What is pip's equivalent of `npm install package --save-dev`?

There isn't an equivalent with pip.

Best way is to pip install package && pip freeze > requirements.txt

You can see all the available options on their documentation page.

If it really bothers you, it wouldn't be too difficult to write a custom bash script (pips) that takes a -s argument and freezes to your requirements.txt file automatically.

Edit 1

Since writing this there has been no change in providing an auto --save-dev option similar to NPM however Kenneth Reitz (author of requests and many more) has released some more info about a better pip workflow to better handle pip updates.

Edit 2

Linked from the "better pip workflow" article above it is now recommended to use pipenv to manage requirements and virtual environments. Having used this a lot recently I would like to summarise how simple the transition is:

Install pipenv (on Mac)

brew install pipenv

pipenv creates and manages it's own virtual environments so in a project with an existing requirements.txt, installing all requirements (I use Python3.7 but you can remove the --three if you do not) is as simple as:

pipenv --three install

Activating the virtualenv to run commands is also easy

pipenv shell

Installing requirements will automatically update the Pipfile and Pipfile.lock

pipenv install <package>

It's also possible to update out-of-date packages

pipenv update

I highly recommend checking it out especially if coming from a npm background as it has a similar feel to package.json and package-lock.json

This simple line is a starting point. You can easily built a bash command to reuse the PACKAGE in the line.

pip install PACKAGE && pip freeze | grep PACKAGE >> requirements.txt

Thanks to @devsnd for the simple bash function example:

function pip-install-save {     pip install $1 && pip freeze | grep $1 >> requirements.txt}

To use it, just run:

pip-install-save some-package

I've created python package that wraps around the actual pip called pipm. All pip commands will work as it is, plus they will be reflected in the requirements file. Unlike pip-save (https://github.com/kgritesh/pip-save unactive for 5 years) similar tool I found and wasn't able to use, it can handle many files and environments(test, dev, production, etc. ). It also has command to upgrade all/any of your dependencies.

installation

pipm install pkg-name

installation as development dependency

pipm install pkg-name --dev

installation as testing dependency

pipm install pkg-name --test

removal

pipm uninstall pkg-name

update all your dependencies

pipm update

install all your dependencies from the requirements file

pipm install

including development dependencies

pipm install --dev