diff --git a/src/action_class_assert.cc b/src/action_class_assert.cc
index 4261524fd..66cf04f56 100644
--- a/src/action_class_assert.cc
+++ b/src/action_class_assert.cc
@@ -137,8 +137,8 @@ static int execute(grib_action* a, grib_handle* h)
     }
     else {
         grib_context_log(h->context, GRIB_LOG_ERROR, "Assertion failure: ");
-        grib_expression_print(h->context, self->expression, h);
-        printf("\n");
+        grib_expression_print(h->context, self->expression, h, stderr);
+        fprintf(stderr, "\n");
         return GRIB_ASSERTION_FAILURE;
     }
 }
diff --git a/src/action_class_if.cc b/src/action_class_if.cc
index c310bada2..ec99ba315 100644
--- a/src/action_class_if.cc
+++ b/src/action_class_if.cc
@@ -154,9 +154,9 @@ static int create_accessor(grib_section* p, grib_action* act, grib_loader* h)
         next = a->block_false;
 
     if (p->h->context->debug > 1) {
-        printf("EVALUATE create_accessor_handle ");
-        grib_expression_print(p->h->context, a->expression, p->h);
-        printf(" [%s][_if%p]\n", (next == a->block_true ? "true" : "false"), (void*)a);
+        fprintf(stderr, "EVALUATE create_accessor_handle ");
+        grib_expression_print(p->h->context, a->expression, p->h, stderr);
+        fprintf(stderr, " [%s][_if%p]\n", (next == a->block_true ? "true" : "false"), (void*)a);
 
         /*grib_dump_action_branch(stdout,next,5);*/
     }
@@ -176,8 +176,8 @@ static int create_accessor(grib_section* p, grib_action* act, grib_loader* h)
 
 static void print_expression_debug_info(grib_context* ctx, grib_expression* exp, grib_handle* h)
 {
-    grib_expression_print(ctx, exp, h); /* writes to stdout without a newline */
-    printf("\n");
+    grib_expression_print(ctx, exp, h, stderr); /* writes without a newline */
+    fprintf(stderr, "\n");
 }
 
 static int execute(grib_action* act, grib_handle* h)
@@ -240,7 +240,7 @@ static void dump(grib_action* act, FILE* f, int lvl)
         grib_context_print(act->context, f, "     ");
 
     printf("if(%s) { ", act->name);
-    grib_expression_print(act->context, a->expression, 0);
+    grib_expression_print(act->context, a->expression, 0, stdout);
     printf("\n");
 
     if (a->block_true) {
@@ -252,7 +252,7 @@ static void dump(grib_action* act, FILE* f, int lvl)
         for (i = 0; i < lvl; i++)
             grib_context_print(act->context, f, "     ");
         printf("else(%s) { ", act->name);
-        grib_expression_print(act->context, a->expression, 0);
+        grib_expression_print(act->context, a->expression, 0, stdout);
         /*     grib_context_print(act->context,f,"ELSE \n" );*/
         grib_dump_action_branch(f, a->block_false, lvl + 1);
     }
diff --git a/src/action_class_when.cc b/src/action_class_when.cc
index 31f377b44..640e75aab 100644
--- a/src/action_class_when.cc
+++ b/src/action_class_when.cc
@@ -149,7 +149,7 @@ static void dump(grib_action* act, FILE* f, int lvl)
         grib_context_print(act->context, f, "     ");
 
     printf("when(%s) { ", act->name);
-    grib_expression_print(act->context, a->expression, 0);
+    grib_expression_print(act->context, a->expression, 0, stdout);
     printf("\n");
 
     grib_dump_action_branch(f, a->block_true, lvl + 1);
@@ -195,8 +195,8 @@ static int notify_change(grib_action* a, grib_accessor* observer, grib_accessor*
     if (self->loop) {
         printf("LOOP detected...\n");
         printf("WHEN triggered by %s %ld\n", observed->name, lres);
-        grib_expression_print(observed->context, self->expression, 0);
-        printf("\n");
+        grib_expression_print(observed->context, self->expression, 0, stderr);
+        fprintf(stderr, "\n");
         return ret;
     }
 #endif
@@ -206,8 +206,8 @@ static int notify_change(grib_action* a, grib_accessor* observer, grib_accessor*
         grib_context_log(hand->context, GRIB_LOG_DEBUG,
                 "------------- SECTION action %s is triggered by [%s] (%s)",
                 a->name, observed->name, a->debug_info ? a->debug_info : "no debug info");
-        //grib_expression_print(observed->context, self->expression, 0);
-        //fprintf(stderr, "\n");
+        grib_expression_print(observed->context, self->expression, 0, stderr);
+        fprintf(stderr, "\n");
     }
 
     if (lres)
diff --git a/src/eccodes_prototypes.h b/src/eccodes_prototypes.h
index 3ce4d0838..61b02b61d 100644
--- a/src/eccodes_prototypes.h
+++ b/src/eccodes_prototypes.h
@@ -830,7 +830,7 @@ int grib_expression_evaluate_long(grib_handle* h, grib_expression* g, long* resu
 int grib_expression_evaluate_double(grib_handle* h, grib_expression* g, double* result);
 const char* grib_expression_evaluate_string(grib_handle* h, grib_expression* g, char* buf, size_t* size, int* err);
 const char* grib_expression_get_name(grib_expression* g);
-void grib_expression_print(grib_context* ctx, grib_expression* g, grib_handle* f);
+void grib_expression_print(grib_context* ctx, grib_expression* g, grib_handle* f, FILE*);
 void grib_expression_free(grib_context* ctx, grib_expression* g);
 void grib_expression_add_dependency(grib_expression* e, grib_accessor* observer);
 grib_arguments* grib_arguments_new(grib_context* c, grib_expression* g, grib_arguments* n);
diff --git a/src/grib_api_internal.h b/src/grib_api_internal.h
index 335da5954..dc0742d8a 100644
--- a/src/grib_api_internal.h
+++ b/src/grib_api_internal.h
@@ -891,7 +891,7 @@ typedef int (*expression_evaluate_double_proc)(grib_expression*, grib_handle*, d
 typedef const char* (*expression_evaluate_string_proc)(grib_expression*, grib_handle*, char*, size_t*, int*);
 typedef const char* (*expression_get_name_proc)(grib_expression*);
 
-typedef void (*expression_print_proc)(grib_context*, grib_expression*, grib_handle*);
+typedef void (*expression_print_proc)(grib_context*, grib_expression*, grib_handle*, FILE*);
 typedef void (*expression_add_dependency_proc)(grib_expression* e, grib_accessor* observer);
 
 typedef struct grib_expression_class grib_expression_class;
diff --git a/src/grib_expression.cc b/src/grib_expression.cc
index 4c1109640..314e5ad8d 100644
--- a/src/grib_expression.cc
+++ b/src/grib_expression.cc
@@ -74,12 +74,12 @@ const char* grib_expression_get_name(grib_expression* g)
     return 0;
 }
 
-void grib_expression_print(grib_context* ctx, grib_expression* g, grib_handle* f)
+void grib_expression_print(grib_context* ctx, grib_expression* g, grib_handle* f, FILE* out)
 {
     grib_expression_class* c = g->cclass;
     while (c) {
         if (c->print) {
-            c->print(ctx, g, f);
+            c->print(ctx, g, f, out);
             return;
         }
         c = c->super ? *(c->super) : NULL;
@@ -163,7 +163,7 @@ void grib_arguments_print(grib_context* c, grib_arguments* g, grib_handle* f)
 {
     if (g) {
         if (g->expression)
-            grib_expression_print(c, g->expression, f);
+            grib_expression_print(c, g->expression, f, stdout);
         if (g->next) {
             printf(",");
             grib_arguments_print(c, g->next, f);
diff --git a/src/grib_expression_class_accessor.cc b/src/grib_expression_class_accessor.cc
index 064cc2051..efa42714a 100644
--- a/src/grib_expression_class_accessor.cc
+++ b/src/grib_expression_class_accessor.cc
@@ -42,7 +42,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*, FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static string  get_name(grib_expression* e);
 static int     native_type(grib_expression*,grib_handle*);
@@ -128,16 +128,16 @@ static string evaluate_string(grib_expression* g, grib_handle* h, char* buf, siz
     return buf;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     const grib_expression_accessor* e = (grib_expression_accessor*)g;
-    printf("access('%s", e->name);
+    fprintf(out, "access('%s", e->name);
     if (f) {
         long s = 0;
         grib_get_long(f, e->name, &s);
-        printf("=%ld", s);
+        fprintf(out, "=%ld", s);
     }
-    printf("')");
+    fprintf(out, "')");
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_binop.cc b/src/grib_expression_class_binop.cc
index 5caefd701..e1c4f3d02 100644
--- a/src/grib_expression_class_binop.cc
+++ b/src/grib_expression_class_binop.cc
@@ -42,7 +42,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*, FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static int     native_type(grib_expression*,grib_handle*);
 static int     evaluate_long(grib_expression*,grib_handle*,long*);
@@ -144,14 +144,28 @@ static int evaluate_double(grib_expression* g, grib_handle* h, double* dres)
     return GRIB_SUCCESS;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     grib_expression_binop* e = (grib_expression_binop*)g;
-    printf("binop(");
-    grib_expression_print(c, e->left, f);
-    printf(",");
-    grib_expression_print(c, e->right, f);
-    printf(")");
+
+    // Cover a subset of the most commonly used functions
+    // TODO(masn): Can be done in a much better way!
+    //             e.g., the yacc parser passing in the functions name
+    if (e->long_func && e->long_func == grib_op_eq) {
+        fprintf(out, "equals(");
+    } else if (e->long_func && e->long_func == grib_op_ne) {
+        fprintf(out, "not_equals(");
+    } else if (e->long_func && e->long_func == grib_op_lt) {
+        fprintf(out, "less_than(");
+    } else if (e->long_func && e->long_func == grib_op_gt) {
+        fprintf(out, "greater_than(");
+    } else {
+        fprintf(out, "binop(");
+    }
+    grib_expression_print(c, e->left, f, out);
+    fprintf(out, ",");
+    grib_expression_print(c, e->right, f, out);
+    fprintf(out, ")");
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_double.cc b/src/grib_expression_class_double.cc
index 89747194b..52d40832f 100644
--- a/src/grib_expression_class_double.cc
+++ b/src/grib_expression_class_double.cc
@@ -38,7 +38,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*, FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static int     native_type(grib_expression*,grib_handle*);
 static int     evaluate_long(grib_expression*,grib_handle*,long*);
@@ -85,10 +85,10 @@ static int evaluate_double(grib_expression* g, grib_handle* h, double* dres)
     return GRIB_SUCCESS;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     const grib_expression_double* e = (grib_expression_double*)g;
-    printf("double(%g)", e->value);
+    fprintf(out, "double(%g)", e->value);
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_functor.cc b/src/grib_expression_class_functor.cc
index 530654ea2..f335966a0 100644
--- a/src/grib_expression_class_functor.cc
+++ b/src/grib_expression_class_functor.cc
@@ -38,7 +38,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static int     native_type(grib_expression*,grib_handle*);
 static int     evaluate_long(grib_expression*,grib_handle*,long*);
@@ -216,12 +216,12 @@ static int evaluate_long(grib_expression* g, grib_handle* h, long* lres)
     return GRIB_NOT_IMPLEMENTED;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     const grib_expression_functor* e = (grib_expression_functor*)g;
-    printf("%s(", e->name);
+    fprintf(out, "%s(", e->name);
     // grib_expression_print(c,e->args,f);
-    printf(")");
+    fprintf(out, ")");
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_is_in_dict.cc b/src/grib_expression_class_is_in_dict.cc
index 142287cdc..46e0833f6 100644
--- a/src/grib_expression_class_is_in_dict.cc
+++ b/src/grib_expression_class_is_in_dict.cc
@@ -39,7 +39,7 @@ or edit "expression.class" and rerun ./make_class.pl
 
 typedef const char* string; /* to keep make_class.pl happy */
 
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static string  get_name(grib_expression* e);
 static int     native_type(grib_expression*,grib_handle*);
@@ -200,16 +200,16 @@ static string evaluate_string(grib_expression* g, grib_handle* h, char* buf, siz
     // return buf;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     const grib_expression_is_in_dict* e = (grib_expression_is_in_dict*)g;
-    printf("access('%s", e->key);
+    fprintf(out, "access('%s", e->key);
     if (f) {
         long s = 0;
         grib_get_long(f, e->key, &s);
-        printf("=%ld", s);
+        fprintf(out, "=%ld", s);
     }
-    printf("')");
+    fprintf(out, "')");
 }
 
 grib_expression* new_is_in_dict_expression(grib_context* c, const char* name, const char* list)
diff --git a/src/grib_expression_class_is_in_list.cc b/src/grib_expression_class_is_in_list.cc
index f5339c309..3e5e3728d 100644
--- a/src/grib_expression_class_is_in_list.cc
+++ b/src/grib_expression_class_is_in_list.cc
@@ -41,7 +41,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static string  get_name(grib_expression* e);
 static int     native_type(grib_expression*,grib_handle*);
@@ -199,16 +199,16 @@ static string evaluate_string(grib_expression* g, grib_handle* h, char* buf, siz
     return buf;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     const grib_expression_is_in_list* e = (grib_expression_is_in_list*)g;
-    printf("access('%s", e->name);
+    fprintf(out, "access('%s", e->name);
     if (f) {
         long s = 0;
         grib_get_long(f, e->name, &s);
-        printf("=%ld", s);
+        fprintf(out, "=%ld", s);
     }
-    printf("')");
+    fprintf(out, "')");
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_is_integer.cc b/src/grib_expression_class_is_integer.cc
index da81cc4cb..bf203bc81 100644
--- a/src/grib_expression_class_is_integer.cc
+++ b/src/grib_expression_class_is_integer.cc
@@ -42,7 +42,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*, FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static string  get_name(grib_expression* e);
 static int     native_type(grib_expression*,grib_handle*);
@@ -140,7 +140,7 @@ static string evaluate_string(grib_expression* g, grib_handle* h, char* buf, siz
     return buf;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     // grib_expression_is_integer* e = (grib_expression_is_integer*)g;
     // printf("access('%s", e->name);
diff --git a/src/grib_expression_class_length.cc b/src/grib_expression_class_length.cc
index c9e967c77..c8c63f8e8 100644
--- a/src/grib_expression_class_length.cc
+++ b/src/grib_expression_class_length.cc
@@ -42,7 +42,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static string  get_name(grib_expression* e);
 static int     native_type(grib_expression*,grib_handle*);
@@ -123,16 +123,16 @@ static string evaluate_string(grib_expression* g, grib_handle* h, char* buf, siz
     return buf;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     const grib_expression_length* e = (grib_expression_length*)g;
-    printf("access('%s", e->name);
+    fprintf(out, "access('%s", e->name);
     if (f) {
         long s = 0;
         grib_get_long(f, e->name, &s);
-        printf("=%ld", s);
+        fprintf(out, "=%ld", s);
     }
-    printf("')");
+    fprintf(out, "')");
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_logical_and.cc b/src/grib_expression_class_logical_and.cc
index 902119a5e..d95de8d92 100644
--- a/src/grib_expression_class_logical_and.cc
+++ b/src/grib_expression_class_logical_and.cc
@@ -39,7 +39,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*, FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static int     native_type(grib_expression*,grib_handle*);
 static int     evaluate_long(grib_expression*,grib_handle*,long*);
@@ -133,14 +133,14 @@ static int evaluate_double(grib_expression* g, grib_handle* h, double* dres)
     return ret;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     grib_expression_logical_and* e = (grib_expression_logical_and*)g;
-    printf("(");
-    grib_expression_print(c, e->left, f);
-    printf(" && ");
-    grib_expression_print(c, e->right, f);
-    printf(")");
+    fprintf(out, "(");
+    grib_expression_print(c, e->left, f, out);
+    fprintf(out, " && ");
+    grib_expression_print(c, e->right, f, out);
+    fprintf(out, ")");
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_logical_or.cc b/src/grib_expression_class_logical_or.cc
index 772a73d8a..aed0618f3 100644
--- a/src/grib_expression_class_logical_or.cc
+++ b/src/grib_expression_class_logical_or.cc
@@ -39,7 +39,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static int     native_type(grib_expression*,grib_handle*);
 static int     evaluate_long(grib_expression*,grib_handle*,long*);
@@ -134,14 +134,14 @@ static int evaluate_double(grib_expression* g, grib_handle* h, double* dres)
     return ret;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     grib_expression_logical_or* e = (grib_expression_logical_or*)g;
-    printf("(");
-    grib_expression_print(c, e->left, f);
-    printf(" || ");
-    grib_expression_print(c, e->right, f);
-    printf(")");
+    fprintf(out, "(");
+    grib_expression_print(c, e->left, f, out);
+    fprintf(out, " || ");
+    grib_expression_print(c, e->right, f, out);
+    fprintf(out, ")");
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_long.cc b/src/grib_expression_class_long.cc
index 3b61a83d1..774f736e6 100644
--- a/src/grib_expression_class_long.cc
+++ b/src/grib_expression_class_long.cc
@@ -38,7 +38,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static int     native_type(grib_expression*,grib_handle*);
 static int     evaluate_long(grib_expression*,grib_handle*,long*);
@@ -85,10 +85,10 @@ static int evaluate_double(grib_expression* g, grib_handle* h, double* dres)
     return GRIB_SUCCESS;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     const grib_expression_long* e = (grib_expression_long*)g;
-    printf("long(%ld)", e->value);
+    fprintf(out, "long(%ld)", e->value);
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_string.cc b/src/grib_expression_class_string.cc
index 79d42e247..379140de7 100644
--- a/src/grib_expression_class_string.cc
+++ b/src/grib_expression_class_string.cc
@@ -37,7 +37,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static int     native_type(grib_expression*,grib_handle*);
 static string  evaluate_string(grib_expression*,grib_handle*,char*,size_t*,int*);
@@ -76,10 +76,10 @@ static const char* evaluate_string(grib_expression* g, grib_handle* h, char* buf
     return e->value;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     const grib_expression_string* e = (grib_expression_string*)g;
-    printf("string('%s')", e->value);
+    fprintf(out, "string('%s')", e->value);
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_string_compare.cc b/src/grib_expression_class_string_compare.cc
index 3003b5ef6..ac54cb6d2 100644
--- a/src/grib_expression_class_string_compare.cc
+++ b/src/grib_expression_class_string_compare.cc
@@ -40,7 +40,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static int     native_type(grib_expression*,grib_handle*);
 static int     evaluate_long(grib_expression*,grib_handle*,long*);
@@ -128,14 +128,14 @@ static int evaluate_double(grib_expression* g, grib_handle* h, double* dres)
     return ret;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     grib_expression_string_compare* e = (grib_expression_string_compare*)g;
-    printf("string_compare(");
-    grib_expression_print(c, e->left, f);
-    printf(",");
-    grib_expression_print(c, e->right, f);
-    printf(")");
+    fprintf(out, "string_compare(");
+    grib_expression_print(c, e->left, f, out);
+    fprintf(out, ",");
+    grib_expression_print(c, e->right, f, out);
+    fprintf(out, ")");
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_sub_string.cc b/src/grib_expression_class_sub_string.cc
index a5a0da732..dd44ec323 100644
--- a/src/grib_expression_class_sub_string.cc
+++ b/src/grib_expression_class_sub_string.cc
@@ -37,7 +37,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static int     native_type(grib_expression*,grib_handle*);
 static string  evaluate_string(grib_expression*,grib_handle*,char*,size_t*,int*);
@@ -76,10 +76,10 @@ static const char* evaluate_string(grib_expression* g, grib_handle* h, char* buf
     return e->value;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     const grib_expression_sub_string* e = (grib_expression_sub_string*)g;
-    printf("string('%s')", e->value);
+    fprintf(out, "string('%s')", e->value);
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_true.cc b/src/grib_expression_class_true.cc
index 6b418cb2c..382dec868 100644
--- a/src/grib_expression_class_true.cc
+++ b/src/grib_expression_class_true.cc
@@ -37,7 +37,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static int     native_type(grib_expression*,grib_handle*);
 static int     evaluate_long(grib_expression*,grib_handle*,long*);
@@ -81,10 +81,10 @@ static int evaluate_double(grib_expression* g, grib_handle* h, double* dres)
     return GRIB_SUCCESS;
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
-    printf("true(");
-    printf(")");
+    fprintf(out, "true(");
+    fprintf(out, ")");
 }
 
 static void destroy(grib_context* c, grib_expression* g)
diff --git a/src/grib_expression_class_unop.cc b/src/grib_expression_class_unop.cc
index ae7678043..0de77a02d 100644
--- a/src/grib_expression_class_unop.cc
+++ b/src/grib_expression_class_unop.cc
@@ -41,7 +41,7 @@ or edit "expression.class" and rerun ./make_class.pl
 typedef const char* string; /* to keep make_class.pl happy */
 
 static void    destroy(grib_context*,grib_expression* e);
-static void    print(grib_context*,grib_expression*,grib_handle*);
+static void    print(grib_context*,grib_expression*,grib_handle*,FILE*);
 static void    add_dependency(grib_expression* e, grib_accessor* observer);
 static string  get_name(grib_expression* e);
 static int     native_type(grib_expression*,grib_handle*);
@@ -105,12 +105,12 @@ static const char* get_name(grib_expression* g)
     return grib_expression_get_name(e->exp);
 }
 
-static void print(grib_context* c, grib_expression* g, grib_handle* f)
+static void print(grib_context* c, grib_expression* g, grib_handle* f, FILE* out)
 {
     grib_expression_unop* e = (grib_expression_unop*)g;
-    printf("unop(");
-    grib_expression_print(c, e->exp, f);
-    printf(")");
+    fprintf(out, "unop(");
+    grib_expression_print(c, e->exp, f, out);
+    fprintf(out, ")");
 }
 
 static void destroy(grib_context* c, grib_expression* g)