本文和大家分享的主要是python中模块动态加载机制相关内容，一起来看看吧，希望对大家学习python有所帮助。
import指令
来看看import sys所产生的指令:
co_consts : (0, None)
co_names : (’sys’,)
0 LOAD_CONST 0 (0)
2 LOAD_CONST 1 (None)
4 IMPORT_NAME 0 (sys)
6 STORE_NAME 0 (sys)
可以看到import的结果是赋值给变量sys并存储在当前frame的local名字空间中,使得后续使用sys.path就能很快找到这个符号了.具体看看IMPORT_NAME指令做了什么动作:
TARGET(IMPORT_NAME) {
PyObject *name = GETITEM(names, oparg); //获得’sys’ PyUnicodeObject
PyObject *fromlist = POP(); // None
PyObject *level = TOP(); // 0
PyObject *res;
res = import_name(f, name, fromlist, level);
Py_DECREF(level);
Py_DECREF(fromlist);
SET_TOP(res);
if (res == NULL)
goto error;
DISPATCH();
}
这部分是收集将要import操作的所需信息,然后调用import_name :
[ceval.c]
static PyObject * import_name(PyFrameObject *f, PyObject *name, PyObject *fromlist, PyObject *level)
{
_Py_IDENTIFIER(__import__);
PyObject *import_func, *res;
PyObject* stack[5];
//获得内建函数__import__
import_func = _PyDict_GetItemId(f->f_builtins, &PyId___import__);
/* Fast path for not overloaded __import__. */
if (import_func == PyThreadState_GET()->interp->import_func) {
int ilevel = _PyLong_AsInt(level);
if (ilevel == -1 && PyErr_Occurred()) {
return NULL;
}
res = PyImport_ImportModuleLevelObject(
name,
f->f_globals,
f->f_locals == NULL ? Py_None : f->f_locals,
fromlist,
ilevel);
return res;
}
...
}
传进来的参数列表分别是,当前frame的PyFrameObject对象,表示’sys’的PyUnicodeObject, Py_None对象,表示0的PyLongObject.首先从内建f->builtins中获取__import__函数.此时它已经是一个包装过了的PyCFunctionObject对象了,在上一篇的builtins初始化时对每一个方法进行了包装.
if (import_func == PyThreadState_GET()->interp->import_func)是用来判断__import__是否被程序员重载了,这里不考虑被重载的情况. PyImport_ImportModuleLevelObject函数内比较复杂,因为它还要处理如import xml.sax这样的结构,好像调用时PyImport_ImportModuleLevelObject压根没有用到import_func这个内建import方法,但其实他们是殊途同归的,还记得内建方法的数组builtin_methods吗:
[bltinmodule.c]
static PyMethodDef builtin_methods[] = {
...
{"__import__", (PyCFunction)builtin___import__, METH_VARARGS | METH_KEYWORDS, import_doc},
...
};
static PyObject * builtin___import__(PyObject *self, PyObject *args, PyObject *kwds)
{
static char *kwlist[] = {"name", "globals", "locals", "fromlist",
"level", 0};
PyObject *name, *globals = NULL, *locals = NULL, *fromlist = NULL;
int level = 0;
if (!PyArg_ParseTupleAndKeywords(args, kwds, "U|OOOi:__import__",
kwlist, &name, &globals, &locals, &fromlist, &level))
return NULL;
return PyImport_ImportModuleLevelObject(name, globals, locals,
fromlist, level);
}
最终调用的其实是同一个PyImport_ImportModuleLevelObject函数.
很混乱有没有,好,重新整理下import的实现过程.
import机制
对于任何import操作, python虚拟机都要做到:
对运行时sys.modules全局模块池的维护
解析和搜索module路径
对不同文件的module的动态加载机制
import的形式也有很多种,最简单的形式如import os ,复杂一点就是import x.y.z ,注入from和as与import结合的,也会被分析后变成import x.y.z的形式.所以我们分析import的实现代码就以import x.y.z作为指令动作.
当以import x.y.z形式时,调用的参数分别是:
res = PyImport_ImportModuleLevelObject(
name, //表示’x.y.z’的PyUnicodeObject
f->f_globals, // frame的global名字空间
f->f_locals == NULL ? Py_None : f->f_locals,// frame的local名字空间
fromlist, // None值
ilevel); // 0
深入这个函数来看:
PyObject * PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
PyObject *locals, PyObject *fromlist,
int level)
{
_Py_IDENTIFIER(_find_and_load);
_Py_IDENTIFIER(_handle_fromlist);
PyObject *abs_name = NULL;
PyObject *final_mod = NULL;
PyObject *mod = NULL;
PyObject *package = NULL;
PyInterpreterState *interp = PyThreadState_GET()->interp;
int has_from;
abs_name = name;
mod = PyDict_GetItem(interp->modules, abs_name);
if (mod != NULL && mod != Py_None) { //如果全局modules里已经有了,说明重复引入模块
...
}
}
else { //该模块第一次引入
mod = _PyObject_CallMethodIdObjArgs(interp->importlib,
&PyId__find_and_load, abs_name,
interp->import_func, NULL);
}
//处理from xxx import xxx语句
has_from = 0;
if (fromlist != NULL && fromlist != Py_None) {
has_from = PyObject_IsTrue(fromlist);
if (has_from < 0)
goto error;
}
if (!has_from) { //不是from xxx形式的
Py_ssize_t len = PyUnicode_GET_LENGTH(name);
if (level == 0 || len > 0) {
Py_ssize_t dot;
//查找是模块名是否含有.不含返回-1,含会返回其索引
dot = PyUnicode_FindChar(name, ’.’, 0, len, 1);
if (dot == -1) {
/* No dot in module name, simple exit */
final_mod = mod;
Py_INCREF(mod);
goto error;
}
if (level == 0) {
PyObject *front = PyUnicode_Substring(name, 0, dot);
if (front == NULL) {
goto error;
}
final_mod = PyImport_ImportModuleLevelObject(front, NULL, NULL, NULL, 0);
Py_DECREF(front);
}
else {
...
}
}
else {
final_mod = mod;
Py_INCREF(mod);
}
}
else {
final_mod = _PyObject_CallMethodIdObjArgs(interp->importlib,
&