MegEngine/python_module/megengine/core/tensor.py

# MegEngine is Licensed under the Apache License, Version 2.0 (the "License")
#
# Copyright (c) 2014-2020 Megvii Inc. All rights reserved.
#
# Unless required by applicable law or agreed to in writing,
# software distributed under the License is distributed on an
# "AS IS" BASIS, WITHOUT ARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
import collections
import functools
import itertools
import weakref
from typing import Callable, Tuple, Union

import numpy as np

import megengine._internal as mgb

from .graph import _use_default_if_none, get_default_graph


def wrap_io_tensor(func):
    r"""A wrapper to make ``func`` compatible with functions in ``_internal.opr``.
    """

    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        comp_graph = None
        for i in itertools.chain(args, kwargs.values()):
            if isinstance(i, Tensor) and i._comp_graph:
                comp_graph = i._comp_graph
                break
        else:

            comp_graph = get_default_graph()
        new_args = (
            arg._attach(comp_graph) if isinstance(arg, Tensor) else arg for arg in args
        )
        new_kwargs = {
            k: v._attach(comp_graph) if isinstance(v, Tensor) else v
            for k, v in kwargs.items()
        }
        ret = func(*new_args, **new_kwargs)
        if isinstance(ret, mgb.SymbolVar):
            ret = Tensor(ret)
        elif isinstance(ret, list):
            ret = [Tensor(t) if isinstance(t, mgb.SymbolVar) else t for t in ret]
        elif isinstance(ret, tuple):
            ret = tuple(Tensor(t) if isinstance(t, mgb.SymbolVar) else t for t in ret)
        return ret

    return wrapper


def _wrap_symbolvar_binary_op(f):
    @functools.wraps(f)
    def wrapped(self, other):
        comp_graph = (
            isinstance(other, Tensor)
            and other._comp_graph
            or self._comp_graph
            or get_default_graph()
        )
        if isinstance(other, Tensor):
            other = other._attach(comp_graph)
        return Tensor(f(self._attach(comp_graph), other))

    return wrapped


def _wrap_slice(inp: slice):
    r"""
    A wrapper to handle Tensor values in ``inp`` slice.
    """
    start = inp.start._symvar if isinstance(inp.start, Tensor) else inp.start
    stop = inp.stop._symvar if isinstance(inp.stop, Tensor) else inp.stop
    step = inp.step._symvar if isinstance(inp.step, Tensor) else inp.step
    return slice(start, stop, step)


def _wrap_idx(idx: Tuple[Union[int, "Tensor"]]):
    r"""
    A wrapper to handle Tensor values in ``idx``.
    """
    if not isinstance(idx, tuple):
        idx = (idx,)

    idx = tuple(i._symvar if isinstance(i, Tensor) else i for i in idx)
    idx = tuple(_wrap_slice(i) if isinstance(i, slice) else i for i in idx)
    return idx


class _MGBIndexWrapper:
    r"""
    A wrapper class to handle ``__getitem__`` for index containing Tensor values.

    :param dest: a destination Tensor to do indexing on.
    :param mgb_index: an ``_internal`` helper function indicating how to index.
    :param val: a optional Tensor parameter used for ``mgb_index``.
    """

    def __init__(self, dest: "Tensor", mgb_index: Callable, val=None):
        self.dest = dest
        self.val = val
        self.mgb_index = mgb_index

    def __getitem__(self, idx):
        if self.val is None:
            return wrap_io_tensor(self.mgb_index(self.dest._symvar).__getitem__)(
                _wrap_idx(idx)
            )
        else:
            return wrap_io_tensor(
                self.mgb_index(self.dest._symvar, self.val._symvar).__getitem__
            )(_wrap_idx(idx))


class _Guard:
    r"""
    A wrapper class with custom ``__del__`` method calling ``deleter``.

    :param deleter: a function to be called in ``__del__``.
    """

    def __init__(self, deleter: Callable):
        self.deleter = deleter

    def __del__(self):
        self.deleter()


class Tensor:
    r"""The main data container in MegEngine.
    Use :func:`~.tensor` to create a Tensor with existed data.
    """
    requires_grad = False
    grad = None

    def __init__(self, val=None, *, requires_grad=None):
        self._reset(val, requires_grad=requires_grad)

    def _reset(self, val=None, *, requires_grad=None):
        self.__sym_override = None
        if val is None:
            self.__val = None
            self.__sym = None
        elif isinstance(val, mgb.SharedND):
            self.__val = val
            self.__sym = None
        elif isinstance(val, mgb.SymbolVar):
            self.__val = None
            self.__sym = val
        else:
            raise TypeError("must be initialized with SymbolVar or SharedND")
        self.requires_grad = requires_grad

    def _as_tensor(self, obj):
        r"""Convert the data into a ``Tensor``. If the data is already a Tensor
        with the same dtype and device, no copy will be performed. Otherwise a
        new Tensor will be returned with computational graph retained.

        """
        if isinstance(obj, Tensor):
            return obj
        if isinstance(obj, mgb.SymbolVar):
            return Tensor(obj)
        if isinstance(obj, mgb.SharedScalar):
            return Tensor(obj._as_sym_var(self._comp_graph, self._comp_node))
        return tensor(data=obj, device=self.device)

    def numpy(self):
        r"""Return the tensor value in numpy.ndarray format.
        """
        if self.__val is not None:
            assert self.__sym is None
            return self.__val.get_value()
        if self.__sym is None:
            raise ValueError("uninitialized")
        if self.__sym.eager_val is not None:
            return self.__sym.eager_val.get_value()
        return self.__sym.inferred_value

    def item(self):
        r"""If tensor only has only one value, return it."""
        return self.numpy().item()

    def _attach(self, comp_graph, *, volatile=True):
        sym = self.__sym_override or self.__sym
        if sym:
            if sym.owner_graph != comp_graph:
                raise RuntimeError("internal error")
            return sym
        if self.__val:
            return self.__val.symvar(comp_graph, volatile=volatile)
        else:
            raise ValueError("uninitialized")

    @property
    def _symvar(self):
        if self.__sym_override:
            return self.__sym_override
        if self.__sym:
            assert not self.__val
            return self.__sym
        if not self.__val:
            raise ValueError("uninitialized")

        return self._attach(get_default_graph())

    def __mgb_symvar__(self, comp_graph=None, **_):
        if self.__sym_override:
            return self.__sym_override
        if self.__val and comp_graph:
            return self._attach(comp_graph)
        return self._symvar  # read by mgb.opr

    def _override_symvar_during_trace(self, trace, symvar):
        assert self.__val and not self.__sym
        assert trace is type(trace)._active_instance
        deleters = trace._user_cache.setdefault(Tensor, set())
        self_ref = weakref.ref(self)

        def restore():
            self = self_ref()
            if self is not None:
                self.__sym_override = None

        deleters.add(_Guard(restore))
        self.__sym_override = symvar

    @property
    def dtype(self):
        r"""Return the data type of the tensor.
        """
        if self.__val is not None:
            return self.__val.dtype
        return self._symvar.dtype

    def set_dtype(self, dtype: str = None):
        r"""Set the data type of the tensor.
        """
        if self.__val is not None:
            self.__val = mgb.make_shared(self.device, value=self.astype(dtype).numpy())
        elif self.__sym is not None:
            self.__sym = self.__sym.astype(dtype)

    @property
    def _comp_node(self):
        if self.__val is not None:
            return self.__val.comp_node
        return self._symvar.comp_node

    device = _comp_node

    @property
    def _comp_graph(self):
        if self.__sym is not None:
            return self.__sym.owner_graph
        return None

    @property
    def shape(self):
        r"""Return an int tuple that is the shape/layout of the tensor.
        Could be invalid in static graph mode.
        """
        from ..jit import trace

        if trace._active_instance:  # pylint: disable=protected-access
            # NOTE: this is an hack
            shape = mgb.opr.get_var_shape(self._symvar)
            return tuple(Tensor(shape[i]) for i in range(self.ndim))
        return self._symvar.imm_shape

    def set_value(self, value, *, sync=True, inplace=False, share=False):
        r"""Set value to the tensor.
        """
        if not self.__val:
            raise ValueError("not detached")
        if isinstance(value, Tensor):
            value = value.__val or value.__sym.eager_val
        self.__val.set_value(value, sync=sync, inplace=inplace, share=share)

    def fill(self, value):
        r"""Fills the tensor with the specified value.
        """
        self.set_value(np.full(self.shape, value, dtype=self.dtype))

    def reset_zero(self):
        r"""Reset the tensor and fills with zeros.
        """
        if not self.__val:
            raise ValueError("not detached")
        self.__val.reset_zero()

    def to(self, device):
        r"""Performs Tensor device conversion, returns Tensor with the specified device.
        """
        return wrap_io_tensor(mgb.opr.copy)(self, comp_node=device)

    # https://docs.python.org/3/reference/datamodel.html#object.__hash__
    # > If a class does not define an __eq__() method it should not define a
    # > __hash__() operation either
    __hash__ = None  # type: ignore[assignment]

    def __eq__(self, rhs):
        rhs = self._as_tensor(rhs)
        return Tensor(self._symvar._binary_opr("EQ", rhs._symvar))

    def __ne__(self, rhs):
        return 1 - self.__eq__(rhs)

    def __len__(self):
        if self._symvar.eager_val is not None:
            return self._symvar.eager_val.shape[0]
        raise TypeError(
            "__len__ and __iter__ is not available for tensors on non eager graph."
        )

    __add__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__add__)
    __radd__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__radd__)
    __sub__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__sub__)
    __rsub__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__rsub__)
    __mul__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__mul__)
    __rmul__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__rmul__)
    __matmul__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__matmul__)
    __rmatmul__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__rmatmul__)
    __lshift__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__lshift__)
    __rshift__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__rshift__)
    __truediv__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__truediv__)
    __rtruediv__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__rtruediv__)
    __floordiv__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__floordiv__)
    __rfloordiv__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__rfloordiv__)
    __mod__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__mod__)
    __rmod__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__rmod__)
    __pow__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__pow__)
    __rpow__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__rpow__)
    __lt__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__lt__)
    __gt__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__gt__)
    __le__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__le__)
    __ge__ = _wrap_symbolvar_binary_op(mgb.SymbolVar.__ge__)
    __neg__ = wrap_io_tensor(mgb.SymbolVar.__neg__)
    sum = wrap_io_tensor(mgb.SymbolVar.sum)
    """
    Sum up the given tensors.
    """
    max = wrap_io_tensor(mgb.SymbolVar.max)
    """
    Return the maximum value of given tensor.
    """
    min = wrap_io_tensor(mgb.SymbolVar.min)
    """
    Return the minimum value of given tensor.
    """
    prod = wrap_io_tensor(mgb.SymbolVar.prod)
    """
    Return the product value of the given tensor.
    """
    mean = wrap_io_tensor(mgb.SymbolVar.mean)
    """
    Return the mean value of the given tensor.
    """
    dimshuffle = wrap_io_tensor(mgb.SymbolVar.dimshuffle)
    """
    See more details in :func:`~.functional.tensor.dimshuffle`.
    """
    astype = wrap_io_tensor(mgb.SymbolVar.astype)
    """
    Cast the tensor to a specified type.
    """

    def reshape(self, *target_shape):
        r"""Return a tensor which has given target shape

        Examples:

        .. testcode::

            import numpy as np
            from megengine import tensor

            inp = tensor(np.arange(1, 17, dtype=np.int32).reshape(4,4))
            out = tensor(np.arange(100, 116, dtype=np.int32).reshape(1,16))
            out = out.reshape(inp.shape)
            print(out.numpy())

        .. testoutput::

           [[100 101 102 103]
            [104 105 106 107]
            [108 109 110 111]
            [112 113 114 115]]
        """

        if isinstance(target_shape[0], tuple):
            if len(target_shape) > 1:
                raise ValueError("Only single tuple is accepted in reshape")
            target_shape = target_shape[0]
        target_shape = (t._symvar if isinstance(t, Tensor) else t for t in target_shape)
        return Tensor(mgb.SymbolVar.reshape(self._symvar, *target_shape))

    def broadcast(self, *target_shape):
        r"""Return a tesnor broadcasted by current tensor to given target shape

        Examples:

        .. testcode::

            import numpy as np
            from megengine import tensor

            data = tensor(np.arange(100, 104, dtype=np.int32).reshape(1,4))
            data = data.broadcast((4,4))
            print(data.numpy())

        .. testoutput::

            [[100 101 102 103]
             [100 101 102 103]
             [100 101 102 103]
             [100 101 102 103]]
        """

        if isinstance(target_shape[0], tuple):
            if len(target_shape) > 1:
                raise ValueError("Only single tuple is accepted in broadcast")
            target_shape = target_shape[0]
        target_shape = (t._symvar if isinstance(t, Tensor) else t for t in target_shape)
        return Tensor(mgb.SymbolVar.broadcast(self._symvar, *target_shape))

    # Prefer operators on Tensor instead of convert to numpy
    __array_priority__ = 1000

    # mgb indexing family
    def __getitem__(self, idx):
        return wrap_io_tensor(self._symvar.__getitem__)(_wrap_idx(idx))

    def set_subtensor(self, val: "Tensor") -> _MGBIndexWrapper:
        r"""
        Return a object which supports using ``__getitem__`` to set subtensor.

        ``c = a.set_subtensor(b)[idx]`` is equivalent to ``c = a.copy()`` and ``c[idx] = b``.
        """
        return _MGBIndexWrapper(self, mgb.opr.set_subtensor, val)

    def incr_subtensor(self, val: "Tensor") -> _MGBIndexWrapper:
        r"""
        Return a object which supports using ``__getitem__`` to increase subtensor.

        ``c = a.incr_subtensor(b)[idx]`` is equivalent to ``c = a.copy()`` and ``c[idx] += b``.
        """
        return _MGBIndexWrapper(self, mgb.opr.incr_subtensor, val)

    @property
    def ai(self) -> _MGBIndexWrapper:
        r"""
        Return a object which supports complex index method to get subtensor.

        Examples:

        .. testcode::

            from megengine import tensor
            a = tensor(np.arange(16, dtype=np.float32).reshape((4, 4)))
            print(a.ai[:, [2, 3]])

        Outputs:

        .. testoutput::

            Tensor([[ 2.  3.]
                    [ 6.  7.]
                    [10. 11.]
                    [14. 15.]])
        """
        return _MGBIndexWrapper(self, mgb.opr.advanced_indexing)

    def set_ai(self, val: "Tensor") -> _MGBIndexWrapper:
        r"""
        Equal to :meth:`~.Tensor.set_subtensor` which supports advanced indexing.
        """
        return _MGBIndexWrapper(self, mgb.opr.set_advanced_indexing, val)

    def incr_ai(self, val: "Tensor") -> _MGBIndexWrapper:
        r"""
        Equal to :meth:`~.Tensor.incr_subtensor` which supports advanced indexing.
        """
        return _MGBIndexWrapper(self, mgb.opr.incr_advanced_indexing, val)

    @property
    def mi(self) -> _MGBIndexWrapper:
        r"""
        Return a object which supports getting subtensor by
        the coordinates which is Cartesian product of given index.

        Examples:

        .. testcode::

            from megengine import tensor
            a = tensor(np.arange(16, dtype=np.float32).reshape((4, 4)))
            print(a.mi[[1, 2], [2, 3]])
            # is equal to elements on [1, 2] * [2, 3] = [[(1,2), (1, 3)], [(2, 2), (2, 3)]]
            # a[1,2] = 6, a[1,3] = 7, a[2,2] = 10, a[2,3] = 11

        Outputs:

        .. testoutput::

            Tensor([[ 6.  7.]
                    [10. 11.]])
        """
        return _MGBIndexWrapper(self, mgb.opr.mesh_indexing)

    def set_mi(self, val: "Tensor") -> _MGBIndexWrapper:
        r"""
        Equal to :meth:`~.Tensor.set_subtensor` which using mesh indexing.
        """
        return _MGBIndexWrapper(self, mgb.opr.set_mesh_indexing, val)

    def incr_mi(self, val: "Tensor") -> _MGBIndexWrapper:
        r"""
        Equal to :meth:`~.Tensor.incr_subtensor` which using mesh indexing.
        """
        return _MGBIndexWrapper(self, mgb.opr.incr_mesh_indexing, val)

    @property
    def batched_mi(self) -> _MGBIndexWrapper:
        r"""
        Return a object which supports getting subtensor by
        batched mesh indexing.

        For Tensor ``a`` and index ``idx``, each value of the ``idx`` need to be a 2-dim matrix or slice.
        Cartesian product ``... * idx[k-1][i] * idx[k][i] * idx[k+1][i] * ...`` will be a subtensor from ``a[i]``.
        Each matrix ``idx[k]`` should have the size of ``batched_dim`` rows as ``idx[0]`` indicated.
        And for slice value, it will apply same slice for each ``batched_dim``. For more details see the example below.

        Examples:

        .. testcode::

            from megengine import tensor
            a = tensor(np.arange(144, dtype=np.float32).reshape((3, 3, 4, 4)))

            print(a.batched_mi[:2, [[0],[1]],[[0,1],[2,3]],[[0],[1]]])
            # is equal to elements from a[0] with ``[0] * [0,1] * [0] = [[[(0,0,0)], [(0,1,0)]]]``(shape is [1,2,1])
            # and from a[1] with ``[1] * [2,3] * [1] = [[[(1,2,1)], [(1,3,1)]]]``(shape is also [1,2,1])
            # a[0,0,0,0] = 0, a[0,0,1,0] = 4, a[1,1,2,1] = 73, a[1,1,3,1] = 77

            print(a.batched_mi[:2, [[0],[1]], :2, :1])
            # is equal to ``a.batched_mi[:2, [[0],[1]], [[0,1],[0,1]],[[0],[0]]]``

        Outputs:

        .. testoutput::

            Tensor([[[[ 0.]
                      [ 4.]]]
                    [[[73.]
                      [77.]]]])
            Tensor([[[[ 0.]
                      [ 4.]]]
                    [[[64.]
                      [68.]]]])
        """
        return _MGBIndexWrapper(self, mgb.opr.batched_mesh_indexing)

    def batched_set_mi(self, val: "Tensor") -> _MGBIndexWrapper:
        r"""
        Equal to :meth:`~.Tensor.incr_subtensor` which using batched mesh indexing.
        """
        return _MGBIndexWrapper(self, mgb.opr.batched_set_mesh_indexing, val)

    def batched_incr_mi(self, val: "Tensor") -> _MGBIndexWrapper:
        r"""
        Equal to :meth:`~.Tensor.incr_subtensor` which using batched mesh indexing.
        """
        return _MGBIndexWrapper(self, mgb.opr.batched_incr_mesh_indexing, val)

    def __array__(self, dtype=None):
        if dtype is None:
            return self.numpy()
        else:
            return self.numpy().astype(dtype, copy=False)

    def __int__(self):
        return int(self.item())

    def __index__(self):
        return int(self.item())

    def __round__(self, ndigits=0):
        if ndigits != 0:
            raise ValueError("ndigits must be 0 for Tensor.round")
        return Tensor(mgb.opr.elemwise([self._symvar], mode="ROUND"))

    round = __round__

    def sqrt(self):
        r"""Return a tensor that each element is the square root of its
        original value.

        """
        return Tensor(mgb.opr.sqrt(self._symvar))

    def shapeof(self, axis=None):
        r"""Return a Tensor that represent the shape of the tensor.
        """
        return Tensor(mgb.opr.get_var_shape(self._symvar, axis=axis))

    @property
    def ndim(self):
        r"""Return the number of dimensions of the tensor.
        """
        return len(self._symvar.imm_shape)

    def __repr__(self):
        piece = "Tensor("
        with np.printoptions(precision=4, suppress=True):
            piece += "{}".format(str(self.numpy()))
        if self.dtype != np.float32:
            piece += ", dtype={}".format(np.dtype(self.dtype).name)
        if self._comp_node.locator_logical != ("XPU", -1, 0):
            piece += ", device={}".format(self.device)
        piece += ")"
        return piece

    def __bool__(self):
        raise RuntimeError(
            "Tensor object should not be converted to bool or used in a if statement. Use .numpy(), int() or float() if you want to use its value in if statement, be aware that this may lead to incorrect result in non-eager mode."
        )

    def __getstate__(self):
        r""" __getstate__ will be called for pickle serialization or deep copy
        """

        assert (self.__val is not None) and (
            self.__sym is None
        ), "Only SharedND initialized Tensor can be serialized or deep copied"
        metadata = {"requires_grad": self.requires_grad}
        state = {
            "data": self.numpy(),
            "device": self.device,
            "dtype": self.dtype,
            "metadata": metadata,
        }
        return state

    def __setstate__(self, state):
        data = state.pop("data")
        device = state.pop("device")
        dtype = state.pop("dtype")
        metadata = state.pop("metadata", {})
        requires_grad = metadata.pop("requires_grad", None)
        snd = mgb.make_shared(device, value=data, dtype=dtype)
        self._reset(snd, requires_grad=requires_grad)


def tensor(
    data: Union[list, np.ndarray] = None,
    *,
    dtype: str = None,
    device: mgb.CompNode = None,
    requires_grad: bool = None
):
    r"""A helper function to create a :class:`~.Tensor` using existing data.

    :param data: an existing data array, must be Python list, NumPy array or None.
    :param dtype: target Tensor data type, one of ``("uint8", "int8", "int16", "int32", "float32", "float16")``.
    :param device: target device for Tensor storing.
    :param requires_grad: whether its gradiant will be calculated during :meth:`~.Optimizer.backward`
    """
    supported_dtypes = ("uint8", "int8", "int16", "int32", "float32", "float16")
    if isinstance(data, Tensor):
        raise NotImplementedError
    if dtype is not None and np.dtype(dtype).name not in supported_dtypes:
        raise TypeError("unsupported dtype {}".format(dtype))
    if data is not None:
        if not isinstance(data, np.ndarray):
            data = np.array(data, dtype=dtype)
            # In order to accept tensor([1]),
            # Automaticlly convert to  32-bit number instead of numpy's default 64-bit when input data is not nparray.
            dtype = mgb.to_mgb_supported_dtype(data.dtype)
        if dtype is None:
            if data.dtype.name not in supported_dtypes:
                raise TypeError("unsupported dtype {}".format(data.dtype))

    device, _ = _use_default_if_none(device, None)
    shared_nd = mgb.make_shared(device, value=data, dtype=dtype)
    return Tensor(shared_nd, requires_grad=requires_grad)


class TensorDict(collections.MutableMapping):
    r"""
    A helper class to maintain dict with Tensor key.
    """

    def __init__(self, *args, **kwargs):
        self.data = {}
        for i in args:
            self.update(i)
        self.update(**kwargs)

    class keyfn:
        def __new__(cls, x: Tensor):
            if not isinstance(x, Tensor):
                return x
            return super().__new__(cls)

        def __init__(self, x: Tensor):
            self._data = x  # do not save id directly to make pickle work

        def __hash__(self):
            return id(self._data)

        def __eq__(self, other):
            return isinstance(other, type(self)) and id(self._data) == id(other._data)

    def __getitem__(self, key):
        _, v = self.data[self.keyfn(key)]
        return v

    def __setitem__(self, key, value):
        self.data[self.keyfn(key)] = key, value

    def __delitem__(self, key):
        del self.data[self.keyfn(key)]

    def __iter__(self):
        for _, (k, _) in self.data.items():
            yield k

    def __len__(self):
        return len(self.data)