🐍 Python for Data Engineers · Chapter 2

Control flow: if, for, while

Three shapes — ask a question, repeat over things, repeat until something changes. Every program you will ever write is these three arranged.

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The basics

Control flow is how a program decides which lines run and how many times. Without it, code runs top to bottom exactly once. With it, you can make decisions and repeat work over a million rows.

The problem it solves: real data is uneven — some rides are cancelled, some fares are negative, some API responses have another page. You need to ask questions and repeat steps without writing the same line a thousand times.

The mental model: a parcel sorter at a conveyor belt. Three shapes, and every program is these three nested:

• if — ask a question about each parcel; pick a chute. elif is only checked if everything above was false.
• for — handle every parcel on the belt. The collection decides how many times.
• while — keep going until a condition changes (trolley full, no next page). The condition decides — so it must be able to become false.

The production habit: skip a bad row, but record that you skipped it.

Explained

if fare > 500:            # ask a question
    band = "high"

for event in events:      # do this to each thing
    process(event)

while cursor:             # keep going until something changes
    page = fetch(cursor)

That's it. Everything else is these three, nested and named.

The parcel sorter

A sorter stands at a conveyor belt.

for — they handle every parcel that arrives. The belt decides how many; the sorter doesn't count. When the belt is empty, they stop.

if — for each parcel they ask a question. Heavy? Left chute. Fragile? Right chute. Otherwise, straight on.

while — a different job: keep collecting parcels until the trolley is full. Nobody knows in advance how many that is. The condition, not the belt, decides when to stop.

The distinction matters: for when you know the collection, while when you know the condition. Choose wrong and you either write awkward code or hang forever.

The shapes, written out

# ---- if / elif / else ----
if fare > 500:
    band = "high"
elif fare > 100:          # only checked if the first was False
    band = "mid"
else:
    band = "low"

# combining conditions
if status == "completed" and fare > 0:
    ...
if city in ("Shillong", "Guwahati"):     # 'in' beats three ors
    ...
if not events:                            # empty list is falsy
    return

# ---- for ----
for event in events:
    print(event)

for i, event in enumerate(events, start=1):   # need the position too
    print(i, event)

for city, revenue in totals.items():          # walking a dict
    print(city, revenue)

for n in range(5):        # 0,1,2,3,4 — a count, not a collection
    ...

# ---- while ----
cursor = None
while True:
    page = fetch(cursor)
    yield from page["data"]
    cursor = page.get("next")
    if not cursor:
        break             # the only exit — make it obvious

Step through a loop with a skip

continue = skip this one. break = stop entirely. Confusing them is a classic beginner bug — and one that silently loses data.

Two loops, one difference

# version A
for e in events:
    if e["fare"] < 0:
        break
    total += e["fare"]

# version B
for e in events:
    if e["fare"] < 0:
        continue
    total += e["fare"]

The list has 1,000 events. Event number 3 has a negative fare (a refund that was written wrongly).

Indentation is the syntax

Other languages use { }. Python uses indentation, and it is not decoration — it is how the language knows what's inside what.

for e in events:
    total += e.fare      # inside the loop — runs every pass
print(total)             # outside — runs once

for e in events:
    total += e.fare
    print(total)         # inside — prints on EVERY pass

Those two are different programs. On a million events, the second prints a million lines.

Rules that avoid all indentation pain:

• Four spaces per level. Not tabs, not two, not eight
• Let your editor insert spaces when you press Tab
• A line ending in : is always followed by an indented block
• If a block feels too deeply nested (three levels or more), that's a signal to extract a function — which is the next lesson

Gotchas

• break where you meant continue — silently drops the rest of the data
• Accumulator inside the loop — total = 0 on the wrong line resets it every pass, so you end up with only the last value
• Modifying a list while looping over it — elements get skipped. Build a new list instead
• while with no way to become false — the job hangs forever, and a hang is worse than a crash because nothing alerts
• if x == True — just write if x. And for None, write is None
• else on a for loop — real Python, runs only if the loop wasn't broken out of. Almost nobody knows it; avoid it in shared code

for when you know the collection. while when you know the condition.

continue = skip this one. break = stop everything.

Indentation is the structure. Four spaces, always.

Words to know

In two minutes

• Three shapes: if asks, for repeats over a collection, while repeats until a condition changes.
• if/elif/else checks top to bottom and runs only the first true branch — order conditions from most specific.
• Use for when you know the collection, while when you know the stopping condition; a while must be able to end.
• continue skips one item; break abandons the rest of the loop — confusing them silently loses data.
• Accumulators go before the loop; inside they reset every pass.
• Indentation is syntax: four spaces decide what runs per item and what runs once.
• Never remove items from a list you are looping over — build a new list.
• Skip bad rows but collect them; silent skipping is a red flag in interviews and production.

Test yourself

There are 10 questions, 3 puzzles and 5 recall cards for this lesson in the app — the recall cards come back on later days, just before you would forget.