Re: helper function in a class' namespace
- From: Stargaming <stargaming@xxxxxxxxx>
- Date: 31 Jan 2008 20:05:44 GMT
On Thu, 31 Jan 2008 20:51:23 +0100, Helmut Jarausch wrote:
Hi,
the following code works fine
def Helper(M) :
return 'H>'+M
String concatenation is generally considered unpythonic, better use
string interpolation::
'H> %s' % (M,)
class A(object):
def __init__(self,Msg) :
print Helper(Msg)
B=A('ZZ')
Watch out your names -- they could confuse other Python programmers
introspecting your namespaces! Names bound to objects should generally be
lowercase (argument ``msg``, object ``b``, argument ``m``, function
``helper``). For details on what the Python community has agreed on is
"good style," see the `Style Guide <http://www.python.org/dev/peps/
pep-0008/>`_.
but the Helper function is in the module's namespace.
Where's the problem? If it is "I don't want Helper to be visible", just
use this convention: "Names beginning with an underscore are private.
Better keep your fingers off."
I'd like to put it into class A's namespace. Note, the Helper function
doesn't need access to any instance attributes.
The first variant
class A(object):
def Helper(M) :
return 'H>'+M
def __init__(self,Msg) :
print Helper(Msg)
doesn't work since Python is looking for a global function Helper (why?)
Because in the scope of the ``__init__`` function, the name ``Helper`` is
not bound. It then jumps out to the global scope.
The alternative
class A(object):
def Helper(M) :
return 'H>'+M
def __init__(self,Msg) :
print A.Helper(Msg)
doesn't work either since now the first argument to A.Helper must be
'A'.
So, isn't it possible to have a helper function in the namespace of a
class without (the overhead of) passing a 'self' or a 'cls' parameter?
Of course it is! You can decorate certain functions with the
`staticmethod <http://docs.python.org/lib/built-in-funcs.html#l2h-69>`_
wrapper, which will do exactly what you wanted::
class A(object):
@staticmethod
def helper(m):
return 'H>%s' % (m,)
def __init__(self, msg):
print A.helper(msg) # or self.helper
HTH,
.
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- From: Helmut Jarausch
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