Control flow: if, for, while

Control flow is the logic of your program. Decide, repeat, stop. Without it you cannot automate anything.

Indentation is the syntax

Python has no braces. A block is the lines indented under a colon. Use 4 spaces. Mixing tabs and spaces is an error.

if salary >= 100000:
    print("senior")        # inside the if
print("done")              # outside the if

if / elif / else

if salary >= 100000:
    level = "senior"
elif salary >= 70000:
    level = "middle"
else:
    level = "junior"

Conditions are checked from the top. The first true one wins. Comparison operators: ==, !=, <, <=, >, >=. Combine with and, or, not:

if remote and salary > 90000:
    print("apply")
if country == "Canada" or country == "Germany":
    print("target market")
if not is_deleted:
    print("active")

Check membership with in: if country in ["Canada", "Germany", "Poland"]:.

for loops

for walks through every item of a list, a dict, a string, a file:

for city in ["Berlin", "Toronto", "Seattle"]:
    print(city)

for i in range(3):          # 0, 1, 2
    print(i)

offer = {"company": "Shopify", "salary_usd": 95000}
for key, value in offer.items():
    print(key, value)

for company, salary in [("Shopify", 95000), ("Startup", 68000)]:
    print(f"{company}: {salary}")          # unpack tuples as you go

Need the position too? for i, city in enumerate(cities):.

The two loop patterns of data work

You will write these two shapes more than anything else. Learn them by heart.

Filter: keep some rows

high = []
for offer in offers:
    if offer["salary_usd"] > 90000:
        high.append(offer)

Accumulate: sum or count

total = 0
count = 0
for offer in offers:
    total += offer["salary_usd"]
    count += 1
print(total / count)

Later you will see sum(), len(), and list comprehensions do this in one line. Understand the loop first.

while loops

Repeat while a condition holds. Less common in data code, but you meet it in API pagination: “keep fetching pages until there is no next page”.

page = 1
while page <= 3:
    print(f"fetching page {page}")
    page += 1

break exits a loop early. continue skips to the next item.