using attributes as defaults

[Wanderer]

I want to give the option of changing attributes in a method or using
the current values of the attributes as the default.

class MyClass():
""" my Class
"""
def __init__(self):
""" initialize
"""
self.a = 3
self.b = 4

def MyMethod(self, a = self.a, b = self.b)
""" My Method
"""
self.a = a
self.b = b
DoSomething(a, b)

The above doesn't work. Is there a way to make it work?

Thanks

 

[Stephen Hansen]

I want to give the option of changing attributes in a method or using
the current values of the attributes as the default.

class MyClass():
""" my Class
"""
def __init__(self):
""" initialize
"""
self.a = 3
self.b = 4

def MyMethod(self, a = self.a, b = self.b)
""" My Method
"""
self.a = a
self.b = b
DoSomething(a, b)

The above doesn't work. Is there a way to make it work?

The normal way to go about it would be to set the a/b arguments to None,
like so:

def MyMethod(self, a = None, b = None):
if a is not None:
self.a = a
if b is not None:
self.b = b

DoSomething(self.a, self.b)

--

Stephen Hansen
... Also: Ixokai
... Mail: me+list/python (AT) ixokai (DOT) io
... Blog: http://meh.ixokai.io/


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[Peter Otten]

I want to give the option of changing attributes in a method or using
the current values of the attributes as the default.

class MyClass():
""" my Class
"""
def __init__(self):
""" initialize
"""
self.a = 3
self.b = 4

def MyMethod(self, a = self.a, b = self.b)
""" My Method
"""
self.a = a
self.b = b
DoSomething(a, b)

The above doesn't work. Is there a way to make it work?

No. The defaults are evaluated once, when the method is created, and self.a
is only known after the instance has been created.

The Python idiom to achieve what you want is

def MyMethod(self, a=None):
if a is not None:
self.a = a

If None is a valid value for self.a you can use a custom sentinel instead:

missing = object()
class MyClass:
def MyMethod(self, a=missing):
if a is not missing:
self.a = a

 

[Emile van Sebille]

On 2/4/2011 1:08 PM Wanderer said...

I want to give the option of changing attributes in a method or using
the current values of the attributes as the default.

class MyClass():
""" my Class
"""
def __init__(self):
""" initialize
"""
self.a = 3
self.b = 4

def MyMethod(self, a = self.a, b = self.b)
""" My Method
"""
self.a = a
self.b = b
DoSomething(a, b)

The typical approach would be to use None:

def myMethod(self,a=None,b=None):
if a is None: a=self.a
...

Emile

 

[Jean-Michel Pichavant]

This doesn't work because you can't reference keyword arguments in the
keyword argument array. This will work:
class MyClass:

def __init__(self):
""" initialize

Really? These are the worst docstrings ever.

"""
self.a = 3
self.b = 4

def MyMethod(self, a=None, b=None)
if a is not None:
self.a = a
if b is not None:
self.b = b
DoSomething(a, b)

There is an alternative to this None thing:

Don't use optional arguments. Optional arguments are fine ,but I found
myself avoiding using them is often a better choice.
Quoting the zen of python: "Explicit is better than implicit."
If the reason for using optional arguments is that it'll take you 2 sec
less to write the method call, then it sounds kind of wrong. Any other
reason would be valid I guess.

I personnaly use optional arguments only to keep backward compatibility
when changing a method signature.

JM