Question

Get this simple python code, same matching with re.compile instance.
I noticed that even though I am using the very same value, it creates two instances, and repeats them accordingly.

I wonder if one can tell the reason for this behavior,

• Why does it create the second instance at all?
• Why only two?
• And why each time picked the other one and not randomly?

the CLI code:

>>> import re
>>>
>>> rec = re.compile("(?:[-a-z0-9]+\.)+[a-z]{2,6}(?:\s|$)")
>>>
>>> rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb238>
>>> rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb1d0>
>>> rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb238>
>>> rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb1d0>
>>> rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb238>
>>> rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb1d0>

Edit:

As @kimvais answered, the reason lays in the _ which holds the latest assignment.
see, if you not assigning, rather printing, it is the same one, all the time.

>>> print rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb1d0>
>>> print rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb1d0>
>>> print rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb1d0>
>>> print rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb1d0>
>>> print rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb1d0>
>>> print rec.match('www.example.com')
<_sre.SRE_Match object at 0x23cb1d0>

Answer 1

My guess is that this has something to do with the return value being assigned to underscore (_) internally in the interactive python shell – i.e. since _ is pointing to <_sre.SRE_Match object at 0x23cb238> ’til the next rec.match is completed the same local cannot be reused until _ points to somewhere else and the old one can be recycled.