Master Python Basics: Variables, Control Flow, Loops, Lists & Dictionaries

21CTO Community Introduction: What is Python? Not a snake, but a powerful, readable programming language created by Guido van Rossum. It emphasizes concise syntax and code readability, allowing developers to express concepts with fewer lines. Python is ideal for data mining, web development, and machine learning; companies like Quora, Google, Pinterest, Spotify, Douban, and Toutiao use it for backend development.

Python Basics

1. Variables

You can think of a variable as a word that stores a value.

Defining a variable in Python is straightforward: one = 1 This assigns the integer 1 to the variable one.

two = 2

some_number = 10000

Variables can hold integers, booleans, strings, floats, etc.

true_boolean = True

false_boolean = False

my_name = "Raymond"

book_price = 15.80

2. Control Flow: Conditional Statements

Use if to evaluate an expression. If it is true, the block runs; otherwise else runs.

if True:

print("Hello Python If")

if 2 > 1:

print("2 if greater than 1")

When the condition is false:

if 1 > 2:

print("1 is greater than 2")

else:

print("1 is not greater than 2")

You can also chain elif:

if 1 > 2:

print("1 is greater than 2")

elif 2 > 1:

print("1 is not greater than 2")

else:

print("1 is equal to 2")

3. Loops / Iterators

Python supports while and for loops.

While loop example (prints 1‑10):

num = 1

while num <= 10:

print(num)

num += 1

Another while example with a condition variable:

loop_condition = True

while loop_condition:

print("Loop Condition keeps: %s" % (loop_condition))

loop_condition = False

For loop example (prints 1‑10):

for i in range(1, 11):

print(i)

4. Lists: Collections / Arrays / Data Structures

Lists store ordered collections of items. my_integers = [1, 2, 3, 4, 5] Access elements by index (0‑based):

print(my_integers[0])  # 1

print(my_integers[4])  # 5

Lists can hold any type, e.g., strings:

relatives_names = ["Toshiaki", "Juliana", "Yuji", "Bruno", "Kaio"]

print(relatives_names[4])  # Kaio

Add elements with append:

bookshelf = []

bookshelf.append("The EffectiveEngineer")

bookshelf.append("The 4 Hour WorkWeek")

print(bookshelf[0])  # The EffectiveEngineer

print(bookshelf[1])  # The 4 Hour WorkWeek

5. Dictionaries: Key‑Value Data Structure

Dictionaries map keys to values.

dictionary_example = {"key1": "value1", "key2": "value2", "key3": "value3"}

Example with personal information:

dictionary_ex = {"name": "Raymond", "nickname": "Luoyi", "nationality": "China"}

print("My name is %s" % (dictionary_ex["name"]))  # My name is Raymond

print("But you can call me %s" % (dictionary_ex["nickname"]))  # But you can call me Luoyi

print("And by the way I'm %s" % (dictionary_ex["nationality"]))  # And by the way I'm China

Add a new key‑value pair:

dictionary_ex["age"] = 38

print(dictionary_ex)  # {'name': 'Raymond', 'nickname': 'Luoyi', 'nationality': 'China', 'age': 38}

Author: 21CTO Community Note: Original content. Stay tuned for more Python development series.