Makefile should use python from venv as default

Makefile

@@ -58,14 +58,15 @@ EXCLUDED := \
 	library/xdrlib.po
 
 # Internal variables
-
 UPSTREAM := https://github.com/python/cpython
-
-PYTHON := $(shell which python3)
-MODE := html
 POSPELL_TMP_DIR := .pospell/
-JOBS := auto
-SERVE_PORT :=
+
+# You can set these variables from the command line.
+VENVDIR      = ./venv
+PYTHON       = $(VENVDIR)/bin/python3
+MODE         = html
+JOBS         = auto
+SERVE_PORT   =
 
 # Detect OS
 
@@ -90,6 +91,8 @@ all: ensure_prerequisites
 	mkdir -p locales/$(LANGUAGE)/LC_MESSAGES/
 	$(CP_CMD) -u --parents *.po */*.po locales/$(LANGUAGE)/LC_MESSAGES/
 	$(MAKE) -C venv/cpython/Doc/ \
+	  PYTHON=$(abspath $(PYTHON)) \
+	  VENVDIR=$(abspath $(VENVDIR)) \
 	  SPHINXOPTS='-j$(JOBS)             \
 	  -D locale_dirs=$(abspath locales) \
 	  -D language=$(LANGUAGE)           \
@@ -109,31 +112,64 @@ venv/cpython/.git/HEAD:
 
 .PHONY: ensure_prerequisites
 ensure_prerequisites: venv/cpython/.git/HEAD
-	@if ! (blurb help >/dev/null 2>&1 && sphinx-build --version >/dev/null 2>&1); then \
+	@if ! ($(VENVDIR)/bin/blurb help >/dev/null 2>&1 && $(VENVDIR)/bin/sphinx-build --version >/dev/null 2>&1); then \
 	    git -C venv/cpython/ checkout $(BRANCH); \
-	    echo "You're missing dependencies please install:"; \
+	    echo "You're missing dependencies please install using:"; \
 	    echo ""; \
-	    echo "  python -m pip install -r requirements.txt -r venv/cpython/Doc/requirements.txt"; \
+	    echo "make update_venv"; \
 	    exit 1; \
 	fi
 
+.PHONY: $(PYTHON)
+$(PYTHON):
+	@if [ ! -x $(PYTHON) ] || \
+		[ "`$(PYTHON) --version | cut --fields=1 --delimiter='.'`" != "Python 3" ] ; then \
+		echo "You don't have a suitable Python in: " $(abspath $(dir $(PYTHON))); \
+		echo "The recommended way to proceed is to install a virtual env in: "$(abspath $(VENVDIR)); \
+		echo "You can achieve this with:"; \
+		echo "make install_venv PYTHON_BASE=/your/version/of/python"; \
+		echo ""; \
+		echo "or you can customize the variable PYTHON of the Makefile"; \
+		exit 1; \
+	fi
+
+PYTHON_BASE = $(shell which python)
+PYTHON_BASE_VERSION = $(shell $(PYTHON_BASE) --version | cut --fields=1 --delimiter='.')
+
+.PHONY: install_venv
+install_venv:
+ifneq ($(PYTHON_BASE_VERSION), 	Python 3)
+	@echo $(PYTHON_BASE) " is not a Python executable" ; \
+	echo $(PYTHON_BASE_VERSION) ; \
+	exit 1
+endif
+	$(PYTHON_BASE) -m venv $(VENVDIR)
+	@echo "Python virtual env installed in $(abspath $(VENVDIR))"
+	make update_venv
+
+.PHONY: update_venv
+update_venv: $(PYTHON) venv/cpython/.git/HEAD
+	$(PYTHON) -m pip install -r requirements.txt -r venv/cpython/Doc/requirements.txt
 
 .PHONY: serve
 serve:
 ifdef SERVE_PORT
-	$(MAKE) -C venv/cpython/Doc/ serve SERVE_PORT=$(SERVE_PORT)
+	$(MAKE) -C venv/cpython/Doc/ \
+	  PYTHON=$(abspath $(PYTHON)) \
+	  SERVE_PORT=$(SERVE_PORT) \
+	  serve
 else
-	$(MAKE) -C venv/cpython/Doc/ serve
+	$(MAKE) -C venv/cpython/Doc/ PYTHON=$(abspath $(PYTHON)) serve
 endif
 
 .PHONY: todo
 todo: ensure_prerequisites
-	potodo --exclude venv .venv $(EXCLUDED)
+	$(VENVDIR)/bin/potodo --exclude venv .venv $(EXCLUDED)
 
 .PHONY: wrap
 wrap: ensure_prerequisites
 	@echo "Re wrapping modified files"
-	powrap -m
+	$(VENVDIR)/bin/powrap -m
 
 SRCS = $(shell git diff --name-only $(BRANCH) | grep '.po$$')
 # foo/bar.po => $(POSPELL_TMP_DIR)/foo/bar.po.out
@@ -145,15 +181,15 @@ spell: ensure_prerequisites $(DESTS)
 $(POSPELL_TMP_DIR)/%.po.out: %.po dict
 	@echo "Pospell checking $<..."
 	mkdir -p $(@D)
-	pospell -p dict -l fr_FR $< && touch $@
+	$(VENVDIR)/bin/pospell -p dict -l fr_FR $< && touch $@
 
 .PHONY: fuzzy
 fuzzy: ensure_prerequisites
-	potodo -f --exclude venv .venv $(EXCLUDED)
+	$(VENVDIR)/bin/potodo -f --exclude venv .venv $(EXCLUDED)
 
 .PHONY: verifs
 verifs: spell
-	powrap --check --quiet *.po */*.po
+	$(VENVDIR)/bin/powrap --check --quiet *.po */*.po
 
 .PHONY: clean
 clean: