Python CLI & REPL

What it is

Python’s command-line interface (CLI) and interactive REPL (Read-Eval-Print Loop) for executing Python code, scripts, and managing packages.

Installation

Python is typically pre-installed on macOS and many Linux distributions. For Windows, download from python.org.

Linux (Debian/Ubuntu):

sudo apt update
sudo apt install python3 python3-pip

Linux (Fedora):

sudo dnf install python3 python3-pip

macOS (using Homebrew):

brew update
brew install python

Windows: Download the installer from python.org and run it. Ensure "Add Python to PATH" is checked during installation.

Verifying Installation:

python3 --version
pip3 --version

(On Windows, you might use python and pip instead of python3 and pip3 if that’s how they were added to your PATH).

Core Concepts

• REPL (Read-Eval-Print Loop): An interactive environment where you type Python code, it’s executed immediately, and the result is displayed. Great for testing small snippets or exploring.
• Scripts: .py files containing Python code that can be executed directly.
• Modules/Packages: Reusable collections of Python code. pip is the standard package installer for Python.

Commands / Usage

Running the Python REPL

Start the REPL:

python3

Starts an interactive Python 3 session.

Start REPL with specific version (if multiple installed):

python3.9

Starts an interactive session using Python 3.9.

Exit the REPL:

exit()

Exits the interactive Python session.

quit()

Exits the interactive Python session.

Import modules within the REPL:

import os

Imports the os module, making its functions available.

Executing Python Scripts

Run a Python script:

python3 my_script.py

Executes the Python code contained in my_script.py.

Run a script with command-line arguments:

python3 process_data.py input.csv output.txt --verbose

Executes process_data.py, passing input.csv, output.txt, and --verbose as arguments accessible via sys.argv.

Run a script and treat it as a command (if executable and has shebang):

./my_script.py

Executes my_script.py directly if it has a shebang line (e.g., #!/usr/bin/env python3) and execute permissions (chmod +x my_script.py).

Executing Python Code Directly

Execute a single Python expression:

python3 -c "print(2 + 2)"

Prints the result of the expression 2 + 2.

Execute multiple Python statements:

python3 -c "import sys; print(sys.version)"

Imports the sys module and prints the Python version.

Python Interpreter Options

Print Python version and exit:

python3 -V

Displays the installed Python version and exits.

Print Python version and exit (alternative):

python3 --version

Displays the installed Python version and exits.

Print detailed configuration information and exit:

python3 -V

Displays detailed information about the Python build and environment.

Run Python code from standard input:

echo "print('Hello from stdin')" | python3

Pipes the string "print('Hello from stdin')" to the Python interpreter, which executes it.

Run a script with -m (module execution):

python3 -m http.server 8000

Runs the built-in http.server module as a script, starting a web server on port 8000.

Enable site-specific sitecustomize.py:

python3 -S

Starts Python without importing the site module, which disables automatic import of site-specific modules like sitecustomize.

Don’t add user site-packages directory to sys.path:

python3 -s

Starts Python without adding the user site-packages directory to sys.path.

Add a directory to the Python path:

python3 -I

Starts Python in "isolated mode". The site module is not imported, and environment variables like PYTHONPATH and PYTHONHOME are ignored. This is useful for testing.

Disable Python’s site-specific optimizations:

python3 -O

Disables __debug__ and .pyc file generation.

Enable Python’s site-specific optimizations (and disable .pyc):

python3 -OO

Disables __debug__ and .pyc file generation, and also removes docstrings.

Print trace of statements being executed:

python3 -t

Prints a trace of statements being executed. Useful for debugging.

Warn about tabs instead of spaces:

python3 -tt

Warns about tabs instead of spaces in the input, and also errors on indentation where tabs and spaces are mixed.

Prevent addition of a directory to sys.path:

python3 -P <directory>

Adds <directory> to sys.path at the beginning. This is useful for testing module search paths.

Set the PYTHONHOME environment variable:

PYTHONHOME=/path/to/python python3

Sets the PYTHONHOME environment variable, which affects module searching.

Set the PYTHONPATH environment variable:

PYTHONPATH=/path/to/my/modules python3 my_script.py

Adds /path/to/my/modules to sys.path, allowing my_script.py to import modules from that directory.

Pip Usage (Package Management)

Install a package:

pip3 install requests

Installs the requests library from PyPI.

Install a specific version of a package:

pip3 install numpy==1.23.5

Installs version 1.23.5 of the numpy library.

Upgrade a package:

pip3 install --upgrade pandas

Upgrades the pandas library to the latest version.

Uninstall a package:

pip3 uninstall beautifulsoup4

Uninstalls the beautifulsoup4 library.

List installed packages:

pip3 list

Shows all installed Python packages and their versions.

Show information about an installed package:

pip3 show Flask

Displays details about the installed Flask package.

Install packages from a requirements file:

pip3 install -r requirements.txt

Installs all packages listed in requirements.txt.

Generate a requirements file:

pip3 freeze > requirements.txt

Creates requirements.txt containing a list of all installed packages and their exact versions.

Install packages in a virtual environment:

# Assuming you are inside an activated virtual environment
pip install django

Installs django into the currently active virtual environment.

Common Patterns

Running a script and capturing its output:

output=$(python3 my_script.py arg1)
echo "Script output: $output"

Executes my_script.py with arg1 and stores its standard output in the output variable.

Piping output to a Python script for processing:

cat data.txt | python3 process_lines.py

Reads data.txt, pipes its content line by line to process_lines.py, which can then process each line.

Executing a Python one-liner to manipulate text:

echo "hello world" | python3 -c "import sys; print(sys.stdin.read().upper())"

Takes "hello world", pipes it to Python, which reads it from stdin, converts it to uppercase, and prints it.

Running a Python script with a specific Python version:

python3.8 my_legacy_script.py

Ensures my_legacy_script.py runs with Python 3.8, useful for compatibility testing.

Using python -m to run installed scripts:

python3 -m jupyter notebook

Launches the Jupyter Notebook server using the installed jupyter module.

Creating and activating a virtual environment:

# Create
python3 -m venv myenv
# Activate (Linux/macOS)
source myenv/bin/activate
# Activate (Windows)
.\myenv\Scripts\activate

Creates an isolated Python environment named myenv, and then activates it so that pip and python commands operate within this environment.

Gotchas

• python vs python3: On many systems, python might still point to Python 2. Always explicitly use python3 and pip3 to ensure you are using Python 3.
• PATH Issues: If python3 or pip3 commands are not found, it likely means Python is not correctly added to your system’s PATH. Reinstall or manually add it.
• Virtual Environments: Forgetting to activate a virtual environment before installing packages can lead to clutter in your global Python installation. Always activate your venv or conda env.
• Shebang Line: For scripts intended to be run directly (./my_script.py), ensure the shebang line (#!/usr/bin/env python3) is correct and the file has execute permissions (chmod +x my_script.py).
• pip version: Ensure your pip is up-to-date (pip3 install --upgrade pip) for the best experience and security.
• Order of sys.path: Python searches for modules in the order they appear in sys.path. Be mindful of this if you have identically named modules in different locations.