Question

为什么我导入NumPy后结果不同？

print(sum(range(5),-1))

答案是9

from numpy import *
print(sum(range(5),-1))

答案是10

Answer 1

内置函数应谨慎操作。

import *可能很危险。

内置的sum和sum中定义的numpy具有不同的用途-因此答案也不同。

Help on built-in function sum in module __builtin__:

sum(...)
    sum(iterable[, start]) -> value

    Return the sum of an iterable or sequence of numbers (NOT strings)
    plus the value of 'start' (which defaults to 0).  When the sequence is
    empty, return start.
(END)


>>> import numpy
>>> help(numpy.sum)
Help on function sum in module numpy.core.fromnumeric:

sum(a, axis=None, dtype=None, out=None, keepdims=<class numpy._globals._NoValue>)
    Sum of array elements over a given axis.

    Parameters
    ----------
    a : array_like
        Elements to sum.
    axis : None or int or tuple of ints, optional
        Axis or axes along which a sum is performed.  The default,
        axis=None, will sum all of the elements of the input array.  If
        axis is negative it counts from the last to the first axis.

        .. versionadded:: 1.7.0

        If axis is a tuple of ints, a sum is performed on all of the axes
        specified in the tuple instead of a single axis or all the axes as
        before.
    dtype : dtype, optional
        The type of the returned array and of the accumulator in which the
        elements are summed.  The dtype of `a` is used by default unless `a`
        has an integer dtype of less precision than the default platform
        integer.  In that case, if `a` is signed then the platform integer
        is used while if `a` is unsigned then an unsigned integer of the
        same precision as the platform integer is used.
    out : ndarray, optional
        Alternative output array in which to place the result. It must have
        the same shape as the expected output, but the type of the output
        values will be cast if necessary.
    keepdims : bool, optional
        If this is set to True, the axes which are reduced are left
        in the result as dimensions with size one. With this option,
        the result will broadcast correctly against the input array.

        If the default value is passed, then `keepdims` will not be
        passed through to the `sum` method of sub-classes of
        `ndarray`, however any non-default value will be.  If the
        sub-classes `sum` method does not implement `keepdims` any
        exceptions will be raised.

    Returns
    -------
    sum_along_axis : ndarray
        An array with the same shape as `a`, with the specified
        axis removed.   If `a` is a 0-d array, or if `axis` is None, a scalar
        is returned.  If an output array is specified, a reference to
        `out` is returned.

    See Also
    .
    .
    .

>>>

Answer 2

之所以会这样，是因为内置的python sum函数被numpy.sum覆盖了。

当您评估内置Python sum(range(5),-1)时，其评估结果类似于-1 + sum([0,1,2,3,4])。

通过对比，numpy.sum假定-1是轴参数，表示输入数组的最后一个（也是唯一的）轴。因此，您实际上得到了np.sum(range(5))。

Answer 3

这是因为根据the documentation，numpy.sum中的第二个参数是axis参数。由于输入是一个1d数组，因此sum(range(5), -1)沿最后一个（也是唯一一个）轴求和，因此等于sum(range(5))，等于10。

在标准库的sum()中，第二个参数是the initial value of the sum，默认为0。

因此您的代码等效于-1 + sum(range(5))，等于9。

为什么在导入NumPy之后sum的行为会有所不同

3 个答案: