Iteration

To perform an action for each element in a collection without manual index management.

Processing items in a list, reading lines from a file, iterating over keys in a dictionary.

Used across virtually all Python codebases, scripts, libraries, and frameworks.

for ; in ;:

for ; in ;:

item: identifier for the loop variable; iterable: any iterable expression (list, tuple, generator, etc.).

• break
• continue
• else clause
• enumerate()
• zip()
• range()

• Using while loop with index
• using list comprehension
• using map/filter.

Modifying the iterable while iterating (causing skipped items or infinite loops), forgetting the colon, using incorrect variable name.

while loop (manual index control), list comprehension (expression-oriented iteration), itertools.chain (combining multiple iterables).

Coming from languages with C-style for loops (e.g., for(i=0;i<n;i++)): may expect index-based iteration and overlook Python's iterator protocol.

• for loop
• while loop
• list comprehension
• generator expression

The loop variable remains accessible after the loop ends (unless shadowed); if the iterable is an iterator, it gets exhausted; modifying mutable iterables can lead to unexpected behavior.

Provides clear, readable iteration semantics; reduces boilerplate index management.

Think 'for each item in my collection'.

The loop variable remains in scope after the loop finishes, which can unintentionally overwrite existing names; also, iterating over a mutable sequence while modifying it can cause skipped elements.

Using enumerate() to get index and value: for i, item in enumerate(iterable): ...