Replacing "tokens" in Python string with alternate values

Question

Imagine a script that receives a string:

http://whatever.org/?title=@Title@&note=@Note@

...and a list of tokens:

['arg:Title=SampleTitle', 'arg:Note=SampleNote']

What is the most Pythonic way to interpolate those tokens into the string, such that, using the above example, the following is produced:

http://whatever.org/?title=SampleTitle&note=SampleNote

I've considered:

1. Loop through the list and, for every string it contains, split out the token name, and do a regex replace on every instance of @TOKEN_NAME found; and

2. Use some sort of templating mechanism (similar to what one can do with Ruby's ERB.template).

Answer 1

To use the Pythonic solution, adopt the str.format specifications for format string syntax:

>>> template = "http://whatever.org/?title={Title}&note={Note}"
>>> template.format(Title="SampleTitle", Note="SampleNote")
'http://whatever.org/?title=SampleTitle&note=SampleNote'

You can also unpack a dictionary of named arguments:

>>> template.format(**{"Title": "SampleTitle", "Note": "SampleNote"})
'http://whatever.org/?title=SampleTitle&note=SampleNote'

---

If you're stuck with your input format, you could easily switch to something more useful with a regular expression:

>>> import re
>>> s = "http://whatever.org/?title=@Title@&note=@Note@"
>>> re.sub(r"@(\w+?)@", r"{\1}", s)
'http://whatever.org/?title={Title}&note={Note}'

(see regex explanation here)

and process the tokens into a dictionary, too:

>>> tokens = ['arg:Title=SampleTitle', 'arg:Note=SampleNote']
>>> dict(s[4:].split("=") for s in tokens)
{'Note': 'SampleNote', 'Title': 'SampleTitle'}