pip install and save dependencies (i.e. Python equivalent of `npm install --save`)

pip-save.sh

# pip install and save to (and sort + dedupe) requirements.txt
function pip-save {
    # loop through all listed requirements
    for var in "$@"
    do
        # attempt to install it
        pip install $var

        # add it to the requirements.txt file
        pip freeze | grep -i "^$var=" >> requirements.txt
    done

    # sort requirements.txt and remove duplicates
    sort -u requirements.txt -o requirements.txt
}

Good tip. You should take a look on Poetry as an easy npm like python all-in-one environment/dependencies manager.
It is great and so easy to use. Forget requirements.txt, conda, pipenv and messing your python environment.