to Python’s String Template Class
Python’s String Template class provides a powerful way to handle string formatting in Python. It simplifies the process of string interpolation and creates string templates that allow you to substitute values dynamically.
In this article, we will explore the String Template class, how to use it and the rules that govern it.
String Template Rules
$ based substitutions
The String Template class uses dollar sign($) as a marker for substitution placeholders within a string template. The substitution placeholders are replaced by actual values at runtime, as specified in a dictionary.
For instance, consider the following code snippet:
“`
from string import Template
dict1 = {‘name’: ‘John Doe’, ‘age’: ’27’, ‘country’: ‘Canada’}
tpl = Template(‘$name is $age years old and is from $country.’)
output_str = tpl.substitute(dict1)
print(output_str)
“`
In this example, the `Template` class’s `substitute` method is used to substitute `$name`, `$age`, and `$country` placeholders with corresponding values specified in the `dict1` dictionary. Rules for different uses of $
The dollar sign has three different uses:
Escape sequence: The dollar sign can be used as an escape sequence that allows you to include the actual character($) in the output string.
For example, consider the following code snippet:
“`
from string import Template
tpl = Template(‘Calculate the total cost as $$ ${price:.2f}’)
output_str = tpl.substitute(price=300.25)
print(output_str)
“`
Here, the `$$` is an escape sequence which produces a single dollar sign ($) in the output. Substitution placeholder: The dollar sign is used to specify substitution placeholders within the string template.
A substitution placeholder starts with `$` followed by an identifier. For instance, consider the following code snippet:
“`
from string import Template
dict1 = {‘name’: ‘John Doe’}
tpl = Template(‘My name is $name’)
output_str = tpl.substitute(dict1)
print(output_str)
“`
Here, `$name` is a substitution placeholder for the value `John Doe`. ValueError Exception: If there is a mismatch between the substitution placeholders’ identifier and the data dictionary’s keys, a `ValueError` exception is raised.
Consider the following code snippet:
“`
from string import Template
dict1 = {‘first_name’: ‘John’, ‘last_name’: ‘Doe’}
tpl = Template(‘My name is $name’)
try:
output_str = tpl.substitute(dict1)
except ValueError as e:
print(e)
“`
In this example, `$name` has no corresponding key in the dictionary object, and thus a ValueError exception occurs.
Conclusion
In this article, we have discussed the String Template Class and how to use it in Python. We have also explored the rules that govern the `$` based substitutions, the escape sequence, the substitution placeholder, and the ValueError exception.
We hope this article has provided you with valuable insights into the powerful capabilities of Python’s String Template class.
Example of Template Substitution
The Template substitution is a method of Python’s String Template class that allows you to substitute values dynamically in a string template. Here’s an example:
“`
from string import Template
dict1 = {‘name’: ‘John’, ‘age’: 25}
tpl = Template(‘Hi, my name is $name and I am $age years old.’)
output_str = tpl.substitute(dict1)
print(output_str)
“`
In this example, we import the `Template` class from Python’s string module and define a dictionary object named `dict1` with the keys `”name”` and `”age”` and their corresponding values `”John”` and `25`. We then define a string template object `tpl` that contains two substitution placeholders `$name` and `$age`.
We then use the `substitute()` method of the `tpl` object to substitute the placeholders with the values specified in the dictionary. The dictionary’s keys serve as the identifiers for the placeholders, and their corresponding values are the actual values that replace them.
The resulting string `output_str` is then printed to the console. If any of the keys in the dictionary are missing, a `ValueError` exception is raised.
For example, if we change `dict1` to the following:
“`
from string import Template
dict1 = {‘name’: ‘John’}
“`
And run the same code, we will get the following error:
“`
Traceback (most recent call last):
File “main.py”, line 7, in
output_str = tpl.substitute(dict1)
ValueError: ‘age’
“`
This error message states that the key `age` is missing from the dictionary object.
String Template Class Methods
Template Constructor
The `Template` constructor is used to create a new string template object. The constructor can take an optional argument, which is the template string itself.
Here’s an example:
“`
from string import Template
tpl = Template(‘Hi, my name is $name’)
print(tpl.template)
“`
In this example, we create a new `Template` object named `tpl` with the string template `”Hi, my name is $name”`. We then use the `template` attribute of the `tpl` object to print the template string to the console.
substitute(mapping, **kwargs)
The `substitute(mapping, **kwargs)` method is used to substitute values in a string template using a dictionary or keyword arguments. The method replaces the placeholders in the template with the corresponding values in the dictionary object or keyword arguments.
Here’s an example of using this method with keyword arguments:
“`
from string import Template
tpl = Template(‘My name is $name and I am ${age} years old.’)
output_str = tpl.substitute(name=’John’, age=’25’)
print(output_str)
“`
In this example, we create a string template object named `tpl`. We then use the `substitute()` method with keyword arguments to substitute the values `John` and `25` for the `$name` and `${age}` placeholders in the template.
If one of the placeholders in the template does not have a corresponding value in the dictionary or keyword arguments, a `KeyError` exception is raised. Here’s an example of raising a `KeyError` exception:
“`
from string import Template
tpl = Template(‘My name is $name and I am ${age} years old.’)
try:
output_str = tpl.substitute(name=’John’)
except KeyError as e:
print(e)
“`
In this example, we try to substitute the `$name` placeholder in the template with the value `’John’`, but we do not provide a value for the `${age}` placeholder. This causes a `KeyError` exception to be raised with the message `’age’`.
safe_substitute(mapping, **kwargs)
The `safe_substitute(mapping, **kwargs)` method is similar to the `substitute(mapping, **kwargs)` method, except that it leaves the original placeholders in place if a value is not specified in the dictionary or keyword arguments. This method is useful when you need to ensure that the template remains intact even if some of the placeholders cannot be substituted.
Here’s an example:
“`
from string import Template
tpl = Template(‘My name is $name and I am ${age} years old.’)
output_str = tpl.safe_substitute(name=’John’)
print(output_str)
“`
In this example, we use the `safe_substitute()` method with keyword arguments to substitute the `$name` placeholder with the value `’John’`. We do not provide a value for the `${age}` placeholder, but because we are using the `safe_substitute()` method, the original `${age}` placeholder remains intact in the output string.
Conclusion
In this article, we explored the String Template class and how to use it in Python. We also examined the rules that govern `$` based substitutions, the escape sequence, the substitution placeholder, and the ValueError exception.
Furthermore, we delved into the different methods of the String Template class, including the Template constructor, substitute, and safe_substitute. We hope this article has provided you with valuable insights into the powerful capabilities of Python’s String Template class.
Template Class Attributes
In addition to its class methods, the Template class also has several attributes that store essential information about the Template object.
template attribute
The `template` attribute is one of the properties of the Template object, which stores the string used to create the object. It allows you to access and modify the template string if need be.
Here’s an example:
“`
from string import Template
tpl = Template(‘Hi, my name is $name. I am from $country.’)
tpl.template = ‘Hello, my name is $name.
I am from $city.’
print(tpl.template)
“`
In this example, we create a new Template object named `tpl` with the template string `”Hi, my name is $name. I am from $country.”`.
We then use the `template` attribute of the `tpl` object to change the template string to `”Hello, my name is $name. I am from $city.”`.
Finally, we print the new template string to the console.
Conclusion
Python’s String Template class provides an easy way to perform string substitution operations in a template string. By utilizing its `$` based substitution, you can easily substitute predetermined variables with the desired value.
This makes it very easy to modify and customize output formats for specific use cases. Additionally, we learned about the different methods the Template class provides, including the `substitute()` and `safe_substitute()` methods, which allow us to substitute variables in the template string dynamically.
Lastly, we examined the class attributes that make the Template object more versatile, including the `template` attribute that facilitates the ability to change the template string. In conclusion, Python’s String Template Class provides developers with a reliable and powerful way to handle string interpolation.
Its simple syntax and capabilities make it an excellent tool for creating more dynamic and customizable output formats. Python’s String Template Class is a powerful tool that simplifies the process of string formatting and makes string interpolation more convenient.
It provides a straightforward syntax for performing string substitution operations in template strings, making it very easy to modify the output formats for specific use cases. The `$` based substitution and the different methods of the Template class, including `substitute()` and `safe_substitute()`, make it easy to implement dynamic string formatting.
In addition, the class attributes make the Template class more versatile, especially the `template` attribute. In conclusion, the String Template Class is undoubtedly a valuable addition to any Python developer’s toolkit, and the knowledge gained in this article will undoubtedly benefit those who use it.