Use names SEEK_SET, etc instead of magic number (GH-12057)

The previous code hardcoded `SEEK_SET`, etc. While it's very unlikely
that these values will change, it's best to use the definitions to avoid
there being mismatches in behavior with the code in the future.
Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>

Lib/_pyio.py

@@ -2386,18 +2386,18 @@ class TextIOWrapper(TextIOBase):
             raise ValueError("tell on closed file")
         if not self._seekable:
             raise UnsupportedOperation("underlying stream is not seekable")
-        if whence == 1: # seek relative to current position
+        if whence == SEEK_CUR:
             if cookie != 0:
                 raise UnsupportedOperation("can't do nonzero cur-relative seeks")
             # Seeking to the current position should attempt to
             # sync the underlying buffer with the current position.
             whence = 0
             cookie = self.tell()
-        if whence == 2: # seek relative to end of file
+        elif whence == SEEK_END:
             if cookie != 0:
                 raise UnsupportedOperation("can't do nonzero end-relative seeks")
             self.flush()
-            position = self.buffer.seek(0, 2)
+            position = self.buffer.seek(0, whence)
             self._set_decoded_chars('')
             self._snapshot = None
             if self._decoder:

Modules/_io/textio.c

@@ -2344,7 +2344,8 @@ _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
         goto fail;
     }
 
-    if (whence == 1) {
+    switch (whence) {
+    case SEEK_CUR:
         /* seek relative to current position */
         cmp = PyObject_RichCompareBool(cookieObj, _PyLong_Zero, Py_EQ);
         if (cmp < 0)
@@ -2362,8 +2363,7 @@ _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
         cookieObj = _PyObject_CallMethodId((PyObject *)self, &PyId_tell, NULL);
         if (cookieObj == NULL)
             goto fail;
-    }
-    else if (whence == 2) {
+    case SEEK_END:
         /* seek relative to end of file */
         cmp = PyObject_RichCompareBool(cookieObj, _PyLong_Zero, Py_EQ);
         if (cmp < 0)
@@ -2401,10 +2401,12 @@ _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
             }
         }
         return res;
-    }
-    else if (whence != 0) {
+    case SEEK_SET:
+        break;
+    default:
         PyErr_Format(PyExc_ValueError,
-                     "invalid whence (%d, should be 0, 1 or 2)", whence);
+                     "invalid whence (%d, should be %d, %d or %d)", whence,
+                     SEEK_SET, SEEK_CUR, SEEK_END);
         goto fail;
     }