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'