Simon Morgan wrote:
> Can somebody please explain to me why:
>
> class SomeClass:
> def __init__(self, contents=[]):
> self.contents = contents[:]
> def add(self, element):
> self.contents.append(element)
>
> when called a second time (i.e. to create a new instance of a SomeClass
> object) results in self.contents being assigned an empty list when say for
> example I've done:
>
> foo = SomeClass()
> foo.add(1)
> foo.add(2)
>
> beforehand?
Maybe, after a little renaming you can see it yourself:
class SomeClass:
def __init__(self, default_contents=[]):
# make a copy of default_contents that is
# kept in a SomeClass instance
self.contents = default_contents[:]
def add(self, element):
# modify the *copy* of default_contents...
self.contents.append(element)
When you want modifiable defaults, you can use a class attribute:
>>> class SomeClass:
... default_contents = []
... def __init__(self, contents=default_contents):
... self.contents = contents[:]
... def add(self, elem): self.contents.append(elem)
... def addDefault(self, elem): self.default_contents.append(elem)
... def __repr__(self):
... return "SomeClass(contents=%s, default_contents=%s)" % (
... self.contents, self.default_contents)
...
>>> foo = SomeClass()
>>> foo
SomeClass(contents=[], default_contents=[])
>>> foo.add(1)
>>> foo.addDefault("x")
>>> foo
SomeClass(contents=[1], default_contents=['x'])
>>>
>>> bar = SomeClass()
>>> bar
SomeClass(contents=['x'], default_contents=['x'])
>>> bar.add(2)
>>> bar
SomeClass(contents=['x', 2], default_contents=['x'])
Peter
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