Python

List

  • You can access values using negative indexes, which will go from right to left. The last element would be at index -1.
  • You can get multiple values with slicing, i.e. l[start:end]. start is inclusive, and end is exclusive.
    • If you are just slicing from the beginning, you can leave it out, e.g. l[:3] will slice from 0 to 3 (exclusive).
    • Similarly, if you are just slicing up to the end of the list, you can leave it out, e.g. l[2:] will slice from 2 (inclusive) to the end.
  • One correct way to copy a list is b = a[:], since slicing gives you a new list.
  • You can combine two lists using +.

Build System

Virtual Environments

  • Key points

    • Virtual environments should be considered as similar to Maven POM to project dependencies, therefore one project per environment.
    • One virtual environment should be dedicated to one project for dependency management purposes.

  • Workflow

    1. Prepare pyenv-installed Python versions

      pyenv install $python_version

    2. Switch to the desired Python version for the current directory

      pyenv local $python_version

      This will create a .python-version file in the current directory (can be version controlled with git).

    3. Prepare Poetry project config (pyproject.toml)

      Make sure to specify the project name and Python version, which will be used in the name of the virtual environment.

      # Create a new Poetry project file
poetry init
      # Example: pyproject.toml
[tool.poetry]
name = "03-visualizing-financial-time-series"
 
[tool.poetry.dependencies]
python = ">=3.9,<3.10"

    4. Switch to the virtual environment using the specified Python version, must meet version constraint in pyproject.toml.

      poetry env use $python_version

      This will create a virtual environment if it does not exist.

    5. Resolve project dependencies

      poetry lock

      This will create a poetry.lock file in the current directory (can be version controlled with git).

    6. Install project dependencies

      poetry install

    7. Use the virtual environment in IDE if needed

      # Get the path of the activated virtual environment
poetry env list --full-path | grep Activated | awk '{print $1}'

Virtual Environments - Cheatsheet

venv - version control

  • venv is bound to local Python installation, so it is not recommended to version control it.
  • Build system declares the Python version to use, and venv is responsible for sourcing the correct version of Python installation.

Installing Packages from requirements.txt

pip install -r requirements.txt

Create a requirements.txt file with all the packages installed in the current environment

pip freeze > requirements.txt

pyenv

pyenv - Cheatsheet

pyenv set up

  • Homebrew

    brew install pyenv

  • zsh

    # env
export PYENV_ROOT="$HOME/.pyenv"
export PATH="$PYENV_ROOT/bin:$PATH"
    # init
eval "$(pyenv init -)"
List all available Python versions
pyenv install -l
List all installed Python versions
pyenv versions
Determine which Pyenv-installed Python executable will actually be used
pyenv which $python_executable
# e.g. pyenv which python3
Select a Pyenv-installed Python for the current shell
pyenv shell $python_version
Select a Pyenv-installed Python for the current directory
pyenv local $python_version

Poetry

  • Key points

    • All virtual environments created by Poetry are stored in virtualenvs.path in Poetry config.

Poetry - Cheatsheet

Poetry set up

Installation

  • Homebrew

    brew install poetry
Create a project file template in the current directory
poetry init
Determine Python version of the current environment
poetry env info
List all environments of the current project
poetry env list --full-path
Switch to a different Python version

Will create a new environment with the specified Python version if it doesn't already exist.

poetry env use $python_version
Delete an environment
# Get the name of the environment
env_name=$(poetry env list | grep $python_version | awk '{print $1}')
 
# Delete the environment
poetry env remove $env_name

Note: you can only delete an environment of the current project.

Display the current config
poetry config --list

uv (opens in a new tab)

uv - Cheatsheet

Convert an existing project to uv
# pyproject.toml
[project]
name = "my_project"
version = "0.1.0"
requires-python = ">=3.9,<3.10"
dependencies = [
    "alpha-vantage>=3.0.0",
    "pandas>=2.0.0"
]
# Test the build
uv build
Add a new dependency
uv add alpha-vantage
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