reworked the windows launcher, should be more robust now

Filmatyk.bat

@@ -1,16 +1,62 @@
-REM fw-local launch script
+REM Filmatyk launch script
 
 @ECHO OFF
 
-REM Try to use python3 explicitly, only falling back to python.exe on problems
-WHERE python3
-IF %ERRORLEVEL% NEQ 0 (
-  SET pyexe=python
-) ELSE (
-  SET pyexe=python3
+REM Now the following behavior of the Windows shell makes it understandable why
+REM why most sane people despise it (just in case having to say "REM" to denote
+REM comments was not enough). Turns out that - by default - ERRORLEVEL variable
+REM will NOT be updated when within an IF clause. So when we want to check some
+REM situation and - while handling that one - check some different one, results
+REM of that other check will NOT be readable.
+REM Solution to that is to enable a "delayed expansion" which, simply speaking,
+REM makes the shell behave as a sane programmer with prior experience with sane
+REM languages would expect.
+REM http://batcheero.blogspot.com/2007/06/how-to-enabledelayedexpansion.html
+SETLOCAL ENABLEDELAYEDEXPANSION
+
+REM Check Python version.
+REM It would be best if we could use pythonw as it does not open any additional
+REM terminal, so we check for that first. If this is not found, we look for the
+REM default python executable. In both cases, a version check is made to ensure
+REM that they satisfy the minimum version (3.6). If neither does, one final try
+REM is to search for python3 and hope that will be a valid interpreter.
+REM Python executable search order:
+SET versions=pythonw python python3
+REM Remember the closest we got to a working version. Worst to least dangerous:
+REM no Python found at all (3), Python 3 not found (2), found but minor version
+REM is below 6 (1), working Python 3.6 found (0).
+SET python_status=3
+REM Iterate over versions
+(FOR %%p IN (%versions%) DO (
+  REM Check for existence of the executable
+  WHERE %%p /q
+  IF !ERRORLEVEL! EQU 0 (
+    REM Executable found. Launch the version checker
+    %%p tools\pytest.py %%p
+    SET test_result=!ERRORLEVEL!
+    REM Keep track of the least serious error encountered yet
+    IF !test_result! LSS %python_status% (
+      SET python_status=!test_result!
+    )
+    REM If the error code was zero, means the version matches
+    IF !test_result! EQU 0 (
+      SET pyexe=%%p
+      GOTO exit_loop
+    )
+  ) ELSE (
+    REM Executable not found - might need to set status too
+    IF 3 LSS %python_status% (
+      SET python_status=3
+    )
+  )
+))
+:exit_loop
+REM Check status and jump to failure handler if it is positive
+IF %python_status% NEQ 0 (
+  GOTO failure
 )
 
-REM First of all, execute the dependency check tool
+REM Check dependencies
 %pyexe% filmatyk\dependency_test.py
 REM If it left a list of packages to install - pass it to pip
 IF EXIST install.txt (
@@ -22,10 +68,25 @@ IF EXIST install.txt (
 
 REM Only now trust the dependencies to be satisfied and launch
 CD filmatyk
-SET command=pythonw gui.py
+SET command=%pyexe% gui.py
 
 IF [%1]==[] GOTO launch
 IF %1==debug SET command=python gui.py debug
 
 :launch
   START %command%
+  GOTO end
+
+:failure
+  REM Basing on the error status, print the right diagnostic message
+  IF %python_status% EQU 1 (
+    SET ermsg=Python 3 found but version below 3.6
+  ) ELSE IF %python_status% EQU 2 (
+    SET ermsg=Python 3 not found
+  ) ELSE IF %python_status% EQU 3 (
+    SET ermsg=Python not found. Perhaps it is not in the PATH?
+  )
+  ECHO Failed to launch program, %ermsg%
+  GOTO end
+
+:end

tools/pytest.py

@@ -0,0 +1,7 @@
+import sys
+#print(sys.argv)
+if sys.version_info.major != 3:
+  sys.exit(2)
+if sys.version_info.minor < 6:
+  sys.exit(1)
+sys.exit(0)