On 04/17/2015 11:51 PM, Ben Finney wrote:
Ben Finney <ben+pyt...@benfinney.id.au> writes:
Bill Allen <walle...@gmail.com> writes:
If I have a list defined as my_list = ['a','b','c'], what is the is
differnce between refering to it as my_list or my_list[:]?
‘my_list’ is a reference to the object you've already described (the
existing object ‘['a', 'b', 'c']’).
‘my_list[:]’ is an operation that takes the original object and
creates a new one by slicing. In this case, the new object happens to
be equal to (but probably not identical to) the original, because of
the slice you specified.
To demonstrate how identity differs from equality, use the appropriate
comparison operators::
$ python3
>>> foo = ['a', 'b', 'c']
>>> bar = foo # Bind the name ‘bar’ to the same object.
>>> bar # Show me the object referred to by ‘bar’.
['a', 'b', 'c']
>>> bar == foo # Is the object ‘bar’ equal to the object ‘foo’?
True
>>> bar is foo # Is ‘bar’ referring to the same object as ‘foo’?
True
>>> baz = foo[:] # Slice ‘foo’, creating a new list; bind ‘baz’ to that.
>>> baz # Show me the object referred to by ‘baz’.
['a', 'b', 'c']
>>> baz == foo # Is the object ‘baz’ equal to the object ‘foo’?
True
>>> baz is foo # Is ‘baz’ referring to the same object as ‘foo’?
False
References which compare identical *are the same* object, guaranteed.
Object identity almost always implies equality.
(“almost always” because some object types have unusual behaviour like
“the object is not equal to itself”. Don't fret about that though, these
exceptions are clearly defined when you find them.)
References which compare equal *may under some conditions* be identical.
This is *not* ever a promise, though, and you should never rely on it,
not even in the same session of a program.
Some of Python's internal optimisations depend on the fact that object
equality *does not* imply object identity. If you happen to notice some
operations producing the same object at one point in time, but the
documentation doesn't promise it, then treat that as an unpredictable
implementation detail and don't rely on it in your code.
Great description, if the OP really wants that level of detail. But I
think what he needs to know first is:
If you change foo, like (untested)
foo[1] = "new"
then you'll see that bar also changes (since it's just another name for
the exact same object).
>>> bar
['a', 'new', 'c']
But baz does not, since it's bound to a copy of the object.
>>> baz
['a', 'b', 'c'\
--
DaveA
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