Repetition (P2)¶

This page introduces repetition: how Python repeats actions using loops.

Repetition is used when:

• processing many rows
• iterating over collections
• performing repeated tasks

Repetition and Uses¶

Repetition means executing the same block of code multiple times.

• Use for for known collections
• Use range() for numeric sequences
• Use while for condition-based repetition
• Use list comprehensions to transfrom each item in a list to a new list

for Loops (Most Common)¶

Use a for loop when you know what you are iterating over.

for name in names:
    print(name)

range() for Numeric Sequences¶

Use range() when you need a sequence of numbers.

for year in range(2020, 2024):
    print(year)

Important:

• range(start, stop) does not include stop
• This prints 2020, 2021, 2022, 2023

Use cases:

• years
• counts
• indexes

while Loops (Condition-Controlled)¶

Use a while loop when repetition depends on a condition.

count = 0
while count < 5:
    print(count)
    count += 1

Rules:

• The condition is checked before each iteration
• You must update the condition

When NOT to Use Loops¶

Do not use loops to schedule work over time (e.g., “run nightly”).

That belongs to:

• operating system schedulers
• workflow tools
• pipelines

Loops are for repetition inside a running program.

List Comprehensions¶

List comprehensions create a new list by transforming another list.

lower_names = [name.lower() for name in names]

Use them when:

• the transformation is simple
• readability stays high

Nested Repetition (Briefly)¶

Loops can be nested:

for row in rows:
    for value in row:
        process(value)

Keep nesting shallow when possible.

Repetition in Data Pipelines¶

Repetition appears everywhere in data work:

• CSV rows
• JSON records
• Excel rows
• text lines

Common Mistakes¶

Off-by-one errors¶

range(1, 5)  # produces 1, 2, 3, 4

Infinite while loops¶

Forgetting to update the condition variable.