implement a python source command as suggested in issue #454

doc/src/python.txt

@@ -14,7 +14,7 @@ python func keyword args ... :pre
 
 func = name of Python function :ulb,l
 one or more keyword/args pairs must be appended :l
-keyword = {invoke} or {input} or {return} or {format} or {length} or {file} or {here} or {exists}
+keyword = {invoke} or {input} or {return} or {format} or {length} or {file} or {here} or {exists} or {source}
   {invoke} arg = none = invoke the previously defined Python function
   {input} args = N i1 i2 ... iN
     N = # of inputs to function
@@ -36,7 +36,12 @@ keyword = {invoke} or {input} or {return} or {format} or {length} or {file} or {
   {here} arg = inline
     inline = one or more lines of Python code which defines func
              must be a single argument, typically enclosed between triple quotes
-  {exists} arg = none = Python code has been loaded by previous python command :pre
+  {exists} arg = none = Python code has been loaded by previous python command
+  {source} arg = {filename} or {inline}
+    filename = file of Python code which will be executed immediately
+    inline = one or more lines of Python code which will be executed immediately
+             must be a single argument, typically enclosed between triple quotes
+:pre
 :ule
 
 [Examples:]
@@ -67,7 +72,8 @@ def loop(lmpptr,N,cut0):
 
 [Description:]
 
-Define a Python function or execute a previously defined function.
+Define a Python function or execute a previously defined function or
+execute some arbitrary python code.
 Arguments, including LAMMPS variables, can be passed to the function
 from the LAMMPS input script and a value returned by the Python
 function to a LAMMPS variable.  The Python code for the function can
@@ -102,7 +108,8 @@ command.
 
 The {func} setting specifies the name of the Python function.  The
 code for the function is defined using the {file} or {here} keywords
-as explained below.
+as explained below. In case of the {source} keyword, the name of
+the function is ignored.
 
 If the {invoke} keyword is used, no other keywords can be used, and a
 previous python command must have defined the Python function
@@ -111,6 +118,13 @@ previously defined arguments and return value processed as explained
 below.  You can invoke the function as many times as you wish in your
 input script.
 
+If the {source} keyword is used, no other keywords can be used.
+The argument can be a filename or a string with python commands,
+either on a single line enclosed in quotes, or as multiple lines
+enclosed in triple quotes. These python commands will be passed
+to the python interpreter and executed immediately without registering
+a python function for future execution.
+
 The {input} keyword defines how many arguments {N} the Python function
 expects.  If it takes no arguments, then the {input} keyword should
 not be used.  Each argument can be specified directly as a value,

src/PYTHON/python_impl.cpp

@@ -119,6 +119,32 @@ void PythonImpl::command(int narg, char **arg)
     return;
   }
 
+  // if source is only keyword, execute the python code
+
+  if (narg == 3 && strcmp(arg[1],"source") == 0) {
+
+    PyGILState_STATE gstate = PyGILState_Ensure();
+
+    // if argument string is file, open it
+    // otherwise process string as python code
+
+    int err = 0;
+    FILE *fp = fopen(arg[2],"r");
+
+    if (fp == NULL)
+      err = PyRun_SimpleString(arg[2]);
+    else
+      err = PyRun_SimpleFile(fp,arg[2]);
+
+    if (err) {
+      PyGILState_Release(gstate);
+      error->all(FLERR,"Could not process Python source command");
+    }
+    if (fp) fclose(fp);
+    PyGILState_Release(gstate);
+    return;
+  }
+
   // parse optional args, invoke is not allowed in this mode
 
   ninput = noutput = 0;