Can I rely on...
Albert Hopkins
marduk at letterboxes.org
Thu Mar 19 12:13:33 EDT 2009
On Thu, 2009-03-19 at 08:42 -0700, Emanuele D'Arrigo wrote:
> Hi everybody,
>
> I just had a bit of a shiver for something I'm doing often in my code
> but that might be based on a wrong assumption on my part. Take the
> following code:
>
> pattern = "aPattern"
>
> compiledPatterns = [ ]
> compiledPatterns.append(re.compile(pattern))
>
> if(re.compile(pattern) in compiledPatterns):
> print("The compiled pattern is stored.")
>
> As you can see I'm effectively assuming that every time re.compile()
> is called with the same input pattern it will return the exact same
> object rather than a second, identical, object. In interactive tests
> via python shell this seems to be the case but... can I rely on it -
> always- being the case? Or is it one of those implementation-specific
> issues?
>
> And what about any other function or class/method? Is there a way to
> discriminate between methods and functions that when invoked twice
> with the same arguments will return the same object and those that in
> the same circumstances will return two identical objects?
>
> If the answer is no, am I right to state the in the case portrayed
> above the only way to be safe is to use the following code instead?
>
> for item in compiledPatterns:
> if(item.pattern == pattern):
In general, no. You cannot rely on objects instantiated with the same
parameters to be equal. Eg.:
>>> class N(object):
def __init__(self, foo):
self.foo = foo
>>> a = N('m')
>>> b = N('m')
>>> a == b
False
If, however, the designer of the class implements it as such
(and documents it as well) then you can. E.g:
>>> del N
>>> class N(object):
def __init__(self, foo):
self.foo = foo
def __eq__(self, other):
return self.foo == other.foo
>>> a = N('m')
>>> b = N('m')
>>> a == b
True
For functions/methods it really depends on the implementation. For
example, do we *really* want the following to always be true? Even
though we passed the same arguments?
>>> import random
>>> random.randint(0, 10) == random.randint(0, 10)
For the re module, unless it's documented that
>>> re.compile(p) == re.compile(p)
is always true then you should not rely on it, because it's an
implementation detail that may change in the future.
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