Language cheatsheet

Python 3 Cheatsheet

Run scripts, manage virtual environments and packages, and test and profile Python code.

Running Python

Check the Python version — Print the interpreter version.
Run a script — Run a Python file. On Windows, py or python usually replaces python3.
Run a one-line command — Run a snippet from the command line.
Open the interactive shell — Start the REPL. Exit with exit() or Ctrl+D.
Serve the current folder — Start a simple web server on port 8000.
Pretty-print JSON — Format a JSON file for reading.

Create a virtual environment — Create an isolated environment in the .venv folder.
Activate it — Use the environment's Python and packages in this shell. Once active, python and pip refer to the environment, so the pip commands below use python. On Windows run .venv\Scripts\activate.
Deactivate it — Leave the virtual environment.
Upgrade pip — Upgrade pip inside the active virtual environment.

Install a package — Install a package from PyPI.
Install a specific version — Install an exact version instead of the latest one.
Install from a requirements file — Install everything listed in requirements.txt.
Upgrade a package — Install the newest version of a package.
Uninstall a package — Remove a package from the environment.
List installed packages — Show packages and their versions.
Show package details — Show a package's version, dependencies and location.
Find outdated packages — List packages that have newer versions.
Write a requirements file — Save the installed packages and exact versions.
Check for broken dependencies — Verify that installed packages have compatible dependencies.

Run tests with pytest — Run every test found under the current folder. Needs pytest installed.
Run one test — Run a single test function and show print output.
Run unittest tests — Find and run tests using the standard library.
Debug a script — Run a script under the pdb debugger.
Time a snippet — Measure how long a statement takes.
Profile a script — Show which functions take the most time.

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