Python 101

In this chapter, we will cover the fundamental building blocks of Python programming. These basics will prepare you for more advanced topics in numerical computing later on.

Hello, World!

Let’s start with a classic. Create a file hello_world.py and write:

Code

print("Hello, world!")

Output

>> Hello, world!

Run your first script using your favorite IDE and by simply typing in your terminal

$ python hello_world.py

Variables and basic types

Python supports several basic data types. You can create variables and assign values to them like this:

x = 42        # integer
pi = 3.14     # float
name = "Alice"  # string
is_valid = True  # boolean

You can check the type of a variable using the type() function:

Code

print(type(x))
print(type(pi))
print(type(name))
print(type(is_valid))

Output

>>> print(type(x))
<class 'int'>
>>> print(type(pi))
<class 'float'>
>>> print(type(name))
<class 'str'>
>>> print(type(is_valid))
<class 'bool'>

Strings

Strings are sequences of characters. You can work with them in several ways:

Code

greeting = "Hello"
name = "Alice"
message = f"{greeting}, {name}!"  # f-strings
print(message)

Output

>>> greeting = "Hello"
>>> name = "Alice"
>>> message = f"{greeting}, {name}!"  # f-strings
>>> print(message)
Hello, Alice!

Useful string methods include:

Code

text = "  Python Basics  "
text.strip()         # Remove whitespace
text.lower()         # Convert to lowercase
text.split()         # Split into a list of words
"-".join(["A", "B"]) # Join into a string

Output

>>> text = "  Python Basics  "
>>> text.strip()
'Python Basics'
>>> text.lower()
'  python basics  '
>>> text.split()
['Python', 'Basics']
>>> "-".join(["A", "B"])
'A-B'

Collections

Lists

Lists are ordered collections of items:

Code

fruits = ["apple", "banana", "cherry"]
fruits.append("kiwi")      # Add an item
print(fruits[0])           # Access by index

Output

>>> fruits = ["apple", "banana", "cherry"]
>>> fruits.append("kiwi")      # Add an item
>>> print(fruits[0])           # Access by index
apple

Useful methods include:

fruits.insert(1, "mango")  # Insert at position
fruits.remove("banana")    # Remove first occurrence
fruits.pop()               # Remove and return last item
fruits.index("apple")      # Find index of item
fruits.count("apple")      # Count occurrences
fruits.sort()              # Sort in-place
fruits.reverse()           # Reverse the list
fruits.copy()              # Shallow copy
fruits.clear()             # Remove all items

List comprehensions offer a concise way to create lists:

squares = [x**2 for x in range(5)]

is the same as doing

squares = [0, 1, 4, 9, 16]

Tuples

Tuples are similar to lists but cannot be changed (immutable):

point = (2, 3)
x, y = point  # tuple unpacking

Dictionaries

Dictionaries store key-value pairs:

person = {"name": "Alice", "age": 30}
print(person["name"])    # Access value by key

Useful methods also include:

person.keys()        # ['name', 'age']
person.values()      # ['Alice', 30]
person.items()       # dict_items([('name', 'Alice'), ('age', 30)]) -> This is more tricky, it returns an iterator
person.get("age")    # Safer access
person.get("height", "unknown")  # With default
person.update({"age": 31})       # Update or add key-value pairs
person.pop("age")    # Remove and return value
person.popitem()     # Remove and return a key-value pair
person.clear()       # Remove all items

Sets

Sets store unique elements:

unique_values = set([1, 2, 2, 3])
print(unique_values) # {1, 2, 3}

Useful methods include:

unique_values.add(4)
unique_values.pop()               # Remove and return an arbitrary element
unique_values.remove(2)           # Remove element, error if not found
unique_values.discard(5)          # Remove element if exists, no error
unique_values.union({4, 5})       # Set union
unique_values.intersection({2, 3})# Set intersection
unique_values.difference({1})     # Set difference
unique_values.clear()             # Remove all elements

Control flow

Conditional statements

Use if, elif, and else to control what your program does based on conditions:

if age > 18:
    print("Adult")
elif age == 18:
    print("Just turned adult")
else:
    print("Minor")

Loops

For loops

for fruit in fruits:
    print(fruit)

While loops

i = 0
while i < 3:
    print(i)
    i += 1

Using range, enumerate, and zip

for i in range(3):
    print(i)

for i, fruit in enumerate(fruits):
    print(i, fruit)

colors = ["red", "yellow", "purple"]
for fruit, color in zip(fruits, colors):
    print(f"{fruit} is {color}")

Functions

Functions are blocks of code that can be reused:

def greet(name):
    return f"Hello, {name}!"

You can define default values for parameters:

def greet(name="world"):
    return f"Hello, {name}!"

Importing modules

Modules allow you to use external code. Here’s how to import some standard library modules.

The math module provides mathematical functions such as square roots, trigonometry, exponentials, and constants like π.

import math

print(math.sqrt(16))      # Ssquare root
print(math.sin(math.pi))  # sine of pi
print(math.log(10))       # natural logarithm

You can also import only the functions you need:

from math import sqrt, pi

print(sqrt(25))
print(pi)

The os module gives you access to operating system features such as file and directory handling.

import os

print(os.getcwd())      # Get current working directory
print(os.listdir())     # List files in the current directory

Other useful examples:

os.mkdir("test_folder")     # Create a new directory
os.rename("old.txt", "new.txt")  # Rename a file

Sometimes, it’s helpful to give a shorter name to a module using as:

import numpy as np  # common convention for NumPy

In the next chapter, we will explore numerical computing using NumPy and SciPy.