[Python-Dev] Extracting variables from string.Template objects (original) (raw)

Isaac Morland ijmorlan at cs.uwaterloo.ca
Fri Jan 4 14:58:16 CET 2008

I found myself wanting to obtain the variables from a string.Template object. The situation is that part of input consists of a filename template, and I want to make sure that all the parameter values I have are actually represented in the filename template. So:

def templateVariables (template): """Extract the variable names from a string.Template.

 Returns a tuple of all variable names found in the template, in the order
 in which they occur.  If an invalid escape sequence occurs, the same
 error will be raised as if an attempt was made to expand the template.
 """
 result = []
 for match in template.pattern.finditer (template.template):
     if match.group ('invalid') is not None:
         # Raises ValueError
         template._invalid (match)
     if match.group ('escaped') is not None:
         continue
     # The "or None" should be moot.  It is there to ensure equivalent
     # treatment for an empty 'named' and an empty 'braced'.
     result.append (match.group ('named') or match.group ('braced') or None)
 return tuple (result)

Note that almost all the work is actually done by calling in to parts of the template object, so I think this should work properly with subclasses and other less common cases.

I would like to add this as a method of string.Template, which I think amounts to changing "template" to "self" and putting it in the Template class in string.py rather than on its own. If the general idea is approved I would be happy to make a patch.

Also, on a related issue, does it make sense to scan the template string for invalid escape sequences in Template.init? For the applications I can imagine of string.Template, I would prefer to get an error upon creating the Template object rather than arbitrarily later when I try to .substitute with it.

Isaac Morland CSCF Web Guru DC 2554C, x36650 WWW Software Specialist

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