound to objects.

Your code "l2 = ['word1','word2']" *rebinds* the name "l2" to a new list
object. It doesn't affect the object that l2 was bound to before this
statement.

However, I can return the list and it works OK:

ls = []
def f1(l2=[]):

... l2 = ['word1','word2']
... print l2
... return l2
...

ls = f1(ls) ['word1', 'word2']
ls

['word1', 'word2']

This works because when you say "ls = f1(ls)" you're rebinding ls to the
object that f1() returns. You still haven't changed the list that ls was
originally bound to--ls now refers to the new object.

-Mike

 

[news.chartermi.net]

Thanks, everyone - I knew you folks could help me understand. This is still
me, JP, posting from another account.

From what I've read in the tutorial and FAQ, I should be able to pass a
mutable object like a list as a function argument, and have the caller see
changes made to the list by the function. I must be doing something wrong -
the function in this case assigns items to the list, but in the main module
it's still empty:

Python 2.3.2 (#1, Oct 23 2003, 11:10:37)
[GCC 2.96 20000731 (Red Hat Linux 7.1 2.96-81)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

ls = []
def f1(l2=[]):

... l2 = ['word1','word2']
... print l2
...

f1(ls) ['word1', 'word2']
ls []

However, I can return the list and it works OK:

ls = []
def f1(l2=[]):

... l2 = ['word1','word2']
... print l2
... return l2
...

ls = f1(ls) ['word1', 'word2']
ls

['word1', 'word2']

Am I missing something or just interpreting the documentation wrong?
Jp