Namespace issues...

[Peter Otten]

cghost wrote:

> i am very confused...why does the following script modify the global list
> "l":
> 
> l=[]
> 
> def x():
>    l.append("xyz")
> 
> x()
> print l
> 
> but the same script applied to a single variable doesnt..:
> 
> l="moe"
> 
> def x():
>    l+="howdy"
> 
> x()
> print l

It's even worse:

>>> def add(a, b):
...     a += b
...

Now consider

>>> lst = []
>>> add(lst, ["alpha"])
>>> lst
['alpha']
>>> add(lst, ["beta", "gamma"])
>>> lst
['alpha', 'beta', 'gamma']

where add(a, b) appends b to a versus 

>>> tpl = ()
>>> add( tpl, ("alpha",))
>>> tpl
()

where no apparent change takes place. The difference is that list is a 
mutable type i. e. its instances can be changed anytime, whereas tuples are
immutable, i. e. they cannot be changed once they are created. For lists a
+= b is implemented to append the items in b to a. The binding of a is not
changed in the process. For immutable types this is not an option - they
cannot be altered. Therefore, for tuples a += b is implemented as a = a +
b, i. e. a new tuple containing the elements of both a and b is created and
the variable a is bound to the new tuple. In the example the binding takes
place inside a function, so you never see the new tuple unless you return
it or declare the variable as global as Vincent already pointed out.
Strings are immutable too, so

>>> s = "abc"
>>> add(s, "def")
>>> s
'abc'

as expected (hopefully).

Peter