What do you mean? If you do "pip freeze >constraints.txt", that locks the versions of all installed packages, no matter where they came from.

As an example, let's create a venv and install some older versions of Django and its dependencies (current versions are 0.4.2, 3.5.2 and 4.0.6)

  $ python3 -m venv env1
  $ ./env1/bin/pip install sqlparse==0.4.0 asgiref==3.5.0 django=4.0.0

...and also Flask just to complicate the constraints file for the example:

  $ ./env1/bin/pip install flask

Lock all dependency versions in constraints.txt:

  $ ./env1/bin/pip freeze >constraints.txt 

Create a requirements file that specifies just "django" and references the constraints file:

  $ echo '-c constraints.txt' >requirements.txt
  $ echo 'django' >>requirements.txt

  $ cat requirements.txt 
  -c constraints.txt
  django

  $ cat constraints.txt 
  asgiref==3.5.0
  click==8.1.3
  Django==4.0
  Flask==2.1.3
  itsdangerous==2.1.2
  Jinja2==3.1.2
  MarkupSafe==2.1.1
  sqlparse==0.4.0
  Werkzeug==2.1.2

Now we can create another venv with the exact same versions of Django and all its dependencies (but not Flask or its dependencies) using just pip and the requirements file:

  $ python3 -m venv env2
  $ ./env2/bin/pip install -r requirements.txt 
  $ ./env2/bin/pip freeze
  asgiref==3.5.0
  Django==4.0
  sqlparse==0.4.0