Python - Templating

Python provides different text formatting features. It including formatting operators, Python's format() function and the f-string. In addition, Python's standard library includes string module that comes with more formatting options.

The Template class in string module is useful for forming a string object dynamically by substitution technique described in PEP 292. Its the simpler syntax and functionality makes it easier to translate in case of internalization than other built-in string formatting facilities in Python.

Template strings use $ symbol for substitution. The symbol is immediately followed by an identifier that follows the rules of forming a valid Python identifier.

Syntax

from string import Template

tempStr = Template('Hello $name')

The Template class defines the following methods −

substitute()

This method performs substitution of value the identifiers in the Template object. Using keyword arguments or a dictionary object can be used to map the identifiers in the template. The method returns a new string.

Example 1

Following code uses keyword arguments for substitute() method.

from string import Template

tempStr = Template('Hello. My name is $name and my age is $age')
newStr = tempStr.substitute(name = 'Pushpa', age = 26)
print (newStr)

It will produce the following output −

Hello. My name is Pushpa and my age is 26

Example 2

In the following example, we use a dictionary object to map the substitution identifiers in the template string.

from string import Template

tempStr = Template('Hello. My name is $name and my age is $age')
dct = {'name' : 'Pushpalata', 'age' : 25}
newStr = tempStr.substitute(dct)
print (newStr)

It will produce the following output −

Hello. My name is Pushpalata and my age is 25

Example 3

If the substitute() method is not provided with sufficient parameters to be matched against the identifiers in the template string, Python raises KeyError.

from string import 

tempStr = Template('Hello. My name is $name and my age is $age')
dct = {'name' : 'Pushpalata'}
newStr = tempStr.substitute(dct)
print (newStr)

It will produce the following output −

Traceback (most recent call last):
File "C:\Users\user\example.py", line 5, in 
newStr = tempStr.substitute(dct)
^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Python311\Lib\string.py", line 121, in substitute
return self.pattern.sub(convert, self.template)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Python311\Lib\string.py", line 114, in convert
return str(mapping[named])
~~~~~~~^^^^^^^
KeyError: 'age'

safe_substitute()

This method behaves similarly to substitute() method, except for the fact that it doesn't throw error if the keys are not sufficient or are not matching. Instead, the original placeholder will appear in the resulting string intact.

Example 4

from string import Template
tempStr = Template('Hello. My name is $name and my age is $age')
dct = {'name' : 'Pushpalata'}
newStr = tempStr.safe_substitute(dct)
print (newStr)

It will produce the following output −

Hello. My name is Pushpalata and my age is $age

is_valid()

Returns false if the template has invalid placeholders that will cause substitute() to raise ValueError.

get_identifiers()

Returns a list of the valid identifiers in the template, in the order they first appear, ignoring any invalid identifiers.

Example 5

from string import Template

tempStr = Template('Hello. My name is $name and my age is $23')
print (tempStr.is_valid())
tempStr = Template('Hello. My name is $name and my age is $age')
print (tempStr.get_identifiers())

It will produce the following output −

False

['name', 'age']

Example 6

The "$" symbol has been defined as the substitution character. If you need $ itself to appear in the string, it has to be escaped. In other words, use $$ to use it in the string.

from string import Template

tempStr = Template('The symbol for Dollar is $$')
print (tempStr.substitute())

It will produce the following output −

The symbol for Dollar is $

Example 7

If you wish to use any other character instead of "$" as the substitution symbol, declare a subclass of Template class and assign −

from string import Template

class myTemplate(Template):
   delimiter = '#'
   
tempStr = myTemplate('Hello. My name is #name and my age is #age')
print (tempStr.substitute(name='Harsha', age=30))
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