How to dynamically parse python string templates?

Question

I have a string with some placeholder in it like:

url = "http://www.myserver.com/$abc/$foo_or_bar/$xy"

I cannot use Templates ( http://is.gd/AKmGxa ), because my placeholder-strings needs to be interpreted by some logic.

I need to iterate over all exisiting placeholders and replace them by a code-generated value.

How can I do this? TIA!

Answer 1

Using re.sub which can accept replacement function as a second argument;

>>> url = "http://www.myserver.com/$abc/$foo_or_bar/$xy"
>>>
>>> def some_logic(match):
...     s = match.group()  # to get matched string
...     return str(len(s) - 1)  # put any login you want here
...
>>> import re
>>> re.sub('\$\w+', some_logic, url)
'http://www.myserver.com/3/10/2'

BTW, string.Template also can be used if you pass custom mapping:

>>> class CustomMapping:
...     def __getitem__(self, key):
...         return str(len(key))
...
>>> import string
>>> url = "http://www.myserver.com/$abc/$foo_or_bar/$xy"
>>> string.Template(url).substitute(CustomMapping())
'http://www.myserver.com/3/10/2'