add python projects with Makefiles

system/Makefiles/python_projects.md

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+---
+title: "Python Projects with Makefiles"
+tags: [ "tutorial", "system", "makefiles", "graphviz", "python" ]
+requires: [ "Makefiles" ]
+---
+
+If you have a python script which requires a packages - e.g. `graphviz` - you can automate the setup with a `Makefile`.
+The `Makefile` will:
+
+1. Make three copies of an 8-line installer script.
+2. Use one of the installer script to install local packages.
+3. Install a symbolic link to python.
+3. Make a script called `activate`, which tells python to use 
+
+# Context
+
+Python coders don't like updating their projects, they just expect everyone to install the same version of everything that they have.
+Historically, people dealt with this by installing only half a dozen copies of `graphviz`; but now each python project uses a local environment, with a local copy of `graphviz`, which means everyone gets to install a new copy of `graphviz` every time they try out a project.
+
+Downloading 40MB of software for each 40-line script you write is called 'virtual environments' because it sounds cool.
+We can make it even cooler with `make`, but not yet, because python - like the fae of old - will not fetch anything until you know its true name.
+
+# Setup
+
+```sh
+command -v python
+realpath `!!`
+```
+
+You must reveal that true path, because `python` is always a relative symbolic link, to an absolute symbolic link, which leads to a shortcut.
+We can finally let `make` know how to invoke python, and where it will install `graphviz`.
+
+If your python's version is '3.14', then python needs its packages placed in `${somewhere}/lib/python3.14/site-packages/`.
+You must create a new, local, name for these packages, because - like the fey of old - python demands a private name in return for revealing its true name.
+
+I'll call mine `camelot`, because the path is long and arduous.
+Set up the Makefiles like this:
+
+```make
+py_link != command -v python
+py != realpath $(py_link)
+version != basename $(py)
+
+virtenv = camelot
+```
+
+Now you can ask for a local `pip` script, which can install the python packages:
+
+```make
+    [...]
+
+$(virtenv)/bin/pip:
+    $(py) -m venv $(virtenv)
+```
+
+Finally, list the packages you want in `requirements.txt`, and make `pip` install from it.
+
+
+```sh
+ppkg=graphviz
+echo ${ppkg} > requirements.txt
+```
+
+```make
+    [...]
+
+pkgs = $(virtenv)/lib/$(version)/site-packages/
+
+$(pkgs): $(virtenv)/bin/pip
+$(pkgs): requirements.txt
+    $(virtenv)/bin/pip install -r $<
+```
+
+The complete Makefile looks like this:
+
+
+```make
+all: .default
+
+py_link != command -v python
+py != realpath $(py_link)
+version != basename $(py)
+
+virtenv = camelot
+
+$(virtenv)/bin/pip:
+    $(py) -m venv $(virtenv)
+
+pkgs = $(virtenv)/lib/$(version)/site-packages/
+
+$(pkgs): $(virtenv)/bin/pip
+$(pkgs): requirements.txt
+    $(virtenv)/bin/pip install -r $<
+
+.PHONY: .default
+.default: $(pkgs)
+```
+