Conditionals and Loops

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Control Flow(DOCUMENTATION)

Introduction

Conditionals and loops are the building blocks of any program's logic. Python's versions are clean and readable by design, and they include a few unique features -- like the loop else clause -- that you will not find in most other languages.

Key Concepts

  • if / elif / else: Branch execution based on boolean conditions.
  • Ternary expression: value_if_true if condition else value_if_false -- a one-line conditional.
  • for loop: Iterates directly over any iterable (list, range, dict, file, etc.).
  • while loop: Repeats as long as a condition is truthy.
  • Loop else clause: Runs when a loop finishes without hitting break.

Real World Context

Every request handler, data pipeline, and CLI tool relies on conditionals and loops. The loop else clause is a Python-specific pattern that simplifies search-and-not-found logic without extra boolean flags. Mastering break, continue, and else lets you write control flow that is both concise and immediately clear to other developers.

Deep Dive

Conditional Statements

python
if condition:
    # runs if condition is truthy
elif other_condition:
    # runs if first was false and this is true
else:
    # runs if all above were false
Ternary Expression
python
result = "yes" if condition else "no"

# Chained ternary (use sparingly)
grade = "A" if score >= 90 else "B" if score >= 80 else "C"

For Loops

Iterate over any iterable:

python
# Over a list
for item in [1, 2, 3]:
    print(item)

# With index using enumerate
for i, item in enumerate(["a", "b", "c"]):
    print(f"{i}: {item}")

# Over a range
for i in range(5):  # 0, 1, 2, 3, 4
    print(i)

# Over dictionary items
for key, value in d.items():
    print(f"{key}: {value}")

While Loops

python
while condition:
    # runs while condition is truthy
    if should_exit:
        break
    if should_skip:
        continue

Loop Else Clause

The else block runs if the loop completes without break:

python
for item in items:
    if item == target:
        print("Found!")
        break
else:
    print("Not found")  # Only if loop didn't break

Common Pitfalls

  1. Chaining too many ternary expressions -- a if x else b if y else c is hard to read. Use a regular if/elif/else block when you have more than two branches.
  2. Modifying a collection during iteration -- Deleting or inserting items while looping over a list causes skipped elements or RuntimeError with dicts. Iterate over a copy or build a new collection.
  3. Confusing loop else with conditional else -- The loop else runs when the loop exits normally (no break), not when the loop body is "false." Think of it as "no break."

Best Practices

  1. Use for loops over while loops when the iterable is known -- for item in items is clearer and less error-prone than manually managing an index with while.
  2. Use enumerate instead of range(len(...)) -- It pairs each element with its index, eliminating off-by-one errors.

Summary

  • Python's if/elif/else and ternary expressions handle all branching needs.
  • for loops iterate directly over iterables; while loops repeat until a condition is false.
  • The loop else clause runs only when the loop completes without break -- useful for search patterns.
  • Avoid modifying collections during iteration and keep ternary chains short.
  • Prefer for with enumerate over while with manual index tracking.

Code Examples

python
# Common patterns
numbers = [1, 2, 3, 4, 5]

# Find first match
for n in numbers:
    if n > 3:
        result = n
        break
else:
    result = None

# Enumerate with start index
for rank, player in enumerate(players, start=1):
    print(f"#{rank}: {player}")
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