Python Language Basics

First PublishedFeb 10, 2026Last UpdatedMar 27, 2026ByAtif Alam

Variables and Types

You don’t declare types or use keywords like var — assign a value and the variable exists.

x = 42           # int
name = "Alice"   # str
pi = 3.14        # float
flag = True      # bool
items = [1, 2, 3]           # list
person = {"name": "Bob", "age": 25}  # dict
pair = (1, 2)    # tuple
tags = {1, 2}    # set

Common types: int, float, str, bool, list, dict, tuple, set. See Data structures for details and when to use each.

Comments

# Single-line comment
x = 1  # inline comment

Indentation and Blocks

Python uses indentation (usually 4 spaces) to define blocks. No braces.

if x > 0:
    print("positive")
else:
    print("non-positive")

Conditionals

if n < 0:
    result = "negative"
elif n == 0:
    result = "zero"
else:
    result = "positive"

Loops

for and while. See the Loops page for more.

for x in [1, 2, 3]:
    print(x)

while n > 0:
    n -= 1

List and Dict Basics

# List: zero-based index, slice
nums = [10, 20, 30]
first = nums[0]      # 10
subset = nums[1:3]   # [20, 30]
nums.append(40)

# Dict: key-value
d = {"a": 1, "b": 2}
d["c"] = 3
value = d.get("x", 0)  # default if missing

String Indexing and Slicing

A string is a sequence of characters, so it uses the same indexing and slicing rules as a list: [start:stop] takes characters from start up to but not including stop. Omitting start means “from the beginning”; omitting stop means “through the end.” Slicing always returns a new string (strings are immutable).

s = "hello"
first = s[0]        # 'h'
mid = s[1:4]        # 'ell' — indexes 1, 2, 3
prefix = s[:3]      # 'hel' — same as s[0:3]
suffix = s[2:]      # 'llo' — from index 2 to the end
snippet = s[:80]    # first 80 characters (shorter strings stay as-is)

None and Truthiness

None is the “no value” object. In conditions, False, 0, "", [], {} are falsy; most else is truthy.

result = None
if items:   # falsy if empty
    process(items)