Python For Loops

Python For Loops: Sequence Iteration, Generators, & Helper Functions

A for loop in Python iterates over elements of any iterable object (lists, tuples, strings, dictionaries, generators, or custom iterators) using the standard iterator protocol.

Iteration Architecture

flowchart LR
    A["Iterable Object (e.g. list)"] --> B["iter(iterable) -> Iterator"]
    B --> C["next(iterator)"]
    C --> D{"StopIteration Raised?"}
    D -- "No" --> E["Execute Loop Body for Item"] --> C
    D -- "Yes" --> F["Loop Terminates Cleanly"]

Iteration Helpers

• range(start, stop, step): Lazy sequence generator for numerical iteration.
• enumerate(iterable, start=0): Yields (index, item) tuples during iteration.
• zip(*iterables): Aggregates elements from multiple iterables in parallel.

Practical Code Example

from typing import List, Tuple

def analyze_student_grades(names: List[str], scores: List[int]) -> None:
    # Parallel Iteration via zip() and enumerate()
    print("--- Student Grade Roster ---")
    for rank, (name, score) in enumerate(zip(names, scores), start=1):
        status = "Honor Roll" if score >= 90 else "Passing"
        print(f"Rank {rank}: {name} - Score: {score} ({status})")

    # Dictionary Key-Value Iteration
    grade_map = dict(zip(names, scores))
    print("
--- Iterating Dictionary Items ---")
    for student, score in grade_map.items():
        print(f"Student: {student:<10} | Score: {score}")

if __name__ == "__main__":
    student_names = ["Alice", "Bob", "Charlie"]
    student_scores = [95, 82, 88]
    analyze_student_grades(student_names, student_scores)

Best Practices & Gotchas

• Do Not Use range(len(seq)): Instead of for i in range(len(items)): print(items[i]), use for item in items: or for i, item in enumerate(items):.
• Use zip(..., strict=True) in Python 3.10+: Prevents silent truncation when zipping iterables of unequal length.
• The for...else Construct: The else block executes only if the loop completes without hitting a break statement.

Self-Check Challenge

Write a loop using enumerate() over ['a', 'b', 'c', 'd'] that prints the 1-based index and uppercase value for every element.