Python String Formatting

Python String Formatting: F-Strings & Specifiers

Python offers modern string interpolation via f-strings (formatted string literals), introduced in Python 3.6 and expanded in 3.8+ (f"{var=}").

Evolution of Python String Formatting

flowchart TD
    A["Format Options"] --> B["Legacy % Operator (e.g. '%s %d' % (a, b))"]
    A --> C["str.format() (e.g. '{} {}'.format(a, b))"]
    A --> D["Modern F-Strings (e.g. f'{a} {b}')"]
    D --> E["Supports Expressions, Debug specifiers, and Format Specs"]

F-String Format Specifier Cheatsheet

• f"{val:.2f}": Float rounded to 2 decimal places.
• f"{val:,}": Large integer with comma thousands separators (1,000,000).
• f"{val:>10}": Right-aligned string padded to width 10.
• f"{val=}": Debug mode printing variable name and value (val=42).

Practical Code Example

from datetime import datetime

def format_financial_report(company: str, revenue: float, margin: float) -> str:
    timestamp = datetime.now()
    
    # Advanced F-String Format Specifiers
    report = f"""
    ========================================
    FINANCIAL STATEMENT: {company.upper():^20}
    Timestamp: {timestamp:%Y-%m-%d %H:%M}
    ========================================
    Gross Revenue : ${revenue:>15,.2f}
    Profit Margin : {margin:>15.1%}
    ========================================
    """
    return report

if __name__ == "__main__":
    sample_report = format_financial_report("Kodersolution Corp", 1250450.80, 0.2345)
    print(sample_report)

    # Debug format specifier (Python 3.8+)
    x = 42
    y = 99
    print(f"Debug Specifier Output: {x=} | {y=} | {x+y=}")

Best Practices & Gotchas

• Prefer F-Strings: F-strings are faster, more readable, and less error-prone than str.format() or %.
• Quote Nesting: Avoid using the same quotes inside f-string expressions as the outer string quotes (e.g., use f"{data['key']}" rather than double quotes everywhere).
• Security Warning: Never use f-strings to format raw SQL query strings; use parameterized queries to prevent SQL Injection.

Self-Check Challenge

Write an f-string formatting a float 1234.5678 as currency $1,234.57 (with comma separators and 2 decimal places).