diff --git a/src/eval.c b/src/eval.c
index c99501a..a2a9986 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -473,9 +473,9 @@
 static void f_argidx __ARGS((typval_T *argvars, typval_T *rettv));
 static void f_arglistid __ARGS((typval_T *argvars, typval_T *rettv));
 static void f_argv __ARGS((typval_T *argvars, typval_T *rettv));
-static void f_assertEqual __ARGS((typval_T *argvars, typval_T *rettv));
-static void f_assertFalse __ARGS((typval_T *argvars, typval_T *rettv));
-static void f_assertTrue __ARGS((typval_T *argvars, typval_T *rettv));
+static void f_assert_equal __ARGS((typval_T *argvars, typval_T *rettv));
+static void f_assert_false __ARGS((typval_T *argvars, typval_T *rettv));
+static void f_assert_true __ARGS((typval_T *argvars, typval_T *rettv));
 #ifdef FEAT_FLOAT
 static void f_asin __ARGS((typval_T *argvars, typval_T *rettv));
 static void f_atan __ARGS((typval_T *argvars, typval_T *rettv));
@@ -8072,9 +8072,9 @@
     {"argidx",		0, 0, f_argidx},
     {"arglistid",	0, 2, f_arglistid},
     {"argv",		0, 1, f_argv},
-    {"assertEqual",	2, 3, f_assertEqual},
-    {"assertFalse",	1, 2, f_assertFalse},
-    {"assertTrue",	1, 2, f_assertTrue},
+    {"assert_equal",	2, 3, f_assert_equal},
+    {"assert_false",	1, 2, f_assert_false},
+    {"assert_true",	1, 2, f_assert_true},
 #ifdef FEAT_FLOAT
     {"asin",		1, 1, f_asin},	/* WJMc */
     {"atan",		1, 1, f_atan},
@@ -9131,15 +9131,67 @@
 					       alist_name(&ARGLIST[idx]), -1);
 }
 
-static void assertError __ARGS((garray_T *gap));
-static void prepareForAssertError __ARGS((garray_T*gap));
-static void assertBool __ARGS((typval_T *argvars, int isTrue));
+static void prepare_assert_error __ARGS((garray_T*gap));
+static void fill_assert_error __ARGS((garray_T *gap, typval_T *opt_msg_tv, char_u *exp_str, typval_T *exp_tv, typval_T *got_tv));
+static void assert_error __ARGS((garray_T *gap));
+static void assert_bool __ARGS((typval_T *argvars, int isTrue));
+
+/*
+ * Prepare "gap" for an assert error and add the sourcing position.
+ */
+    static void
+prepare_assert_error(gap)
+    garray_T	*gap;
+{
+    char buf[NUMBUFLEN];
+
+    ga_init2(gap, 1, 100);
+    ga_concat(gap, sourcing_name);
+    sprintf(buf, " line %ld", (long)sourcing_lnum);
+    ga_concat(gap, (char_u *)buf);
+    ga_concat(gap, (char_u *)": ");
+}
+
+/*
+ * Fill "gap" with information about an assert error.
+ */
+    static void
+fill_assert_error(gap, opt_msg_tv, exp_str, exp_tv, got_tv)
+    garray_T	*gap;
+    typval_T	*opt_msg_tv;
+    char_u      *exp_str;
+    typval_T	*exp_tv;
+    typval_T	*got_tv;
+{
+    char_u	numbuf[NUMBUFLEN];
+    char_u	*tofree;
+
+    if (opt_msg_tv->v_type != VAR_UNKNOWN)
+    {
+	ga_concat(gap, tv2string(opt_msg_tv, &tofree, numbuf, 0));
+	vim_free(tofree);
+    }
+    else
+    {
+	ga_concat(gap, (char_u *)"Expected ");
+	if (exp_str == NULL)
+	{
+	    ga_concat(gap, tv2string(exp_tv, &tofree, numbuf, 0));
+	    vim_free(tofree);
+	}
+	else
+	    ga_concat(gap, exp_str);
+	ga_concat(gap, (char_u *)" but got ");
+	ga_concat(gap, tv2string(got_tv, &tofree, numbuf, 0));
+	vim_free(tofree);
+    }
+}
 
 /*
  * Add an assert error to v:errors.
  */
     static void
-assertError(gap)
+assert_error(gap)
     garray_T	*gap;
 {
     struct vimvar   *vp = &vimvars[VV_ERRORS];
@@ -9150,92 +9202,69 @@
     list_append_string(vimvars[VV_ERRORS].vv_list, gap->ga_data, gap->ga_len);
 }
 
-    static void
-prepareForAssertError(gap)
-    garray_T	*gap;
-{
-    char buf[NUMBUFLEN];
-
-    ga_init2(gap, 1, 100);
-    ga_concat(gap, sourcing_name);
-    sprintf(buf, " line %ld", (long)sourcing_lnum);
-    ga_concat(gap, (char_u *)buf);
-}
-
 /*
- * "assertEqual(expected, actual[, msg])" function
+ * "assert_equal(expected, actual[, msg])" function
  */
     static void
-f_assertEqual(argvars, rettv)
+f_assert_equal(argvars, rettv)
     typval_T	*argvars;
     typval_T	*rettv UNUSED;
 {
     garray_T	ga;
-    char_u	*tofree;
-    char_u	numbuf[NUMBUFLEN];
 
     if (!tv_equal(&argvars[0], &argvars[1], FALSE, FALSE))
     {
-	prepareForAssertError(&ga);
-	ga_concat(&ga, (char_u *)": Expected ");
-	ga_concat(&ga, tv2string(&argvars[0], &tofree, numbuf, 0));
-	vim_free(tofree);
-	ga_concat(&ga, (char_u *)" but got ");
-	ga_concat(&ga, tv2string(&argvars[1], &tofree, numbuf, 0));
-	vim_free(tofree);
-	assertError(&ga);
+	prepare_assert_error(&ga);
+	fill_assert_error(&ga, &argvars[2], NULL, &argvars[0], &argvars[1]);
+	assert_error(&ga);
 	ga_clear(&ga);
     }
 }
 
+/*
+ * Common for assert_true() and assert_false().
+ */
     static void
-assertBool(argvars, isTrue)
+assert_bool(argvars, isTrue)
     typval_T	*argvars;
     int		isTrue;
 {
     int		error = FALSE;
     garray_T	ga;
-    char_u	*tofree;
-    char_u	numbuf[NUMBUFLEN];
 
     if (argvars[0].v_type != VAR_NUMBER
 	    || (get_tv_number_chk(&argvars[0], &error) == 0) == isTrue
 	    || error)
     {
-	prepareForAssertError(&ga);
-	ga_concat(&ga, (char_u *)": Expected ");
-	if (isTrue)
-	    ga_concat(&ga, (char_u *)"True ");
-	else
-	    ga_concat(&ga, (char_u *)"False ");
-	ga_concat(&ga, (char_u *)"but got ");
-	ga_concat(&ga, tv2string(&argvars[0], &tofree, numbuf, 0));
-	vim_free(tofree);
-	assertError(&ga);
+	prepare_assert_error(&ga);
+	fill_assert_error(&ga, &argvars[1],
+		(char_u *)(isTrue ? "True " : "False "),
+		NULL, &argvars[0]);
+	assert_error(&ga);
 	ga_clear(&ga);
     }
 }
 
 /*
- * "assertFalse(actual[, msg])" function
+ * "assert_false(actual[, msg])" function
  */
     static void
-f_assertFalse(argvars, rettv)
+f_assert_false(argvars, rettv)
     typval_T	*argvars;
     typval_T	*rettv UNUSED;
 {
-    assertBool(argvars, FALSE);
+    assert_bool(argvars, FALSE);
 }
 
 /*
- * "assertTrue(actual[, msg])" function
+ * "assert_true(actual[, msg])" function
  */
     static void
-f_assertTrue(argvars, rettv)
+f_assert_true(argvars, rettv)
     typval_T	*argvars;
     typval_T	*rettv UNUSED;
 {
-    assertBool(argvars, TRUE);
+    assert_bool(argvars, TRUE);
 }
 
 #ifdef FEAT_FLOAT