Create where_is utility to locate a binary in the standard places

This is particularly useful on Mac OS X where Gramps is passed a PATH
that does not include elements added by the terminal shell.

gramps/gen/plug/docgen/graphdoc.py

@@ -44,7 +44,7 @@ from subprocess import Popen, PIPE
 #-------------------------------------------------------------------------
 from ...const import GRAMPS_LOCALE as glocale
 _ = glocale.translation.gettext
-from ...utils.file import search_for
+from ...utils.file import search_for, where_is
 from . import BaseDoc
 from ..menu import NumberOption, TextOption, EnumeratedListOption, \
     BooleanOption
@@ -103,11 +103,7 @@ if win():
         _GS_CMD = ""
 else:
     _DOT_FOUND = search_for("dot")
-
-    if search_for("gs") == 1:
-        _GS_CMD = "gs"
-    else:
-        _GS_CMD = ""
+    _GS_CMD = where_is("gs")
 
 
 #------------------------------------------------------------------------------

gramps/gen/utils/file.py

@@ -61,7 +61,7 @@ def find_file( filename):
     try:
         if os.path.isfile(filename):
             return(filename)
-    except UnicodeError:
+    except UnicodeError as err:
         LOG.error("Filename %s raised a Unicode Error %s.", repr(filename), err)
 
     LOG.debug("Filename %s not found.", repr(filename))
@@ -228,6 +228,24 @@ def search_for(name):
                 return 1
     return 0
 
+
+def where_is(name):
+    """ This command is similar to the Linux "whereis -b file" command.
+    It looks for an executable file (name) in the PATH python is using, as
+    well as several likely other paths.  It returns the first file found,
+    or an empty string if not found.
+    """
+    paths = set(os.environ['PATH'].split(os.pathsep))
+    if not win():
+        paths.update(("/bin", "/usr/bin", "/usr/local/bin", "/opt/local/bin",
+                      "/opt/bin"))
+    for i in paths:
+        fname = os.path.join(i, name)
+        if os.access(fname, os.X_OK) and not os.path.isdir(fname):
+            return fname
+    return ""
+
+
 def create_checksum(full_path):
     """
     Create a md5 hash for the given file.