To add one more third party dependency when you already have some is as simple as adding one more to whatever solution you are already using (eg another line in requirements or running a command).
When you have no third-party dependencies, then adding the first one requires picking amongst trade offs and lots of work. A subset of choices include using virtualenv, using pip, using higher layer tools, copying the code to the project, using a Python distribution that includes them, writing code to avoid needing the first dependency ...
* You have to document to humans and to the computer which of the approaches is being used
* Compiled extensions are a pain
* You have to consider multiple platforms and operating systems
* You have to consider Python version compatibility (eg third party could support fewer Python versions than the current code base)
* And the version compatibility of the tools used to reference the dependency
* And a way of checking license compatibility
* The dependency may use different test, doc, type checking etc tools so they may have to be added to the project workflow too
* Its makes it harder for collaborators since there is more complexity than "install Python and you are done"
I stand by my claim that the first paragraph (adding another dependency) is way less work, than the rest which is adding the very first one.
