Python Syntax

Python Syntax: Indentation, Statements, & Structure

Python relies on whitespace indentation rather than curly braces {} or keywords (begin/end) to delimit block structures. This indentation model enforces uniform readability across codebases.

Scope & Indentation Structure

flowchart TD
    A["Outer Module Scope (0 spaces)"] --> B["Function / Class Def (4 spaces)"]
    B --> C["Conditional / Loop Block (8 spaces)"]
    C --> D["Nested Block (12 spaces)"]

Indentation Rules in Python

1. Standard PEP 8 indentation level is 4 spaces per indentation level. Never mix tabs and spaces.
2. Code statements on the same block level must share exact alignment.
3. Colon : marks the start of an indented block (functions, classes, loops, conditionals, try/except).

Practical Code Example

from typing import List

def process_scores(scores: List[int], passing_grade: int = 70) -> List[str]:
    """Evaluate student scores and return classification results."""
    results: List[str] = []
    
    for score in scores:
        # Indented 8 spaces (inside function + inside loop)
        if score >= passing_grade:
            status = f"Pass ({score})"
        else:
            status = f"Fail ({score})"
        
        results.append(status)
    
    return results

if __name__ == "__main__":
    sample_scores = [85, 62, 90, 45, 78]
    summary = process_scores(sample_scores)
    print("Evaluation Results:", summary)

Best Practices & Gotchas

• TabError: Mixing tabs and spaces in Python 3 triggers an immediate TabError syntax exception during parsing.
• Implicit Line Continuation: Enclose long expressions inside parentheses () instead of using backslashes \ for line wrapping.
• Explicit Block Termination: Python blocks end automatically when indentation returns to a previous level.

Self-Check Challenge

Write a function check_even_odd(numbers: list[int]) -> None that iterates over a list, uses if/else with 4-space indentation, and prints whether each integer is even or odd.