Formatting Floating-Point Numbers in Python

When working with floating-point numbers in Python, you often need to control how they are displayed. This is particularly important when presenting numerical data to users or when ensuring consistent formatting for calculations and output. This tutorial covers several common methods to format floats to a specific number of decimal places.

String Formatting with `%`

One of the earliest methods for string formatting in Python involves the % operator. This approach allows you to specify the desired format within a string. For floats, you can use %.nf, where n represents the number of decimal places you want to display.

number = 5.5
formatted_number = "%.2f" % number
print(formatted_number)  # Output: 5.50

number = 5
formatted_number = "%.2f" % number
print(formatted_number) # Output: 5.00

In this example, %.2f instructs Python to format the number as a floating-point number with two decimal places. The result is a string.

`str.format()` Method

The str.format() method provides a more flexible and readable way to format strings, including floating-point numbers.

number = 5.5
formatted_number = "{:.2f}".format(number)
print(formatted_number)  # Output: 5.50

number = 5
formatted_number = "{:.2f}".format(number)
print(formatted_number) # Output: 5.00

Here, :.2f within the curly braces {} acts as a format specifier, indicating that the number should be formatted as a floating-point number with two decimal places. Again, the result is a string.

F-strings (Python 3.6+)

F-strings, introduced in Python 3.6, offer a concise and readable way to embed expressions directly within string literals. They are often the preferred method for string formatting due to their clarity and performance.

number = 5.5
formatted_number = f"{number:.2f}"
print(formatted_number)  # Output: 5.50

number = 5
formatted_number = f"{number:.2f}"
print(formatted_number) # Output: 5.00

Within the f-string, :.2f serves the same purpose as in str.format(), formatting the number as a float with two decimal places.

Using `round()`

The built-in round() function can also be used to achieve a similar result. However, it returns a floating-point number, not a string. This is important to consider if you need the formatted output as a string.

number = 5.5
rounded_number = round(number, 2)
print(rounded_number)  # Output: 5.5

number = 5.555
rounded_number = round(number, 2)
print(rounded_number) # Output: 5.56

If you need the output as a string after using round(), you can convert the result using str():

number = 5.5
rounded_number = round(number, 2)
formatted_number = str(rounded_number)
print(formatted_number) #Output: 5.5

Choosing the Right Method

For simple formatting, f-strings (Python 3.6+) are generally the most readable and efficient option.
str.format() is a good choice for older Python versions or when you need more complex formatting options.
The % operator is a legacy approach and is less recommended for new code.
Use round() when you need a numerical result with a specific number of decimal places, but remember to convert it to a string if you require string output.