string.Template

The string.Template class allows to do $-style substitutions:

from string import Template
t = Template('Hello, $channel!')

t.substitute(dict(channel='@pythonetc'))
# 'Hello, @pythonetc!'

t.safe_substitute(dict())
# 'Hello, $channel!'

Initially, it was introduced to simplify translations of strings. However, now PO-files natively support python-format flag. It indicates for translators that the string has str.format-style substitutions. And on top of that, str.format is much more powerful and flexible.

Nowadays, the main purpose of Template is to confuse newbies with one more way to format a string. Jokes aside, there are a few more cases when it can come in handy:

• Template.safe_substitute can be used when the template might have variables that aren't defined and should be ignored.
• The substitution format is similar to the string substitution in bash (and other shells), which is useful in some cases. For instance, if you want to write your own dotenv.