Handle checks of string padding functions better.

Allow widths represented by subclasses of int and throw an exception
instead of aborting if the wrong types are given.

src/runtime/str.cpp

@@ -1226,9 +1226,13 @@ Box* str_richcompare(Box* lhs, Box* rhs, int op) {
 #define JUST_RIGHT 1
 #define JUST_CENTER 2
 static Box* pad(BoxedString* self, Box* width, Box* fillchar, int justType) {
-    assert(width->cls == int_cls);
-    assert(PyString_Check(fillchar));
-    assert(static_cast<BoxedString*>(fillchar)->size() == 1);
+    if (!PyInt_Check(width)) {
+        raiseExcHelper(TypeError, "an integer is required");
+    }
+    if (!PyString_Check(fillchar) || static_cast<BoxedString*>(fillchar)->size() != 1) {
+        raiseExcHelper(TypeError, "must be char, not %s", fillchar->cls->tp_name);
+    }
+
     int64_t curWidth = self->size();
     int64_t targetWidth = static_cast<BoxedInt*>(width)->n;
 

test/tests/str_functions.py

@@ -112,6 +112,31 @@ def test_just_funcs(s, w):
     t5 = s.rjust(w)
     t6 = s.center(w)
 
+    try:
+        print s.ljust("a string")
+    except TypeError as e:
+        print e
+    try:
+        print s.rjust("a string")
+    except TypeError:
+        print e
+    try:
+        print s.center("a string")
+    except TypeError:
+        print e
+    try:
+        print s.ljust(10, 12345)
+    except TypeError:
+        print e
+    try:
+        print s.rjust(10, 12345)
+    except TypeError:
+        print e
+    try:
+        print s.center(10, 12345)
+    except TypeError:
+        print e
+
     print t1, t1 == s, t1 is s, type(t1)
     print t2, t2 == s, t2 is s, type(t2)
     print t3, t3 == s, t3 is s, type(t3)