Python3.x 使用 Python 3 打印不带括号的不同错误消息

看看source code for exceptions.c，就在_set_legacy_print_statement_msg的正上方，有一条很好的街区 comments :

/* To help with migration from Python 2, SyntaxError.__init__ applies some
 * heuristics to try to report a more meaningful exception when print and
 * exec are used like statements.
 *
 * The heuristics are currently expected to detect the following cases:
 *   - top level statement
 *   - statement in a nested suite
 *   - trailing section of a one line complex statement
 *
 * They're currently known not to trigger:
 *   - after a semi-colon
 *
 * The error message can be a bit odd in cases where the "arguments" are
 * completely illegal syntactically, but that isn't worth the hassle of
 * fixing.
 *
 * We also can't do anything about cases that are legal Python 3 syntax
 * but mean something entirely different from what they did in Python 2
 * (omitting the arguments entirely, printing items preceded by a unary plus
 * or minus, using the stream redirection syntax).
 */

这里有一些有趣的信息.此外，在同一文件中的SyntaxError_init方法中，我们可以看到

    /*
     * Issue #21669: Custom error for 'print' & 'exec' as statements
     *
     * Only applies to SyntaxError instances, not to subclasses such
     * as TabError or IndentationError (see issue #31161)
     */
    if ((PyObject*)Py_TYPE(self) == PyExc_SyntaxError &&
            self->text && PyUnicode_Check(self->text) &&
            _report_missing_parentheses(self) < 0) {
        return -1;
    }

还要注意的是，上面提到的issue #21669 on the python bugtracker篇文章中，作者和Guido就如何实现这一点进行了一些讨论.所以我们跟着兔子(也就是_report_missing_parentheses)走，它在文件的最底部，看到...

legacy_check_result = _check_for_legacy_statements(self, 0);

然而，也有一些情况下，这是绕过和正常的SyntaxError信息打印，请参阅MSeifert's answer了解更多信息.如果我们将一个函数增加到_check_for_legacy_statements，我们将看到actual check用于传统打印语句.

/* Check for legacy print statements */
if (print_prefix == NULL) {
    print_prefix = PyUnicode_InternFromString("print ");
    if (print_prefix == NULL) {
        return -1;
    }
}
if (PyUnicode_Tailmatch(self->text, print_prefix,
                        start, text_len, -1)) {

    return _set_legacy_print_statement_msg(self, start);
}

所以，为了回答这个问题:"为什么Python不能更早地检测到这个问题？"，我想说，括号的问题不是检测到什么；它实际上被解析为after个语法错误.这一直都是一个语法错误，但关于括号的实际小片段在之后被捕捉，只是为了给出一个额外的提示.