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Python

Python is a general-purpose, beginner-friendly programming language widely used in research, data science, web development, automation, and AI. Its readable syntax makes it an excellent first language, and its vast ecosystem of libraries makes it powerful enough for professional use.

Installation​

The recommended way to manage Python versions is with pyenv. However, you can also install Python directly:

  • Windows — Download from python.org or via the Microsoft Store
  • macOS — brew install python (via Homebrew)
  • Linux — Usually pre-installed; or sudo apt install python3 (Debian/Ubuntu)

Verify your installation:

python --version
# or
python3 --version
tip

On many systems, python refers to Python 2 (outdated) and python3 refers to Python 3. Always use Python 3. If python on your system is Python 2, use python3 and pip3 instead of python and pip.

Running Python​

Interactive Mode (REPL)​

Type python to open an interactive session. This is great for quickly testing small pieces of code:

$ python
>>> 2 + 2
4
>>> name = "world"
>>> print(f"Hello, {name}!")
Hello, world!
>>> exit()

Running a Script​

Write your code in a .py file and run it:

python my_script.py

Python Basics​

Variables and Types​

# Numbers
age = 21
gpa = 3.8

# Strings
name = "Alice"
greeting = f"Hello, {name}!"  # f-strings for embedding variables

# Booleans
is_enrolled = True

# None (equivalent to null)
result = None

# Lists (ordered, mutable)
grades = [95, 87, 92, 78]

# Dictionaries (key-value pairs)
student = {"name": "Alice", "age": 21, "gpa": 3.8}

# Tuples (ordered, immutable)
coordinates = (44.4759, -73.2121)

# Sets (unordered, unique values)
unique_tags = {"python", "research", "open-source"}

Control Flow​

# If/elif/else
score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
else:
    grade = "C"

# For loop
for item in ["apple", "banana", "cherry"]:
    print(item)

# For loop with range
for i in range(5):
    print(i)  # prints 0, 1, 2, 3, 4

# While loop
count = 0
while count < 3:
    print(count)
    count += 1

Functions​

def greet(name, greeting="Hello"):
    """Return a greeting string. (This is a docstring.)"""
    return f"{greeting}, {name}!"

print(greet("Alice"))           # "Hello, Alice!"
print(greet("Bob", "Hi"))       # "Hi, Bob!"

Error Handling​

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")
except Exception as e:
    print(f"Unexpected error: {e}")
finally:
    print("This always runs")

Reading and Writing Files​

# Write a file
with open("output.txt", "w") as f:
    f.write("Hello, file!\n")

# Read a file
with open("output.txt", "r") as f:
    contents = f.read()
    print(contents)

# Read line by line
with open("data.txt", "r") as f:
    for line in f:
        print(line.strip())

The with statement automatically closes the file when the block exits, even if an error occurs.

Package Management with pip​

pip is Python's package manager. It installs libraries from the Python Package Index (PyPI).

# Install a package
pip install requests

# Install a specific version
pip install requests==2.31.0

# Install from a requirements file
pip install -r requirements.txt

# List installed packages
pip list

# Show info about a package
pip show requests

# Uninstall a package
pip uninstall requests

# Save current packages to a requirements file
pip freeze > requirements.txt

See the Package Management concepts page for more detail.

Virtual Environments​

Always use a virtual environment for projects so their dependencies don't conflict with each other.

# Create a virtual environment
python -m venv venv

# Activate it (macOS/Linux)
source venv/bin/activate

# Activate it (Windows)
venv\Scripts\activate

# Your prompt will change to show (venv)
# Now install packages — they go into the venv, not globally
pip install pandas matplotlib

# Deactivate when done
deactivate

Add venv/ to your .gitignore.

Common Libraries​

Data and Science​

LibraryPurpose
numpyNumerical arrays and math
pandasData manipulation with DataFrames
matplotlibPlotting and visualization
scipyScientific computing
scikit-learnMachine learning

Web and APIs​

LibraryPurpose
requestsMaking HTTP requests
flaskLightweight web framework
fastapiModern, fast API framework
beautifulsoup4HTML parsing / web scraping

Utilities​

LibraryPurpose
pathlibModern file path handling (built-in)
jsonJSON parsing (built-in)
osOperating system interface (built-in)
argparseCommand-line argument parsing (built-in)
loggingApplication logging (built-in)

Style Guidelines​

Python has an official style guide called PEP 8. The key points:

  • Use 4 spaces for indentation (not tabs)
  • Variable and function names use snake_case
  • Class names use PascalCase
  • Constants use ALL_CAPS
  • Lines should be 79 characters or fewer
  • Add docstrings to all functions, classes, and modules

Automate formatting with tools like Black or Ruff:

pip install black
black my_script.py

Type Hints​

Python supports optional type hints, which make code more readable and enable better IDE support and error checking:

def calculate_average(numbers: list[float]) -> float:
    return sum(numbers) / len(numbers)

def greet(name: str, times: int = 1) -> str:
    return (f"Hello, {name}!\n") * times

References​