Can I rely on...

Emanuele D'Arrigo manu3d at gmail.com
Thu Mar 19 16:45:10 CET 2009


Sorry for the double-post, the first one was sent by mistake before
completion.

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?

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):
        print("The compiled pattern is stored.")
        break

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? Or is it one
of those implementation-specific issues?

Manu



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