[Python] list as paremeters...

"shagshag13" <shagshag13 at yahooPLUSDESPAM.fr> schrieb im Newsbeitrag
news:3f51ce6e$0$26843$626a54ce at news.free.fr...
Try this:
>>>
def doit(s, stack=None):
if stack == None:
stack=[]
if s == '':
return stack

stack.append(s[:1])
return doit(s[1:], stack)

The problem is the default value auf stack.

IIRC Default values are not created each time the function called but rather
only the first time (am i correct here?). And since a list is mutable it
"rememberes" the previous value.

Ciao Ulrich

Ulrich Petri wrote:
...
Almost, but not quite: they're created when the function is DEFINED,
*not* the first time it's called. For example:
...
uuu

The fact that global variable x is worth 8 by the time f is first called
is irrelevant: what governs is that x was worth 3 when the def statement
was executed -- THAT instant is the one in which the default value of d
is termined and stored in the function-object f... at creation time for the
function-object f, i.e. execution time for the def statement.
Yes, this is correct: if the default value of an argument is mutable, it
"remembers" whatever mutations you choose to perform on it.


Alex

In article <bislss$ckt13$1 at ID-67890.news.uni-berlin.de>,
"Ulrich Petri" wrote:
Default values are created when the 'def' statement is
executed, for example:


def f1 (t, usual=[]):
usual.append (t)
print 'F1:', usual

for x in [1, 2, 3, 4, 5]:
def f2 (t, usual=[]):
usual.append (t)
print 'F2:', usual

f1 (x)
f2 (x)


Results in


F1: [1]
F2: [1]
F1: [1, 2]
F2: [2]
F1: [1, 2, 3]
F2: [3]
F1: [1, 2, 3, 4]
F2: [4]
F1: [1, 2, 3, 4, 5]
F2: [5]


Regards. Mel.

"Mel Wilson" <mwilson at the-wire.com> schrieb im Newsbeitrag
news:obgU/ks/KbsM089yn at the-wire.com...
rather
This seems to be generally a somewhat underestimated issue...

See this example:
while c:
b=Button(...)
b.config(command=lambda x=b: doSomething(x))

This would not work otherwise...


Kindly
Michael P

"shagshag13" <shagshag13 at yahooPLUSDESPAM.fr> schrieb im Newsbeitrag
news:3f51ce6e$0$26843$626a54ce at news.free.fr...

This is a feature, nor a bug ;-)
As Python has no 'static' local variables, you can use this trickily to
emulate them. Note however that the preset value has to be mutable, {} e.g.

Kindly
Michael P

"shagshag13" <shagshag13 at yahooPLUSDESPAM.fr> wrote in message news:<3f51ce6e$0$26843$626a54ce at news.free.fr>...
The problem is your function declaration:
def doit(s, stack = []):

That empty list is evaluated only once, so each call to doit without a
stack argument uses the same list.

You should change that to:

def doit(s, stack=None):
if s == '':
return stack

if stack is None:
stack = [] # A new empty list is created each time

stack.append(s[:1])
return doit(s[1:], stack)

-Caleb Land