add lammps_expand() method and make available in python

python/lammps/core.py

@@ -178,6 +178,9 @@ class lammps(object):
 
     self.lib.lammps_error.argtypes = [c_void_p, c_int, c_char_p]
 
+    self.lib.lammps_expand.argtypes = [c_void_p, c_char_p]
+    self.lib.lammps_expand.restype = c_char_p
+
     self.lib.lammps_file.argtypes = [c_void_p, c_char_p]
     self.lib.lammps_file.restype = None
 
@@ -613,6 +616,28 @@ class lammps(object):
 
   # -------------------------------------------------------------------------
 
+  def expand(self,line):
+    """Expand a single LAMMPS string like an input line
+
+    This is a wrapper around the :cpp:func:`lammps_expand`
+    function of the C-library interface.
+
+    :param cmd: a single lammps line
+    :type cmd:  string
+    :return: expanded string
+    :rtype: string
+    """
+    if line: newline = line.encode()
+    else: return None
+
+    with ExceptionCheck(self):
+      strptr = self.lib.lammps_expand(self.lmp, newline)
+      rval = strptr.decode()
+      return rval
+
+    return None
+  # -------------------------------------------------------------------------
+
   def file(self, path):
     """Read LAMMPS commands from a file.
 

src/library.cpp

@@ -501,6 +501,51 @@ void lammps_error(void *handle, int error_type, const char *error_text)
   }
 }
 
+/* ---------------------------------------------------------------------- */
+
+/** expand a single LAMMPS input line from a string.
+ *
+\verbatim embed:rst
+
+This function tells LAMMPS to expand the string in *cmd* like it would process
+an input line fed to :cpp:func:`lammps_command` **without** executing it.
+The entire string is considered as command and need not have a
+(final) newline character.  Newline characters in the body of the
+string, however, will be treated as part of the command and will **not**
+start a second command.
+
+The function returns the expanded string in a new string buffer that
+must be freed with :cpp:func:`lammps_free` after use to avoid a memory leak.
+
+\endverbatim
+ *
+ * \param  handle  pointer to a previously created LAMMPS instance
+ * \param  cmd     string with a single LAMMPS input line
+ * \return         string with expanded line */
+
+char *lammps_expand(void *handle, const char *line)
+{
+  auto lmp = (LAMMPS *) handle;
+  char *copy, *work;
+  int n, maxcopy, maxwork;
+
+  if (!line) return nullptr;
+
+  BEGIN_CAPTURE
+  {
+    n = strlen(line) + 1;
+    copy = (char *) malloc(n * sizeof(char));
+    work = (char *) malloc(n * sizeof(char));
+    maxwork = maxcopy = n;
+    memcpy(copy, line, maxcopy);
+    lmp->input->substitute(copy, work, maxcopy, maxwork, 0);
+    free(work);
+  }
+  END_CAPTURE
+
+  return copy;
+}
+
 // ----------------------------------------------------------------------
 // Library functions to process commands
 // ----------------------------------------------------------------------

src/library.h

@@ -133,6 +133,8 @@ void lammps_python_finalize();
 
 void lammps_error(void *handle, int error_type, const char *error_text);
 
+char *lammps_expand(void *handle, const char *line);
+
 /* ----------------------------------------------------------------------
  * Library functions to process commands
  * ---------------------------------------------------------------------- */