How to freeze a requirement with pipenv?

Pipenv do natively implement freezing requirements.txt. It is as simple as:

pipenv lock -r > requirements.txt

Assuming you have your virtual environment activated, you have three simple approaches. I will list them from less verbose to more verbose.

pip

$ pip freeze > requirements.txt

pip3

$ pip3 freeze > requirements.txt

If a virtual environment is active, pip is most certainly equivalent to pip3.

pipenv run

$ pipenv run pip freeze > requirements.txt
$ pipenv run pip3 freeze > requirements.txt

pipenv run spawns a command installed into the virtual environment, so these commands are equivalent to the ones run without pipenv run. Once again, it is assumed that your virtual environment is active.