Question

Here is some sample Python code:

import re
some_regex = re.compile(r"\s+1\s+")
result = some_regex.search(" 1 ")
dir(result)

I get back the following using Python 2.6.1:

 ['__copy__', '__deepcopy__', 'end', 'expand', 'group', 'groupdict', 'groups', 'span', 'start']

Yet result.re exists (from the interpreter):

>>> result.re
<_sre.SRE_Pattern object at 0x10041bc90>

How can an attribute not be listed when using the dir() function?

This page confirms the existence of the re attribute:

http://docs.python.org/library/re.html#re.MatchObject.re

Now I understand that if one tries to access an attribute which is not listed via dir(), then __getattr__ is called, but I don’t see __getattr__ listed as one of the object’s attributes either, so I’m left scratching my head.

Update

And here is proof of the existence of matchobject.re in the Python 2.6.1 documentation:
http://docs.python.org/release/2.6.1/library/re.html#re.MatchObject.re

Answer 1

You see this behavior because the class is implemented in C, and in the same way that dir() is unreliable with a custom __getattr__(), it is also unreliable when the C code defines a getattr function.

Here is a link to the Python 2.6 C code for the SRE_Match getattr function:

http://hg.python.org/cpython/file/f130ce67387d/Modules/_sre.c#l3565

Note that the methods defined in the match_methods array have Python implementations and are visible in the dir() output, but handled by an if in the match_getattr() function is not visible.

In Python 2.6, it looks like this includes the following attributes: lastindex, lastgroup, string, regs, re, pos, and endpos.

Here is a link to some of the Python 2.7 code which is slightly different. Here there is not a getattr function implemented for SRE_Match, and all methods and attributes can be found in the match_methods, match_members, and match_getset arrays, and everything is visible in dir().

http://hg.python.org/cpython/file/60a7b704de5c/Modules/_sre.c#l3612