File handling |
Python Methods and Functions
Code # 1:
| # define CHUNK_SIZE 8192
static PyObject * py_consume_file (PyObject * self, PyObject * args) { PyObject * obj; PyObject * read_meth; PyObject * result = NULL; PyObject * read_args; if (! PyArg_ParseTuple (args, " O " , & amp; obj)) { return NULL; } if ((read_meth = PyObject_GetAttrString (obj, "read" )) == NULL) { return NULL; } read_args = Py_BuildValue ( "(i)" , CHUNK_SIZE); while (1) { PyObject * data; PyObject * enc_data; char * buf; Py_ssize_t len; if ((data = PyObject_Call (read_meth, read_args, NULL)) == NULL) { goto final; } if (PySequence_Length (data) == 0) { Py_DECREF (data); break ; } if ((enc_data = PyUnicode_AsEncodedString (data, "utf-8" , "strict" )) == NULL) { Py_DECREF (data); goto final; } PyBytes_AsStringAndSize (enc_data, & amp; buf, & amp; len); write (1, buf, len); Py_DECREF (enc_data); Py_DECREF (data); } result = Py_BuildValue ( " " );
final: Py_DECREF (read_meth); Py_DECREF (read_args); return result; } |
A file object such as a StringIO instance is prepared for code testing and then passed to:
Code # 2:
| import io f = io.StringIO ( 'Hello World' ) import sample sample.consume_file (f) |
Output:
Hello World
Unlike a regular system file, a file-like object is not necessarily built on a low-level file descriptor. Thus, normal C library functions cannot be used to access it. Instead, the Python C API is used to manipulate an object-like object in the same way as in Python.
Thus, the read () method is retrieved from the passed object. The argument list is PyObject_Call () and then re-passed to PyObject_Call () to call the method. To detect end of file (EOF), PySequence_Length () is used to see if the returned result is of zero length.
For all I / O operations, the problem is encoding and distinguishing between bytes and Unicode. This recipe shows how to read a file in text mode and decode the resulting text into a byte encoding that can be used by C. If the file is read in binary mode, only minor changes will be made, as shown in the code below.
Code # 3:
| if ((data = PyObject_Call (read_meth, read_args, NULL)) == NULL) { goto final; }
if (PySequence_Length (data ) == 0) { Py_DECREF (data); break ; } if (! PyBytes_Check (data)) { Py_DECREF (data); PyErr_SetString (PyExc_IOError, "File must be in binary mode " ); goto final; }
PyBytes_AsStringAndSize (data, & amp; buf, & amp; len); |
