Cheat SheetsPython A–ZControl Flow

Control Flow — Cheat Sheet

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Cheat Sheet · AiCanCode.org
Control Flow
Python A–Z3 topicsQuick revision reference
1

Conditionals — if/elif/else & match

Branching in Python is if/elif/else (there is no switch keyword) — plus structural pattern matching with match/case since Python 3.10 for elegant multi-shape logic.

  • ✓No switch keyword — use elif ladders or match/case (3.10+)
  • ✓Guard clauses (early returns) keep code flat and readable
  • ✓match destructures sequences/dicts and binds variables; _ is the default case
  • ✓Conditions use truthiness — "if data:" covers None AND empty
elif ladders + guard clauses
def grade(marks):
    if marks >= 90:
        return "A"
    elif marks >= 75:
        return "B"
    elif marks >= 40:
        return "C"
    else:
        return "F"

# Guard clauses — flat beats nested
def process(order):
    if not order:                 # empty/None -> reject early
        return "no order"
    if not order.get("paid"):
        return "unpaid"
    return f"shipping {order['id']}"   # happy path, unindented
2

Loops — for, while, range, enumerate & zip

Python's for loop iterates over ANY iterable directly — no index bookkeeping. range generates number sequences; enumerate gives index+value; zip walks multiple sequences together.

  • ✓for iterates values directly; enumerate() when you need indexes — avoid range(len(x)) + indexing
  • ✓range is lazy (constant memory); range(n, -1, -1) counts down
  • ✓zip stops at the shortest input; use itertools.zip_longest to pad
  • ✓Dict loops: "for k in d", "for k, v in d.items()"
for-each first; enumerate when you need the index
skills = ["java", "python", "sql"]

for skill in skills:              # items directly — no index
    print(skill)

for i, skill in enumerate(skills, start=1):
    print(f"{i}. {skill}")        # 1. java  2. python  3. sql

print(list(range(5)))             # [0, 1, 2, 3, 4]
print(list(range(2, 11, 2)))      # [2, 4, 6, 8, 10]
print(list(range(5, 0, -1)))      # [5, 4, 3, 2, 1] — countdown

# Classic index loop — only when you MUST mutate by index
nums = [3, 1, 4]
for i in range(len(nums)):
    nums[i] *= 10
3

break, continue & the loop-else

break exits a loop, continue skips to the next iteration, and Python's unusual for...else runs the else block only when the loop finished WITHOUT break — perfect for search loops.

  • ✓break exits the nearest enclosing loop only (no labeled breaks — refactor to a function and return)
  • ✓continue jumps to the next iteration — great for guard conditions
  • ✓for...else / while...else: else runs only if NO break occurred
  • ✓Need to break out of nested loops? Extract to a function and return
Guards with continue, early exit with break
# continue — skip invalid rows early
rows = ["12", "", "7", "abc", "30"]
total = 0
for r in rows:
    if not r.isdigit():      # skip blanks & junk
        continue
    total += int(r)
print(total)                 # 49

# break — stop at first match
primes = [2, 3, 5, 7, 11, 13]
target = 7
for p in primes:
    if p == target:
        print("found", p)
        break
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