🐍 Python for Data Engineers · Chapter 1

Values, names and types

A variable is not a box you put a value in. It is a label you stick on a value. Everything else follows from that.

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The basics

A value is a piece of data — 250, "ride_9001", True. A name (variable) is a label you stick on a value so you can refer to it later. A type says what kind of value it is, and therefore what you can do with it.

The problem it solves: every program is values flowing through names. Get the model wrong now and later lessons — lists that change "by themselves", filters that match nothing — look like magic.

The mental model: luggage tags, not boxes. fare = 250 ties the tag fare to the bag holding 250. total = fare ties a second tag to the same bag — no copy. fare = 300 moves the fare tag to a new bag; total doesn't move.

Explained

fare = 250
fare = 300

Most people read that as "the box called fare now holds 300".

Python reads it as "take the label fare off the value 250 and stick it on the value 300".

Same result here. Completely different result in about six lessons' time — so learn it the right way now.

Luggage tags at the airport

Bags sit on the belt. Each bag exists whether or not anyone has claimed it.

A name is a tag you tie to a bag. fare = 250 ties the tag fare to the bag containing 250.

Two tags can hang off the same bag — total = fare doesn't create a second 250, it adds a second tag to the one that's there.

And when the last tag comes off a bag, the airport takes it away. That is garbage collection, and it is the only memory management you ever have to think about.

The types you will use every day

| Type | Example | What it's for | | --- | --- | --- | | int | 250 | whole numbers — counts, ids | | float | 250.75 | decimals — never for money | | str | "ride_9001" | text | | bool | True / False | yes/no | | None | None | "there is no value here" |

Python figures out the type from the value — you never declare it. But the type is real and it decides what works:

"250" + 10        # TypeError: can only concatenate str to str
int("250") + 10   # 260  ✅
type(fare)        # <class 'int'>

None is worth its own moment. It is not zero, not False, not an empty string — it means absent. In data work that distinction matters constantly: a fare of 0 is a free ride; a fare of None is a ride we have no fare for.

Strings: the 10 things you'll do

ride = "ride_9001"
city = "  Shillong  "

len(ride)                 # 9
ride.upper()              # 'RIDE_9001'
city.strip()              # 'Shillong'   — kills the spaces
ride.startswith("ride")   # True
ride.replace("ride", "trip")   # 'trip_9001'
ride.split("_")           # ['ride', '9001']
"_".join(["ride", "9001"])     # 'ride_9001'

ride[0]                   # 'r'      — first character
ride[-1]                  # '1'      — last character
ride[5:]                  # '9001'   — from index 5 to the end

# f-strings: put values inside text
fare = 250
print(f"{ride} in {city.strip()} cost ₹{fare}")
# ride_9001 in Shillong cost ₹250

Notice city.strip() doesn't change city — it returns a new string. Strings in Python can never be modified in place. city = city.strip() is how you keep the result.

Watch the tags move

Step 3 is the one to hold on to: reassigning a name never disturbs other names.

Predict the output

a = 10
b = a
b = b + 5
print(a, b)

x = "250"
y = 250
print(x == y)
print(int(x) == y)

Gotchas that cost beginners hours

• = vs == — one assigns, one compares. if fare = 250 is a syntax error; if fare == 250 is a question
• Numbers from files are text — CSV and JSON hand you "250". Convert deliberately
• float for money — 0.1 + 0.2 is 0.30000000000000004. Use integers of paise, or Decimal
• None isn't 0 — and if fare: treats 0, "", [] and None as all false. Use if fare is None: when you mean absent
• Indentation is syntax — four spaces, consistently. Mixing tabs and spaces is a real error, not a style opinion
• Case matters — Fare and fare are different names

A name is a tag on a value, not a box holding it.

Re-tagging one name never moves another.

"250" != 250. Fix types at the door.

Words to know

In two minutes

• A name is a tag on a value, not a box; assignment moves the tag, never copies the value.
• Re-tagging one name never changes another name that pointed at the old value.
• Core types: int, float, str, bool, None — Python infers them, but they decide what operations work.
• Data from CSV/JSON arrives as strings; '250' == 250 is False, so convert types at the door.
• Strings are immutable: methods like strip, upper and replace return a new string you must assign.
• None means absent; 0 means a real zero. Use 'is None' when you mean missing.
• Never use float for money — store paise as int or use Decimal.

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.