在python中读取大型csv文件的行

2019年8月4日 354次阅读

我有一个非常大的csv文件,我无法在内存中完整加载.所以我想逐个阅读,将其转换为numpy数组,然后再进行一些处理.

我已经检查过：
Lazy Method for Reading Big File in Python?

但问题是它是一个普通的读者,我无法在csvReader中找到任何指定大小的选项.

此外,因为我想将行转换为numpy数组,我不想读取任何一半的行,所以不是指定大小,我想要一些我可以在读者中指定“no of rows”的东西.

有没有内置功能或简单的方法来做到这一点.

最佳答案
csv.reader不会将整个文件读入内存.当您遍历读者对象时,它会逐行地遍历文件.所以你可以像往常一样使用阅读器,但是在你阅读之后要从你的迭代中断,无论你想读多少行.您可以在
C-code used to implement the reader object中看到这一点.

Initializer for the reader objecT:
static PyObject *
csv_reader(PyObject *module, PyObject *args, PyObject *keyword_args)
{
    PyObject * iterator, * dialect = NULL;
    ReaderObj * self = PyObject_GC_New(ReaderObj, &Reader_Type);

    if (!self)
        return NULL;

    self->dialect = NULL;
    self->fields = NULL;
    self->input_iter = NULL;
    self->field = NULL;
    // stuff we dont care about here
    // ...
    self->input_iter = PyObject_GetIter(iterator);  // here we save the iterator (file object) we passed in
    if (self->input_iter == NULL) {
        PyErr_SetString(PyExc_TypeError,
                        "argument 1 must be an iterator");
        Py_DECREF(self);
        return NULL;
    }

static PyObject *
Reader_iternext(ReaderObj *self)  // This is what gets called when you call `next(reader_obj)` (which is what a for loop does internally)
{
    PyObject *fields = NULL;
    Py_UCS4 c;
    Py_ssize_t pos, linelen;
    unsigned int kind;
    void *data;
    PyObject *lineobj;

    if (parse_reset(self) < 0)
        return NULL;
    do {
        lineobj = PyIter_Next(self->input_iter);  // Equivalent to calling `next(input_iter)`
        if (lineobj == NULL) {
            /* End of input OR exception */
            if (!PyErr_Occurred() && (self->field_len != 0 ||
                                      self->state == IN_QUOTED_FIELD)) {
                if (self->dialect->strict)
                    PyErr_SetString(_csvstate_global->error_obj,
                                    "unexpected end of data");
                else if (parse_save_field(self) >= 0)
                    break;
            }
            return NULL;
        }

如您所见,next(reader_object)在内部调用next(file_object).所以你要逐行迭代,而不是将整个内容读入内存.

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