# Computer "scientist"

Alex Clemmer is a computer programmer. Other programmers love Alex, excitedly describing him as "employed here" and "the boss's son".

Alex is also a Hacker School alum. Surely they do not at all regret admitting him!

# I needed to use a regex on an airplane without Internet. So I reverse engineered the API.

June 14, 2014

I’m sitting here on the 5 hour 40 minute flight from SEA to PHL. I need to use a regular expression to do something.

But, while normally I’d use the re module, I’ve forgetten the details of the API. And to top it off, I’m certainly not ponying up $14 for the crappy plane Internet! I guess that means I have three problems. Looks like I have to go in through the back door. [Ed. note: Allison Kaptur, everyone’s favorite Python nerd, writes in to tell us that we didn’t have to go through the back door, because the front door was in fact open. Ok, well, in my defense I don’t know what the hell I’m doing.] ## What is the name of the method, again? I start by cracking open my trusty Python interpreter and importing the standard regex module, re. $ python
>>> import re


What methods are inside the module?

I honestly forget.

I remember I can call dir(re) to get “approximately” all the members of the re module.

I think dir is a function — it might be a metaclass or something, though. No Internet, remember?

Anyway, calling it results in:

>>> dir(re)
['DEBUG', 'DOTALL', 'I', 'IGNORECASE', 'L', 'LOCALE', 'M', 'MULTILINE', 'S',
'Scanner', 'T', 'TEMPLATE', 'U', 'UNICODE', 'VERBOSE', 'X', '_MAXCACHE',
'__all__', '__builtins__', '__doc__', '__file__', '__name__', '__package__',
'__version__', '_alphanum', '_cache', '_cache_repl', '_compile',
'_compile_repl', '_expand', '_pattern_type', '_pickle', '_subx', 'compile',
'copy_reg', 'error', 'escape', 'findall', 'finditer', 'match', 'purge',
'search', 'split', 'sre_compile', 'sre_parse', 'sub', 'subn', 'sys',
'template']


Cool! Look at all those members! I wonder what they all do!

Looking closely, re.match and re.findall look familiar. That’s a good start.

## How do I call this method, again?

Unfortunately, while those methods look familiar, I have no idea how to call or use them.

So, without docs, I need to figure out how to do that.

dir to the rescue again! This time we’ll call dir with the method re.findall as a parameter:

>>> dir(re.findall)
['__call__', '__class__', '__closure__', '__code__', '__defaults__',
'__delattr__', '__dict__', '__doc__', '__format__', '__get__',
'__getattribute__', '__globals__', '__hash__', '__init__', '__module__',
'__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
'__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'func_closure',
'func_code', 'func_defaults', 'func_dict', 'func_doc', 'func_globals',
'func_name']


WHOA! Look at those last few members! re.findall.func_code? re.findall.func_doc? WHAT EVEN ARE ALL THOSE THINGS, THEY LOOK AWESOME!

re.findall.func_doc seems like it might talk about the variables. Let’s try it:

>>> print re.findall.func_doc
Return a list of all non-overlapping matches in the string.

If one or more groups are present in the pattern, return a
list of groups; this will be a list of tuples if the pattern
has more than one group.

Empty matches are included in the result.


It doesn’t! Damn. And let that be a lesson to you. Document your parameters when you document your method. Still, it does tell us a bit about the method. So, not completely useless.

What about re.findall.func_code?

>>> print re.findall.func_code
<code object findall at 0x105b36730, file "/Users/alex/PYTHON_STD/lib/python2.7/re.py", line 169>


What do we do with a “code object”? [ed. note: You might say, “the code object clearly lists a source file, it’s on your machine.” But it was 4 in the morning, and my brain wasn’t quite ready to think about this sanely yet.]

dir to the rescue, yet again:

>>> dir(re.findall.func_code)
['__class__', '__cmp__', '__delattr__', '__doc__', '__eq__', '__format__',
'__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__',
'__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
'__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'co_argcount',
'co_cellvars', 'co_code', 'co_consts', 'co_filename', 'co_firstlineno',
'co_flags', 'co_freevars', 'co_lnotab', 'co_name', 'co_names', 'co_nlocals',
'co_stacksize', 'co_varnames']


OOOOOOOOOOOOOOOoooooooo, co_varnames seems like it might be interesting!

>>> re.findall.func_code.co_varnames
('pattern', 'string', 'flags')


Bingo. Those are the function arguments. The signature is re.findall(pattern, string, flags)

## So what does the flags parameter do?

Having discovered that findall has the signature re.findall(pattern, string, flags), we now have to figure out what that last argument means.

I don’t remember having used it before, so I suspect it’s a default parameter.

When we look over the entries to dir, we see that in fact the default value for flags is 0.

>>> re.findall.func_defaults
(0,)


Can we confirm this is all true? Turns out we can get both the line and the file the source code is in:

>>> re.findall.func_code.co_filename
'/Users/alex/PYTHON_STD/lib/python2.7/re.py'
>>> re.findall.func_code.co_firstlineno
169


Going back to this source, we see:

def findall(pattern, string, flags=0):
"""Return a list of all non-overlapping matches in the string.

If one or more groups are present in the pattern, return a
list of groups; this will be a list of tuples if the pattern
has more than one group.

Empty matches are included in the result."""
return _compile(pattern, flags).findall(string)


## Did we even need dir at all?

Ok, no, we didn’t. Your machine probably just has the source. But, I thought it would be fun to show off dir anyway. It’s still useful for the case when you have no documentation or code available at all!

If you’re up for more exploring, I suggest you keep poking at things with dir. The most interesting part is re.findall.func_code, which contains (among other things) members with the actual bytecode.