Module 1 · Python · Deep Dive

Python Foundations

From PHP habits to fluent Python — variables, types, strings, and the collections you'll use in every backend you ever write.

BasicIntermediateBuild

1 · Running Python

Python runs your file top to bottom, like a PHP script without the HTML. Two ways to run it:

# 1. Run a file
python hello.py

# 2. The REPL — an interactive shell. Type code, see results instantly.
python
>>> 2 + 2
4

The REPL (Read-Eval-Print Loop) is your laboratory — use it constantly to test a line before you commit it to a file. There's no PHP equivalent you used day-to-day; lean on it.

2 · Variables & dynamic typing

No $, no ;, no var/let. You assign with = and the variable simply exists:

name = "Sam"
age  = 29
is_active = True

Like PHP, Python is dynamically typed — a variable's type is whatever you last assigned. Unlike PHP, names are case-sensitive and conventionally written snake_case (not $camelCase).

3 · Core types & truthiness

Check a type with type(x). Convert with int("5"), str(5), float("1.5").

Truthiness matters because you'll lean on it constantly. These are all "falsy": False, None, 0, "", [], {}. Everything else is "truthy".

items = []
if not items:
    print("the list is empty")   # runs — [] is falsy

4 · Strings & f-strings

Forget the . concatenation operator. The modern way to build strings is the f-string — a string prefixed with f, with {expressions} inlined:

name = "Sam"
count = 3
print(f"Hi {name}, you have {count} docs")
print(f"Next year: {count + 1}")   # expressions work too

Strings are objects with useful methods — these come up in interviews and in real ingestion code (you'll clean document text in the RAG module):

s = "  Hello World  "
s.strip()            # "Hello World"  (trim whitespace)
s.lower()            # "  hello world  "
s.replace("World", "UAE")
s.split()            # ['Hello', 'World']  (split on whitespace)
",".join(["a", "b"])  # "a,b"  (join is called on the separator!)

5 · The four collections

This is the heart of the lesson. Master these four and most Python clicks into place.

list ordered, changeable

docs = ["intro.pdf", "report.pdf"]
docs.append("notes.pdf")   # add to end
docs[0]                    # "intro.pdf"  (zero-indexed)
docs[-1]                   # "notes.pdf"  (negative = from the end)
docs[0:2]                  # ['intro.pdf', 'report.pdf']  (a "slice")
len(docs)                 # 3

dict key → value pairs

doc = {"title": "Intro", "pages": 12}
doc["title"]            # "Intro"
doc["author"] = "Sam"   # add/update a key
doc.get("missing")       # None  (safe — no error)
"title" in doc          # True
doc.keys(), doc.values(), doc.items()

tuple ordered, unchangeable

point = (10, 20)     # round brackets, fixed once created
x, y = point        # "unpacking" — x=10, y=20

Use a tuple when a group of values shouldn't change (coordinates, a row from a database). Unpacking is everywhere in Python.

set unique, unordered

tags = {"ai", "uae", "ai"}   # {"ai", "uae"} — duplicates dropped
"ai" in tags                 # True (very fast membership test)

Reach for a set when you need uniqueness or fast "is this in here?" checks.

6 · Control flow

Blocks are defined by a colon + indentation, not braces. Four spaces per level. This is syntax, not style — get it wrong and Python raises IndentationError.

if count > 10:
    tier = "heavy"
elif count > 0:
    tier = "light"
else:
    tier = "empty"

# Loop over ITEMS directly — not an index counter
for doc in docs:
    print(doc)

# Need the index too? enumerate gives both
for i, doc in enumerate(docs):
    print(i, doc)

while count > 0:
    count -= 1      # no count-- in Python

7 · Build it

Try it yourself first, then compare. A clean version:

wordcount.py

text = """the cat sat on the mat the cat ran"""

counts = {}
for word in text.split():
    counts[word] = counts.get(word, 0) + 1

# sort items by count, descending, take top 3
top = sorted(counts.items(), key=lambda kv: kv[1], reverse=True)[:3]
for word, n in top:
    print(f"{word}: {n}")

Don't worry if lambda is fuzzy — you'll meet it properly in Lesson 1.3. The point: a dict + a loop solves a real problem in six lines.

8 · Check yourself

Answer from memory — retrieval is what moves this from "I read it" to "I know it".