Pip constraints files

In python, it is common practice to write all the application dependencies that are installed via pip into a separate text file called requirements.txt.

It’s good practice to fully specify package versions in your requirements file. And in our case, everything will be there — both direct dependencies of our application and dependency dependencies, etc.

But sometimes, especially on a long-lived project, it’s hard to understand what dependencies were original. It is necessary to update them on time, not depend on packages that are outdated or no longer needed for some reason.

For example, which of the following dependencies are the original ones?

# requirements.txt
numpy==1.17.4
pandas==0.24.2
python-dateutil==2.8.1
pytz==2019.3
six==1.13.0

Yes, it’s just pandas.

One of the mechanisms for separating dependencies is implemented using another text file called constants.txt.

It looks exactly like requirements.txt:

# constants.txt
numpy==1.17.4
python-dateutil==2.8.1
pytz==2019.3
six==1.13.0

Constraints files differ from requirements files in one key way: putting a package in the constraints file does not cause the package to be installed, whereas a requirements file will install all packages listed…