Default method arguments

[gregory.petrosyan]

Hello everybody!
I have little problem:

class A:
def __init__(self, n):
self.data = n
def f(self, x = ????)
print x

All I want is to make self.data the default argument for self.f(). (I
want to use 'A' class as following :

myA = A(5)
myA.f()

and get printed '5' as a result.)

[Bill Mill]

On 15 Nov 2005 08:03:26 -0800, gr************* **@gmail.com
<gr************ ***@gmail.com> wrote:

Hello everybody!
I have little problem:

class A:
def __init__(self, n):
self.data = n
def f(self, x = ????)
print x

All I want is to make self.data the default argument for self.f(). (I
want to use 'A' class as following :

myA = A(5)
myA.f()

and get printed '5' as a result.)

class A:
def __init__(self, n):
self.data = n

def f(self, x=None):
if not x:
x = self.data
print x

myA = A(5)
myA.f()

5

Peace
Bill Mill
bill.mill at gmail.com

[Nicola Larosa]

> I have little problem:

class A:
def __init__(self, n):
self.data = n
def f(self, x = ????)
print x

All I want is to make self.data the default argument for self.f(). (I
want to use 'A' class as following :

myA = A(5)
myA.f()

and get printed '5' as a result.)

# use new-style classes, if there's no cogent reason to do otherwise
class A(object):
def __init__(self, n):
self.data = n
def f(self, x = None)
# do NOT use "if not x" !
if x is None:
print self.data
else:
print x

--
Nicola Larosa - ni*******@m-tekNico.net

....Linux security has been better than many rivals. However, even
the best systems today are totally inadequate. Saying Linux is
more secure than Windows isn't really addressing the bigger issue
- neither is good enough. -- Alan Cox, September 2005

[Nicola Larosa]

> def f(self, x=None):

if not x:
Ha! You fell for it! ;-D
(Hint: what about x being passed with a value of zero? :-) )
x = self.data
print x

--
Nicola Larosa - ni*******@m-tekNico.net

....Linux security has been better than many rivals. However, even
the best systems today are totally inadequate. Saying Linux is
more secure than Windows isn't really addressing the bigger issue
- neither is good enough. -- Alan Cox, September 2005

[Duncan Booth]

Nicola Larosa wrote:

# use new-style classes, if there's no cogent reason to do otherwise
class A(object):
def __init__(self, n):
self.data = n
def f(self, x = None)
# do NOT use "if not x" !
if x is None:
print self.data
else:
print x

Using None might be problematic if None could be a valid argument. The
safest way all round is to use a unique object created just for this
purpose:

_marker = object()

class A(object):

def __init__(self, n):
self.data = n

def f(self, x=_marker)
if x is _marker:
x = self.data
print x

[Gregory Petrosyan]

Thanks a lot, but that's not what I do really want.
1) f() may have many arguments, not one
2) I don't whant only to _print_ x. I want to do many work with it, so
if I could simply write

def f(self, x = self.data) (*)

it would be much better.

By the way, using

class A(object):
data = 0
....
def f(self, x = data)

solves this problem, but not nice at all

So I think (*) is the best variant, but it doesn't work :(

[Gregory Petrosyan]

I want to find REALLY NICE AND PYTHONIC solution (if any)

[Bill Mill]

On 11/15/05, Nicola Larosa <ni*******@m-teknico.net> wrote:

def f(self, x=None):
if not x:

Ha! You fell for it! ;-D
(Hint: what about x being passed with a value of zero? :-) )

I wasn't sure if you saw my post before you posted - good call. I just
tossed off an answer without thinking much, and we see the result. It
could have been a good debugging lesson for him if he'd tried to pass
0; I think I'll use that as my excuse.

Peace
Bill Mill
bill.mill at gmail.com

[Nicola Larosa]

> Using None might be problematic if None could be a valid argument.

That's like saying that NULL could be a significant value in SQL. In
Python, "None" *is* the empty, not significant value, and should always be
used as such. Specifically, never exchange "None" for "False".

--
Nicola Larosa - ni*******@m-tekNico.net

....Linux security has been better than many rivals. However, even
the best systems today are totally inadequate. Saying Linux is
more secure than Windows isn't really addressing the bigger issue
- neither is good enough. -- Alan Cox, September 2005

[bruno at modulix]

gr************* **@gmail.com wrote:

Hello everybody!
I have little problem:

class A:
def __init__(self, n):
self.data = n
def f(self, x = ????)
print x

All I want is to make self.data the default argument for self.f(). (I
want to use 'A' class as following :

myA = A(5)
myA.f()

and get printed '5' as a result.)

class A(object): # Stop using old-style classes, please
def __init__(self, n):
self.data = n
def f(self, x = None):
if x is None:
x = self.data
print x

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'o****@xiludom. gro'.split('@')])"