Implement pyvenv-tracking-mode

The pyvenv-tracking-mode changes the virtual environment on changing focus
between buffers.

CHANGELOG.develop

@@ -286,6 +286,7 @@ sane way, here is the complete list of changed key bindings
 - New keybinding for =org-table-field-info= ~SPC m t f~
   (thanks to Dominik Schrempf)
 ***** Python
+- fix/implement pyvenv-tracking-mode (thanks to Daniel Nicolai)
 - Key bindings (thanks to Danny Freeman):
   - Changed ~SPC m s F~ to ~SPC m e F~ for =lisp-eval-defun-and-go=
   - Changed ~SPC m s R~ to ~SPC m e R~ for =lisp-eval-region-and-go=

layers/+lang/python/README.org

@@ -32,6 +32,7 @@
   - [[#pyvenv-pyenv-and-pipenv][Pyvenv, pyenv and pipenv]]
 - [[#management-of-python-versions-and-virtual-environments][Management of Python versions and virtual environments]]
   - [[#manage-virtual-environments-with-pyvenv][Manage virtual environments with pyvenv]]
+    - [[#automatic-activation-of-local-virtual-environment][Automatic activation of local virtual environment]]
   - [[#manage-multiple-python-versions-with-pyenv][Manage multiple Python versions with pyenv]]
     - [[#automatic-activation-of-local-pyenv-version][Automatic activation of local pyenv version]]
   - [[#manage-environments-and-packages-with-pipenv][Manage environments and packages with pipenv]]
@@ -364,6 +365,26 @@ by the [[https://github.com/jorgenschaefer/pyvenv][pyvenv]] package. It provides
 | ~SPC m V d~ | deactivate active virtual environment           |
 | ~SPC m V w~ | work on virtual environment in =WORKON_HOME=    |
 
+*** Automatic activation of local virtual environment
+By default Spacemacs uses the [[https://github.com/jorgenschaefer/pyvenv][pyvenv]] package to manage virtual environments.
+Additionally it uses =pyvenv-tracking-mode= to activate a buffer's local virtual
+environment on change of focus. Pyvenv determines which virtual environment to
+use from the value of the =pyvenv-workon= or the =pyvenv-activate=
+buffer-local-variable. Spacemacs scans the project directory for a pattern
+=.venv=. If the found =.venv= is a directory then it sets that directory as the
+local virtual environment path. If the =.venv= pattern is a file then it checks if
+its first line matches an existing path and if so, it sets it as the local
+virtual environment path. Finally it checks if it finds an existing directory
+with the name of the first line in the ~pyvenv-workon-home~ directory. By default
+Spacemacs scans for a virtual environment and sets the local =pyvenv-workon= or
+the =pyvenv-activate= variables on visiting a file, but switches virtual
+environment on every change of focus using the local variables. The buffer
+tracking behavior can be disabled by setting the value of the customizable
+variable =pyvenv-tracking-mode= to =nil=. The scanning behavior can be set via the
+variable =python-auto-set-local-pyvenv-virtualenv= to:
+- =on-visit= (default) set the virtualenv when you visit a python buffer,
+- =on-project-switch= set the virtualenv when you switch projects,
+- =nil= to disable.
 ** Manage multiple Python versions with pyenv
 If you need multiple Python versions (e.g. Python 2 and Python 3) then take a
 look at [[https://github.com/yyuu/pyenv][pyenv]]. It enables the installation and management of multiple
@@ -390,15 +411,6 @@ the pyenv version. The behavior can be set with the variable
 - =on-project-switch= set the version when you switch projects,
 - =nil= to disable.
 
-The same is also possible on pyvenv with a file called =.venv= that specifies
-either an absolute or relative path to a virtualenv directory. A relative path
-is checked relative to the location of =.venv=, then relative to
-=pyvenv-workon-home=. =.venv= can also be a virtualenv directory. The behavior
-can be set with the variable =python-auto-set-local-pyvenv-virtualenv= to:
-- =on-visit= (default) set the virtualenv when you visit a python buffer,
-- =on-project-switch= set the virtualenv when you switch projects,
-- =nil= to disable.
-
 ** Manage environments and packages with pipenv
 [[https://pipenv.kennethreitz.org/en/latest/][Pipenv]] is the new standard tool to manage your virtual environments. It can act as
 a replacement for both =pyenv= and =venv= on a per-repository basis. An overview

layers/+lang/python/funcs.el

@@ -231,21 +231,22 @@ location of \".venv\" file, then relative to pyvenv-workon-home()."
   (let ((root-path (locate-dominating-file default-directory ".venv")))
     (when root-path
       (let ((file-path (expand-file-name ".venv" root-path)))
-        (if (file-directory-p file-path)
-            (pyvenv-activate file-path)
-          (let* ((virtualenv-path-in-file
-                  (with-temp-buffer
-                    (insert-file-contents-literally file-path)
-                    (buffer-substring-no-properties (line-beginning-position)
-                                                    (line-end-position))))
-                 (virtualenv-abs-path
-                  (if (file-name-absolute-p virtualenv-path-in-file)
-                      virtualenv-path-in-file
-                    (format "%s/%s" root-path virtualenv-path-in-file))))
-            (if (file-directory-p virtualenv-abs-path)
-                (pyvenv-activate virtualenv-abs-path)
-              (pyvenv-workon virtualenv-path-in-file))))))))
-
+        (cond ((file-directory-p file-path)
+               (pyvenv-activate file-path) (setq-local pyvenv-activate file-path))
+              (t (let* ((virtualenv-path-in-file
+                         (with-temp-buffer
+                           (insert-file-contents-literally file-path)
+                           (buffer-substring-no-properties (line-beginning-position)
+                                                           (line-end-position))))
+                        (virtualenv-abs-path
+                         (if (file-name-absolute-p virtualenv-path-in-file)
+                             virtualenv-path-in-file
+                           (format "%s/%s" root-path virtualenv-path-in-file))))
+                   (cond ((file-directory-p virtualenv-abs-path)
+                          (pyvenv-activate virtualenv-abs-path)
+                          (setq-local pyvenv-activate virtualenv-abs-path))
+                         (t (pyvenv-workon virtualenv-path-in-file)
+                            (setq-local pyvenv-workon virtualenv-path-in-file))))))))))
 
 ;; Tests
 

layers/+lang/python/packages.el

@@ -267,6 +267,7 @@
     :defer t
     :init
     (progn
+      (add-hook 'python-mode-hook #'pyvenv-tracking-mode)
       (pcase python-auto-set-local-pyvenv-virtualenv
         (`on-visit
          (dolist (m spacemacs--python-pyvenv-modes)