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# Python 的编译结果——Code 对象与 pyc 文件
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## Python 脚本的执行过程
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- 编译
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Python 编译器将 py 文件编译成字节码（PyCodeObject）。
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- 执行
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Python 虚拟机执行字节码。
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## PyCodeObject
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PyCodeObject 定义：
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```C
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// Include/code.h
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typedef struct {
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PyObject_HEAD
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int co_argcount; /* #arguments, except *args */
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int co_nlocals; /* #local variables */
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int co_stacksize; /* #entries needed for evaluation stack */
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int co_flags; /* CO_..., see below */
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PyObject *co_code; /* instruction opcodes */
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PyObject *co_consts; /* list (constants used) */
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PyObject *co_names; /* list of strings (names used) */
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PyObject *co_varnames; /* tuple of strings (local variable names) */
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PyObject *co_freevars; /* tuple of strings (free variable names) */
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PyObject *co_cellvars; /* tuple of strings (cell variable names) */
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/* The rest doesn't count for hash/cmp */
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PyObject *co_filename; /* string (where it was loaded from) */
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PyObject *co_name; /* string (name, for reference) */
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int co_firstlineno; /* first source line number */
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PyObject *co_lnotab; /* string (encoding addr<->lineno mapping) */
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void *co_zombieframe; /* for optimization only (see frameobject.c) */
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} PyCodeObject;
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```
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Python 会对源代码中的每个代码块（Code Block）生成一个 PyCodeObject 对象，一个作用域（名字空间）就算做一个代码块。
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代码块对应的字节码就存放在 PyCodeObject 的 `co_code`字段。
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## 生成 pyc 文件
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pyc文件包含三部分信息：
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- magic number
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用于标识 pyc 版本，不同Python 版本的 magic number 不一样。通过 magic number 可以避免 Python 加载错误版本的 pyc 文件。
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Python2.5 的 magic number 为：
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```C
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#define MAGIC (62131 | ((long)'\r'<<16) | ((long)'\n'<<24))
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/* Magic word as global; note that _PyImport_Init() can change the
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value of this global to accommodate for alterations of how the
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compiler works which are enabled by command line switches. */
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static long pyc_magic = MAGIC;
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```
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- mtime
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文件修改时间，用于和 py 文件的修改时间进行对比。Python 在导入模块时优先导入pyc 文件，但是有可能 pyc 文件对应的 py 文件已经被修改了，所以 Python 需要对比 pyc 文件和 py 文件的修改时间，如果 py 文件的修改时间更新，则需要重新编译 pyc 文件。
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- 字节码，即 PyCodeObject。
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生成 pyc 文件的代码如下：
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```C
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// Python/import.c
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/* Write a compiled module to a file, placing the time of last
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modification of its source into the header.
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Errors are ignored, if a write error occurs an attempt is made to
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remove the file. */
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static void
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write_compiled_module(PyCodeObject *co, char *cpathname, time_t mtime)
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{
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FILE *fp;
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fp = open_exclusive(cpathname);
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if (fp == NULL) {
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if (Py_VerboseFlag)
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PySys_WriteStderr(
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"# can't create %s\n", cpathname);
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return;
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}
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PyMarshal_WriteLongToFile(pyc_magic, fp, Py_MARSHAL_VERSION);
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/* First write a 0 for mtime */
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PyMarshal_WriteLongToFile(0L, fp, Py_MARSHAL_VERSION);
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PyMarshal_WriteObjectToFile((PyObject *)co, fp, Py_MARSHAL_VERSION);
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if (fflush(fp) != 0 || ferror(fp)) {
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if (Py_VerboseFlag)
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PySys_WriteStderr("# can't write %s\n", cpathname);
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/* Don't keep partial file */
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fclose(fp);
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(void) unlink(cpathname);
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return;
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}
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/* Now write the true mtime */
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fseek(fp, 4L, 0);
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assert(mtime < LONG_MAX);
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PyMarshal_WriteLongToFile((long)mtime, fp, Py_MARSHAL_VERSION);
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fflush(fp);
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fclose(fp);
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if (Py_VerboseFlag)
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PySys_WriteStderr("# wrote %s\n", cpathname);
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}
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```
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`PyMarshal_WriteObjectToFile`最终调用 `w_object`将 PyCodeObject 写入文件中，`w_object`的代码如下（有删减）：
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```C
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// Python/marchal.c
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// w_object 的代码很长，都是 if/else if 判断 object 的具体类型。
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static void
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w_object(PyObject *v, WFILE *p)
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{
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...
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else if (PyCode_Check(v)) {
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PyCodeObject *co = (PyCodeObject *)v;
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w_byte(TYPE_CODE, p);
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w_long(co->co_argcount, p);
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w_long(co->co_nlocals, p);
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w_long(co->co_stacksize, p);
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w_long(co->co_flags, p);
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w_object(co->co_code, p);
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w_object(co->co_consts, p);
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w_object(co->co_names, p);
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w_object(co->co_varnames, p);
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w_object(co->co_freevars, p);
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w_object(co->co_cellvars, p);
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w_object(co->co_filename, p);
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w_object(co->co_name, p);
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w_long(co->co_firstlineno, p);
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w_object(co->co_lnotab, p);
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}
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...
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```
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## dis
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Python 标准库中的 dis 模块可以用来获取代码对应的字节码。
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