When I try to use print without parentheses on a simple name in Python 3.4 I get:
>>> print max
Traceback (most recent call last):
...
File "<interactive input>", line 1
print max
^
SyntaxError: Missing parentheses in call to 'print'
好的，现在我明白了，我只是忘了移植我的Python 2代码。
但是现在当我尝试打印函数的结果时：
>>> print max([1,2])
Traceback (most recent call last):
...
print max([1,2])
^
SyntaxError: invalid syntax
要么：
print max.__call__(23)
^
SyntaxError: invalid syntax
（请注意，在这种情况下，光标指向第一个点之前的字符。）
The message is different (and slightly misleading, since the marker is below the max function).
为什么Python无法更早地发现问题？
注意：这个问题是由以下问题引起的：Pandas read.csv语法错误，其中一些Python专家由于误导性错误消息而错过了真正的问题。
最佳答案
Looking at the source code for exceptions.c, right above _set_legacy_print_statement_msg there's this nice block comment:
/* 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).
*/
So there's some interesting info. In addition, in the SyntaxError_init method in the same file, we can see
/*
* 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;
}
Note also that the above references issue #21669 on the python bugtracker with some discussion between the author and Guido about how to go about this. So we follow the rabbit (that is, _report_missing_parentheses) which is at the very bottom of the file, and see...
legacy_check_result = _check_for_legacy_statements(self, 0);
However, there are some cases where this is bypassed and the normal SyntaxError message is printed, see MSeifert's answer for more about that. If we go one function up to _check_for_legacy_statements we finally see the actual check for legacy print statements.
/* 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);
}
因此，要回答这个问题：“为什么Python无法更早地检测到该问题？”，我会说括号中的问题不是检测到的；它实际上是在语法错误之后进行解析的。一直以来这都是一个语法错误，但是后来捕获了关于括号的实际小片段，只是为了给出一个额外的提示。