Adding doc and don't write bytecode files in native image build time.

Author: ppisl

CHANGELOG.md

@@ -5,7 +5,7 @@ language runtime. The main focus is on user-observable behavior of the engine.
 
 ## Version 20.1.1
 * When a `*.py` file is imported, `*.pyc` file is created. It contains binary data to speed up parsing.
-* Adding option `PyCachePrefix`, which is equivalent to PYTHONPYCACHEPREFIX environment variable. 
+* Adding option `PyCachePrefix`, which is equivalent to PYTHONPYCACHEPREFIX environment variable, which is also accepted now. 
 * Adding optin `DontWriteBytecodeFlag`. Equivalent to the Python -B flag. Don't write bytecode files.
 * Command option -B works
 

graalpython/com.oracle.graal.python.shell/src/com/oracle/graal/python/shell/GraalPythonMain.java

@@ -87,7 +87,7 @@ public static void main(String[] args) {
     private List<String> relaunchArgs;
     private boolean wantsExperimental = false;
     private Map<String, String> enginePolyglotOptions;
-    private boolean dontWriteBytecode = ImageInfo.inImageBuildtimeCode();
+    private boolean dontWriteBytecode = false;
 
     @Override
     protected List<String> preprocessArguments(List<String> givenArgs, Map<String, String> polyglotOptions) {
@@ -608,6 +608,8 @@ protected void printHelp(OptionCategory maxCategory) {
                         "   as specifying the -R option: a random value is used to seed the hashes of\n" +
                         "   str, bytes and datetime objects.  It can also be set to an integer\n" +
                         "   in the range [0,4294967295] to get hash values with a predictable seed.\n" +
+                        "PYTHONPYCACHEPREFIX: if this is set, GraalPython will write .pyc files in a mirror\n" +
+                        "   directory tree at this path, instead of in __pycache__ directories within the source tree.\n" +
                         "GRAAL_PYTHON_ARGS: the value is added as arguments as if passed on the\n" +
                         "   commandline. There is one special case: any `$$' in the value is replaced\n" +
                         "   with the current process id. To pass a literal `$$', you must escape the\n" +

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/SysModuleBuiltins.java

@@ -192,13 +192,13 @@ public void postInitialize(PythonCore core) {
             sys.setAttribute("executable", context.getOption(PythonOptions.Executable));
             sys.setAttribute("_base_executable", context.getOption(PythonOptions.Executable));
         }
-        sys.setAttribute("dont_write_bytecode", context.getOption(PythonOptions.DontWriteBytecodeFlag));
+        sys.setAttribute("dont_write_bytecode", ImageInfo.inImageBuildtimeCode() || context.getOption(PythonOptions.DontWriteBytecodeFlag));
         String pycachePrefix = context.getOption(PythonOptions.PyCachePrefix);
         sys.setAttribute("pycache_prefix", pycachePrefix.isEmpty() ? PNone.NONE : pycachePrefix);
         sys.setAttribute("__flags__", core.factory().createTuple(new Object[]{
                         false, // bytes_warning
                         !context.getOption(PythonOptions.PythonOptimizeFlag), // debug
-                        context.getOption(PythonOptions.DontWriteBytecodeFlag),  // dont_write_bytecode
+                        ImageInfo.inImageBuildtimeCode() || context.getOption(PythonOptions.DontWriteBytecodeFlag),  // dont_write_bytecode
                         false, // hash_randomization
                         context.getOption(PythonOptions.IgnoreEnvironmentFlag), // ignore_environment
                         context.getOption(PythonOptions.InspectFlag), // inspect