Python String Formatting

1. Introduction to String Formatting in Python

String formatting is an essential skill for any Python developer. It allows you to create dynamic, well-formatted strings by combining static text with variables or expressions. In this tutorial, you will learn about various techniques for formatting strings in Python, including %-formatting, str.format(), and f-strings. By the end of this tutorial, you will have a solid understanding of string formatting in Python and be able to apply it in your own projects.

2. Old-style %-formatting

The old-style string formatting, also known as %-formatting, has been available in Python since its early versions. It is similar to the printf style formatting used in languages like C. In %-formatting, you use placeholders in the format string, which are then replaced by the values provided after the % operator. Here’s an example:

name = "John"
age = 30

formatted_string = "My name is %s, and I am %d years old." % (name, age)
print(formatted_string)

In this example, %s is a placeholder for a string, and %d is a placeholder for an integer. The values in the tuple after the % operator are substituted into the placeholders in the order they appear.

3. The str.format() Method

The str.format() method is a more modern and flexible way to format strings in Python. It was introduced in Python 2.6 and is preferred over %-formatting. The str.format() method uses curly braces {} as placeholders in the format string, which are then replaced by the arguments passed to the method.

Here’s an example of using the str.format() method:

name = "John"
age = 30

formatted_string = "My name is {}, and I am {} years old.".format(name, age)
print(formatted_string)

You can also use positional and keyword arguments to specify the order in which the values should be substituted:

formatted_string = "My name is {0}, and I am {1} years old. {0} is a Python developer.".format(name, age)
print(formatted_string)

formatted_string = "My name is {name}, and I am {age} years old.".format(name="John", age=30)
print(formatted_string)

4. F-strings (Formatted String Literals)

F-strings, or formatted string literals, were introduced in Python 3.6 and provide a more concise and readable way to format strings. To create an f-string, simply prefix the string with an f or F character, and use curly braces {} to embed expressions directly into the string.

Here’s an example of using an f-string:

name = "John"
age = 30

formatted_string = f"My name is {name}, and I am {age} years old."
print(formatted_string)

In f-strings, you can include any valid Python expressions inside the curly braces, such as arithmetic operations, function calls, or even object methods:

price = 49.99
tax = 0.07

formatted_string = f"The total price with tax is {price * (1 + tax):.2f}."
print(formatted_string)

In this example, we calculated the total price with tax inside the curly braces and used the format specifier :.2f to display the result with two decimal places.

5. Formatting Options and Specifiers

Both str.format() and f-strings support a variety of formatting options and specifiers that allow you to control the appearance of the substituted values. These options are placed after the value or expression inside the curly braces, separated by a colon :. Here are some common formatting options:

• {:d}: Formats an integer as a decimal number
• {:f}: Formats a floating-point number as a fixed-point number
• {:e}: Formats a floating-point number in scientific notation
• {:x}: Formats an integer as a hexadecimal number
• {:o}: Formats an integer as an octal number
• {:b}: Formats an integer as a binary number

You can also use additional specifiers to control the width, alignment, and precision of the formatted values:

• {:>10}: Right-aligns the value within a 10-character wide field
• {:<10}: Left-aligns the value within a 10-character wide field
• {:^10}: Centers the value within a 10-character wide field
• {:0>10}: Right-aligns the value within a 10-character wide field, padding with zeros
• {:,.2f}: Formats a floating-point number with a thousands separator and two decimal places

Here’s an example that demonstrates some of these formatting options:

name = "John"
age = 30
salary = 85000.50

formatted_string = f"{name:^10} | {age:>5d} | ${salary:,.2f}"
print(formatted_string)

6. Practice Questions on String Formatting in Python

To test your understanding of string formatting in Python, try solving the following practice questions:

1. Write a Python program that formats a number as a percentage with two decimal places.
2. Write a Python program that formats a date as a string in the format “YYYY-MM-DD”.
3. Write a Python program that formats a list of names in a table with columns for first name, last name, and length of the full name.
4. Write a Python program that formats a number as a currency string with a thousands separator and two decimal places.

7. Frequently Asked Questions (FAQs)

Q1: Which string formatting method should I use in Python?

A: If you are using Python 3.6 or later, it is recommended to use f-strings for their simplicity, readability, and performance. For earlier versions of Python, you can use the str.format() method, which offers more flexibility than old-style %-formatting.

Q2: Can I use f-strings with dictionaries?

A: Yes, you can use f-strings with dictionaries by placing the dictionary key inside the curly braces. For example, f"My name is {person['name']}".

Q3: How do I format a number with a thousands separator?

A: You can format a number with a thousands separator using the format specifier :,. For example, `f”The total price is ${price:,.2f}”`.

8. Conclusion

In this python tutorial, you have learned about various techniques for formatting strings in Python, including %-formatting, str.format(), and f-strings. You have also learned about different formatting options and specifiers that allow you to control the appearance of the substituted values. With this knowledge, you can now create well-formatted strings in your Python projects, improving the readability and presentation of your code.

Now that you have a solid understanding of string formatting in Python, you can apply these techniques in your own projects and continue learning about other Python topics. Continue exploring Whitewood Media & Web Development to expand your skills!