为什么一个文件对象刷新,而另一个没有？

Question

我想要一个文件对象,在写入数据时直接刷新文件,并写下:

class FlushingFileObject(file):
    def write(self,*args,**kwargs):
        return_val= file.write(self,*args,**kwargs)
        self.flush()
        return return_val

    def writelines(self,*args,**kwargs):
        return_val= file.writelines(self,*args,**kwargs)
        self.flush()
        return return_val

但有趣的是,当我写它时它没有刷新,所以我尝试了一些包括这个的东西:

class FlushingFileObject(object):
    def __init__(self,*args,**kwargs):
        self.file_object= file(*args,**kwargs)

    def __getattr__(self, item):
        return getattr(self.file_object,item)

    def write(self,*args,**kwargs):
        return_val= self.file_object.write(*args,**kwargs)
        self.file_object.flush()
        return return_val

    def writelines(self,*args,**kwargs):
        return_val= self.file_object.writelines(*args,**kwargs)
        self.file_object.flush()
        return return_val

冲洗.

为什么不在file此实例中进行子类化工作？

Answer 1

好问题.

这是因为Python 通过绕过Python级方法并直接调用来优化write对file对象的write调用fputs.

要了解这一点,请考虑:

$ cat file_subclass.py
import sys
class FileSubclass(file):
    def write(self, *a, **kw):
        raise Exception("write called!")
    writelines = write
sys.stdout = FileSubclass("/dev/null", "w")
print "foo"
sys.stderr.write("print succeeded!\n")
$ python print_magic.py
print succeeded!

这个write方法从未被调用过!

现在,当对象不是子类时file,事情按预期工作:

$ cat object_subclass.py
import sys
class ObjectSubclass(object):
    def __init__(self):
        pass
    def write(self, *a, **kw):
        raise Exception("write called!")
    writelines = write
sys.stdout = ObjectSubclass()
print "foo"
sys.stderr.write("print succeeded!\n")
$ python object_subclass.py
Traceback (most recent call last):
  File "x.py", line 13, in <module>
    print "foo"
  File "x.py", line 8, in write
    raise Exception("write called!")
Exception: write called!

稍微深入研究Python源代码,看起来罪魁祸首是PyFile_WriteString由print语句调用的函数,该函数检查被写入的对象是否是一个实例file,如果是,则fputs直接绕过对象的方法和调用:

int
PyFile_WriteString(const char *s, PyObject *f)
{

    if (f == NULL) {
        /* … snip … */
    }
    else if (PyFile_Check(f)) { //-- `isinstance(f, file)`
        PyFileObject *fobj = (PyFileObject *) f;
        FILE *fp = PyFile_AsFile(f);
        if (fp == NULL) {
            err_closed();
            return -1;
        }
        FILE_BEGIN_ALLOW_THREADS(fobj)
        fputs(s, fp); //-- fputs, bypassing the Python object entirely
        FILE_END_ALLOW_THREADS(fobj)
        return 0;
    }
    else if (!PyErr_Occurred()) {
        PyObject *v = PyString_FromString(s);
        int err;
        if (v == NULL)
            return -1;
        err = PyFile_WriteObject(v, f, Py_PRINT_RAW);
        Py_DECREF(v);
        return err;
    }
    else
        return -1;
}