Extend string formatting through the string.Formatter class.
How It Works
string.Formatter exposes the parsing and formatting behavior used by str.format(). Subclass methods such as format_field() for controlled custom formatting, but never treat untrusted format templates as safe application code.
Step-by-Step Example
from string import Formatter
formatter = Formatter()
print(formatter.format('Product: {name} ({count:03d})', name='Cable', count=7))Understanding the Example
- The expression 'Formatter()' supplies the value stored as 'formatter'. This value is then used or checked by the later statements.
- The print() call exposes the result so you can compare it with the behavior described above. Check the value and its type when experimenting.
A successful run of this example prints the following output:
Product: Cable (007)Where This Fits in a Real Program
In a real import pipeline, parsing is only the first stage. Convert each field to the intended type, reject missing or unexpected values and report which record failed. Keep the original source file unchanged until the normalized data has passed validation and an export has been checked.
Practice: Try It Yourself
Favor simple f-strings when runtime template customization is unnecessary.
For this exercise, predict the result before running the changed code. Then explain the difference between the original and modified cases using the actual values, not just whether the program completed.
Further Reading
Read the official Python documentation for complete parameters, return types and exceptions.
Common Beginner Mistakes
- Accepting untrusted format strings that can access unexpected object attributes.
- Use Formatter to render a fixed reporting template.
Key Takeaways
- Extend string formatting through the string.Formatter class.
- string.Formatter exposes the parsing and formatting behavior used by str.format().
- Apply the demonstrated operation to the practice input and verify its result before depending on it in a larger program.