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How to use Conditional Formatting with f-Strings in Python

Python's f-strings (formatted string literals) provide a powerful way to embed expressions and variables directly within strings.

This guide explains how to use conditional logic (ternary operators) within f-strings to dynamically format output based on conditions.

Basic Conditional Formatting with Ternary Operators​

The core concept is to use a ternary operator (also known as a conditional expression) inside the f-string's curly braces {}:

my_str = 'tutorialreference.com'

result = f'Result: {my_str.upper() if len(my_str) > 1 else my_str.capitalize()}'
print(result)  # Output: Result: TUTORIALREFERENCE.COM
  • The ternary operator my_str.upper() if len(my_str) > 1 else my_str.capitalize() will first evaluate the boolean value of len(my_str) > 1.

The f-string f'Result: { }' embeds the result of a ternary operator (conditional expression). The general form is:

f'{ <value_if_true> if <condition> else <value_if_false> }'

In this example:

  • Condition: len(my_str) > 1 (checks if the string length is greater than 1)
  • Value if True: my_str.upper() (converts the string to uppercase)
  • Value if False: my_str.capitalize() (capitalizes the first letter)

Here is another simple example:

my_bool = True
result = 'hello' if my_bool else 'bye'
print(result) # Output: hello
  • If the condition is True, the ternary returns the value to the left, and if it's False it returns the value to the right.

Conditional Formatting with Numerical Values​

You can combine conditional logic with f-string's formatting capabilities for numbers:

my_num = 4.56789
result = f'The number is: {my_num:.2f}' # Standard formatting, 2 decimal places
print(result)                           # Output: The number is: 4.57

my_num = 4.56789
result = f'The number is: {my_num:.2f if my_num > 1 else ""}'   # Conditional formatting
print(result)                                                   # Output: The number is: 4.57

f'{my_num:{".2f" if my_num > 1 else ""}}'

  • The outer curly braces {} are for the f-string.
  • The inner curly braces {} are for the format specifier.
  • The ternary operator ".2f" if my_num > 1 else "" chooses the format string based on the value of my_num. If my_num is greater than 1 it will use the .2f, otherwise it will return an empty string.

Quoting and Escaping within f-strings​

When using string literals within an f-string's expression, be careful with quoting:

  • Alternate quote types: If your f-string is enclosed in double quotes ("), use single quotes (') inside the expression (and vice-versa). This is the best and most readable approach.
my_num = 4.56789
my_bool = True
result = f'{my_num:".2f" if my_bool else ""}'   # Correct: single quotes in f-string
print(result)                                   # Output: 4.57

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