python - 符号表的限制？

Question

我一直在研究将 python(OSX 上的 2.7)文件作为配置文件加载的一些特殊情况。如果我循环运行 execfile，我想看看行为是什么。我预计会发生内存不足错误或大量交换，但当我得到不同的结果时，我感到非常惊讶。

我设置了一个测试场景如下:

'd' python 脚本:

#!/usr/bin/python
x = 0
execfile("d1")

'd1' python 脚本:

#!/usr/bin/python
x += 1
print "x = %d" % x
execfile("d2")

'd2' python 脚本:

#!/usr/bin/python
x += 1
print "x = %d" % x
execfile("d1")

结果:

$ ./d
x = 1
x = 2
x = 3
... removed for brevity ...
x = 997
x = 998
x = 999
Traceback (most recent call last):
  File "./d", line 5, in <module>
    execfile("d1")
  File "d1", line 5, in <module>
    execfile("d2")
  File "d2", line 5, in <module>
    execfile("d1")
... removed for brevity ...
  File "d1", line 5, in <module>
    execfile("d2")
  File "d2", line 5, in <module>
    execfile("d1")
  File "d1", line 5, in <module>
    execfile("d2")
KeyError: 'unknown symbol table entry'

我很好奇是否有人可以解释这里发生的事情？为什么执行execfile~1000次就停止了？

Answer 1

来自 Python 源代码，Objects/dictobject.c:

/* Note that, for historical reasons, PyDict_GetItem() suppresses all errors
 * that may occur (originally dicts supported only string keys, and exceptions
 * weren't possible).  So, while the original intent was that a NULL return
 * meant the key wasn't present, in reality it can mean that, or that an error
 * (suppressed) occurred while computing the key's hash, or that some error
 * (suppressed) occurred when comparing keys in the dict's internal probe
 * sequence.  A nasty example of the latter is when a Python-coded comparison
 * function hits a stack-depth error, which can cause this to return NULL
 * even if the key is present.
 */

So, PyDict_GetItem() does not always report errors correctly. Interesting... so in the following code in Python/symtable.c,

v = PyDict_GetItem(st->st_blocks, k);
if (v) {
    assert(PySTEntry_Check(v));
    Py_INCREF(v);
}
else {
    PyErr_SetString(PyExc_KeyError,
                    "unknown symbol table entry");
}

查找符号时发生的任何错误(包括内存不足错误)都将变成KeyError。这可能是一个错误。