Detect re (regexp) object in Python

Question

I wonder what is the proper pythonic backward- and forward-compatible method how check if an object is compiled re object.

isinstance method cannot be easily used, while the resulting object claims to be _sre.SRE_Pattern object:

>>> import re
>>> rex = re.compile('')
>>> rex
<_sre.SRE_Pattern object at 0x7f63db414390>

but there is no such one:

>>> import _sre
>>> _sre.SRE_Pattern
AttributeError: 'module' object has no attribute 'SRE_Pattern'

>>> import sre
__main__:1: DeprecationWarning: The sre module is deprecated, please import re.
>>> sre.SRE_Pattern
AttributeError: 'module' object has no attribute 'SRE_Pattern'

>>> re.SRE_Pattern
AttributeError: 'module' object has no attribute 'SRE_Pattern'

I don't want to use duck typing (i.e. checking for the availability of some specific methods), because this could collide with some other types.

For now, I'm using:

>>> RegexpType = type(re.compile(''))
>>> type(rex) == RegexpType
True

but there might be a better way..

Answer 1

re._pattern_type exists, and appears to do what you want:

>>> isinstance(re.compile(''), re._pattern_type)
True

But this is not a good idea - per Python convention, names starting with _ are not part of the public API of a module and not part of the backward compatibility guarantees. So, using type(re.compile('')) is your best bet - though notice that this isn't guaranteed to work either, since the re module makes no mention of the object returned from re.compile() being of any particular class.

And indeed, even if this was guaranteed, the most Pythonic and back- and forward- compatible way would be to rely on the interface, rather than the type. In other words, embracing duck typing and EAFP, do something like this:

try:
     rex.match(my_string)
except AttributeError:
     # rex is not an re
else:
     # rex is an re

Answer 2

Following some of the recommendations you can come up with:

import re

# global constant
RE_TYPE = re.compile('').__class__

def is_regex(a):
    return isinstance(a, RE_TYPE)