TypeError：“map”类型的对象没有 len() Python3

Posted 2023-03-12

【中文标题】TypeError：“map”类型的对象没有 len() Python3

【英文标题】：TypeError: object of type 'map' has no len() Python3

【发布时间】：2017-06-13 17:14:13

【问题描述】：

我正在尝试使用 Pyspark 实现 KMeans 算法，它在 while 循环的最后一行给出了上述错误。它在循环之外工作正常但是在我创建循环之后它给了我这个错误 我该如何解决这个问题？

#  Find K Means of Loudacre device status locations
#
# Input data: file(s) with device status data (delimited by '|')
# including latitude (13th field) and longitude (14th field) of device locations
# (lat,lon of 0,0 indicates unknown location)
# NOTE: Copy to pyspark using %paste

# for a point p and an array of points, return the index in the array of the point closest to p
def closestPoint(p, points):
    bestIndex = 0
    closest = float("+inf")
    # for each point in the array, calculate the distance to the test point, then return
    # the index of the array point with the smallest distance
    for i in range(len(points)):
        dist = distanceSquared(p,points[i])
        if dist < closest:
            closest = dist
            bestIndex = i
    return bestIndex

# The squared distances between two points
def distanceSquared(p1,p2):
    return (p1[0] - p2[0]) ** 2 + (p1[1] - p2[1]) ** 2

# The sum of two points
def addPoints(p1,p2):
    return [p1[0] + p2[0], p1[1] + p2[1]]

# The files with device status data
filename = "/loudacre/devicestatus_etl/*"

# K is the number of means (center points of clusters) to find
K = 5

# ConvergeDist -- the threshold "distance" between iterations at which we decide we are done
convergeDist=.1

# Parse device status records into [latitude,longitude]
rdd2=rdd1.map(lambda line:(float((line.split(",")[3])),float((line.split(",")[4]))))
# Filter out records where lat/long is unavailable -- ie: 0/0 points
# TODO
filterd=rdd2.filter(lambda x:x!=(0,0))
# start with K randomly selected points from the dataset
# TODO
sample=filterd.takeSample(False,K,42)
# loop until the total distance between one iteration's points and the next is less than the convergence distance specified
tempDist =float("+inf")
while tempDist > convergeDist:
    # for each point, find the index of the closest kpoint.  map to (index, (point,1))
    # TODO
    indexed =filterd.map(lambda (x1,x2):(closestPoint((x1,x2),sample),((x1,x2),1)))

    # For each key (k-point index), reduce by adding the coordinates and number of points

    reduced=indexed.reduceByKey(lambda x,y: ((x[0][0]+y[0][0],x[0][1]+y[0][1]),x[1]+y[1]))
    # For each key (k-point index), find a new point by calculating the average of each closest point
    # TODO
    newCenters=reduced.mapValues(lambda x1: [x1[0][0]/x1[1], x1[0][1]/x1[1]]).sortByKey()
    # calculate the total of the distance between the current points and new points
    newSample=newCenters.collect() #new centers as a list
    samples=zip(newSample,sample) #sample=> old centers
    samples1=sc.parallelize(samples)
    totalDistance=samples1.map(lambda x:distanceSquared(x[0][1],x[1]))
    # Copy the new points to the kPoints array for the next iteration
    tempDist=totalDistance.sum()
    sample=map(lambda x:x[1],samples) #new sample for next iteration as list
sample

【问题讨论】：

错误消息对我来说看起来非常清楚 - map 返回一个生成器，而不是 Python 2 中的列表。

请张贴踪迹。您发布了 100 行代码，但没有提及哪一行有问题。

相关（可能被骗？）：***.com/a/12319034/748858

【参考方案1】：

您收到此错误是因为您试图获取不支持len 的map 对象（生成器类型） 中的len。例如：

>>> x = [[1, 'a'], [2, 'b'], [3, 'c']]

# `map` returns object of map type
>>> map(lambda a: a[0], x)
<map object at 0x101b75ba8>

# on doing `len`, raises error
>>> len(map(lambda a: a[0], x))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'map' has no len()

为了找到长度，您必须将map 类型转换为list（或tuple），然后您可以调用len。例如：

>>> len(list(map(lambda a: a[0], x)))
3

或者使用列表理解（不使用map）简单地创建一个列表会更好：

>>> my_list = [a[0] for a in x]

# since it is a `list`, you can take it's length
>>> len(my_list)
3

【讨论】：

或者使用列表理解：sample = [x[1] for x in samples]

不是每个迭代器都是生成器，map 尤其不是。