How can I read Perl data structures from Python?

Question

I've often seen people use Perl data structures in lieu of configuration files; i.e. a lone file containing only:

%config = (
    'color' => 'red',
    'numbers' => [5, 8],
    qr/^spam/ => 'eggs'
);

What's the best way to convert the contents of these files into Python-equivalent data structures, using pure Python? For the time being we can assume that there are no real expressions to evaluate, only structured data.

Answer 1

Not sure what the use case is. Here's my assumption: you're going to do a one-time conversion from Perl to Python.

Perl has this

%config = (
    'color' => 'red',
    'numbers' => [5, 8],
    qr/^spam/ => 'eggs'
);

In Python, it would be

config = {
    'color' : 'red',
    'numbers' : [5, 8],
    re.compile( "^spam" ) : 'eggs'
}

So, I'm guessing it's a bunch of RE's to replace

• %variable = ( with variable = {
• ); with }
• variable => value with variable : value
• qr/.../ => with re.compile( r"..." ) : value

However, Python's built-in dict doesn't do anything unusual with a regex as a hash key. For that, you'd have to write your own subclass of dict, and override __getitem__ to check REGEX keys separately.

class PerlLikeDict( dict ):
    pattern_type= type(re.compile(""))
    def __getitem__( self, key ):
        if key in self:
            return super( PerlLikeDict, self ).__getitem__( key )
        for k in self:
            if type(k) == self.pattern_type:
                if k.match(key):
                    return self[k]
        raise KeyError( "key %r not found" % ( key, ) )

Here's the example of using a Perl-like dict.

>>> pat= re.compile( "hi" )
>>> a = { pat : 'eggs' } # native dict, no features.
>>> x=PerlLikeDict( a )
>>> x['b']= 'c'
>>> x
{<_sre.SRE_Pattern object at 0x75250>: 'eggs', 'b': 'c'}
>>> x['b']
'c'
>>> x['ji']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 10, in __getitem__
KeyError: "key 'ji' not found"
>>> x['hi']
'eggs'