Aside from the string constants and the deprecated string functions (moved to string methods), Python's string module also includes string templates. The template itself is a class which receives a string as its argument. The object instantiated from that class is called a template string object. Template strings were first introduce in Python 2.4. Where string-formatting operators used the percentage sign ('%') for substitutions, the Template object uses dollar signs ('$'):

• "$$" is an escape sequence; it is replaced with a single "$".

• "$<identifier>" names a substitution placeholder matching a mapping key of "<identifier>". By default, "<identifier>" must spell a Python identifier. The first non-identifier character after the "$" character terminates this placeholder specification.
• "${<identifier>}" is equivalent to "$<identifier>". It is required when valid identifier characters follow the placeholder but are not part of the placeholder, such as "${noun}ification".

Outside of these uses of the dollar sign, any appearance of '$' will cause a ValueError to be raised. The methods available through template strings are as follows:

• class Template(template): The constructor takes a single argument which is the template string.
• substitute(mapping[, **keywords]): Method that substitutes the string values (mapping for the template string values. mapping is a dictionary, and its values may be accessed as a dictionary. If the keywords argument is used, it represents placeholders. Where both mapping and keywords are used, the latter takes precedence. If a placeholder is missing from mapping or keywords, a KeyError is thrown.

• safe_substitute(mapping[, **keywords]): Functions similarly to substitute() (above). However, if a placeholder is missing from mapping or keywords, the original placeholder will be used by default, thus avoiding the KeyError. Also, any occurrence of '$' will return a dollar sign.

Template objects also have one publicly available attribute. template is the object passed to the constructor's template argument. While read-only access is not enforced, it is best not to change this attribute in your program. The sample shell session below serves to illustrate template string objects.
>>> from string import Template>>> s = Template('$when, $who $action $what.')>>> s.substitute(when='In the summer', who='John', action='drinks', what='iced tea')'In the summer, John drinks iced tea.'>>> s.substitute(when='At night', who='Jean', action='eats', what='popcorn')'At night, Jean eats popcorn.'>>> s.template'$when, $who $action $what.'>>> d = dict(when='in the summer')>>> Template('$who $action $what $when').safe_substitute(d)'$who $action $what in the summer'