What is a Template Class in Python? Everything You Need To Know

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Python features effective high-level data structures and an easy-to-use but powerful method for object-oriented programming. Programmers use a unique concept called class to group related objects and implement majority of code in the Python programming language. The template class finds a similar use in Python.

Python classes

Now narrowing our focus to templates, in general, they are documents that act as a foundation for new ones and are pre-formatted when you open them. 

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What Are Python Templates?

Python 2.4 saw the introduction of the template class to the string module. A template in Python is a class description that specifies the variables and methods present in the class.  Templates define parameters for those classes that can have any data. As a result, you may build adaptable classes and work with various data types, avoiding the need to modify the application.

Template Class In Python

Python frameworks frequently use the Template Method pattern, and developers use it to give framework users a quick way to add inheritance to basic functionality.

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How To Identify A Template Method?

If you notice a method in a base class that calls numerous additional methods, either abstract or empty, that method is probably a template method. Cass’s implementation uses regular expressions to match a broad pattern of acceptable template strings. Two components make up a valid placeholder or template string:

  • The $ sign
  • A verified Python identity. Any combination of capital and lowercase characters, underscores (_), and numerals 0 to 9 constitutes an identification. An identifier cannot be a Python keyword or include a digit as its first character.

What Are Template Strings Exactly?

Standard Python strings with specific placeholders make up the template string, and the $ sign and a legitimate Python identifier find its use in constructing these placeholders. Once we have a suitable template string, we can add our values to the placeholders to produce a more complex string.

Python’s Template System In Action

  • Template class initially uses a string as a template.  The template string utilizes a placeholder variable name with the $ symbol within the preceding string. Here, the dollar sign indicates that a placeholder is present. The code employes $$ to represent $ itself.
  • By utilizing the dictionary-based substitute () function, it substitutes a value into the Template. The key must match the placeholder name and be found in the dictionary. As a result, the entire Template returns, not just the placeholder.
  • According to the convention, we use the placeholder variable name as the variable name in Python.

Basic Example of a Template string in code:

Template String

Using The substitute() Method

To conduct string substitution up until now, we’ve been using a Template instance’s substitute() method. Using this technique, the placeholders in a template string are substituted using keyword parameters or mapping with identifier-value pairs.

The IDs used to define the placeholders in the template string must agree with the keyword arguments or identifiers in the mapping. Any Python type that can adequately convert to a string can be used as a value.

The roles of substitution are as follows:

  • A substitution placeholder with the name “$identifier” maps the “identifier” mapping key.
  • We can conclude the specification of the placeholder by the “$” and the first non-identifier character.
  • We use a single $ in place of “$$“, which regards as an escape.

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Errors In Python Template

These are the two categories of errors in Python:

Key Error: When a placeholder value is missing from a template, No Placeholder Match is displayed.

Value Error: Bad Placeholder appears when the placeholder’s first character is invalid.

Correction Of Template Errors

The Template has a safe subsitute() method to handle these problems while returning a string.

If data is lacking, the safe subsitute() method will leave placeholders alone. If there is a problem, we can include the placeholders in the returned string.

Python Template Class Customization

We can customize and subclass a Python Template class. This enables us to alter the class’s regular expression patterns and other characteristics to suit our requirements. ‘Delimiter‘ is widely used for customization. The class attribute delimiter stores the placeholder’s initial character, and its default value is $, as we have already seen. We can subclass Template and override the delimiter default value because the Python Template class intends for inheritance.


The Python Template class finds its application in string interpolation or substitution. The class offers a powerful and user-friendly interface and operates with regular expressions. When building intricate string-based templates, it’s a solid alternative to other built-in string substitution methods.

We’ve studied how the Python Template class functions in this article. We also learned to avoid some of the more frequent mistakes we can make when utilizing Template. Finally, we discussed subclassing as a means of personalizing the class.

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