Python's String Templates

Python's template class processes strings en masse

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Python is an interpreted, object-oriented, high-level programming language. It is easy to learn because its syntax emphasizes readability, which reduces the expense of program maintenance. Many programmers love working with Python because without the compilation step, testing and debugging go quickly.​

Templating, and especially web templating, represents data in forms usually intended to be readable to a viewer.

The simplest form of a templating engine substitutes values into the template to produce the output. 

Aside from the string constants and the deprecated string functions, which moved to string methods, Python's string module also includes string templates. The template itself is a class that receives a string as its argument. The object instantiated from that class is called a template string object. Template strings were first introduced 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 "$" causes a ValueError to be raised. The methods available through template strings are as follows:

    • class string. 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-like object, 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(). However, if a placeholder is missing from mapping or keywords, the original placeholder is used by default, thus avoiding the KeyError. Also, any occurrence of "$" returns 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' 
     
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    Your Citation
    Lukaszewski, Al. "Python's String Templates." ThoughtCo, Nov. 17, 2016, thoughtco.com/pythons-string-templates-2813675. Lukaszewski, Al. (2016, November 17). Python's String Templates. Retrieved from https://www.thoughtco.com/pythons-string-templates-2813675 Lukaszewski, Al. "Python's String Templates." ThoughtCo. https://www.thoughtco.com/pythons-string-templates-2813675 (accessed November 18, 2017).