附加到元组覆盖以前的值 [英] Appending to a tuple overwriting previous values
问题描述
我正在使用 arcpy 来获取形状文件的所有折线.SearchCursor 返回一个光标，以便我可以遍历 shape 文件的所有特征.问题是我想保存游标返回的所有对象供以后使用.
I'm using arcpy to get all the polylines of a shape file. SearchCursor returns a cursor so that I can iterate over all the features of shape file. Problem is I want to save all the objects returned by cursor for later use.
import arcpy
from arcpy import env
env.workspace = r"C:\GIS Data\GIS data"
desc = arcpy.Describe("River.shp")
shapefieldname = desc.ShapeFieldName
rows = arcpy.SearchCursor("River.shp")
featureList = ()
for row in rows:
feat = row.getValue(shapefieldname)
featureList = featureList + (feat, )
print "%i %i" % (featureList[-1].firstPoint.X, featureList[-1].firstPoint.Y)
print "%i %i" % (featureList[-1].lastPoint.X, featureList[-1].lastPoint.Y)
print
print "---------------------------------------------------------------"
for feat in featureList:
print "%i %i" % (feat.firstPoint.X, feat.firstPoint.Y)
print "%i %i" % (feat.lastPoint.X, feat.lastPoint.Y)
print
元组应该包含游标返回的所有对象.但它只有元组重复size次的最后一个元素.
Tuple supposed to contain all the objects returned by cursor. But it has only the last elements repeated size of tuple number of times.
3610930 2135882 3611593 2134453
3610930 2135882 3611593 2134453
3611806 2134981 3611593 2134453
3611806 2134981 3611593 2134453
3614160 2136164 3617432 2131734
3614160 2136164 3617432 2131734
3611593 2134453 3617432 2131734
3611593 2134453 3617432 2131734
3617432 2131734 3620568 2127591
3617432 2131734 3620568 2127591
3620568 2127591 3620785 2127423
3620568 2127591 3620785 2127423
3617980 2126657 3620568 2127591
3617980 2126657 3620568 2127591
3616768 2129454 3617948 2126649
3616768 2129454 3617948 2126649
3617948 2126649 3617980 2126657
3617948 2126649 3617980 2126657
3615102 2128889 3617587 2126510
3615102 2128889 3617587 2126510
3617587 2126510 3617948 2126649
3617587 2126510 3617948 2126649
3617624 2126416 3617980 2126657
3617624 2126416 3617980 2126657
3613129 2128176 3615155 2125617
3613129 2128176 3615155 2125617
3615155 2125617 3617587 2126510
3615155 2125617 3617587 2126510
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
3615086 2125515 3615155 2125617
起初，我尝试过使用列表.当我使用 'append()' 方法时，也为列表找到了相同的输出.由于元组是不可变的数据结构，+ 如何覆盖元组之前的所有元素.虽然这段代码是为arcpy写的，但我猜问题不是arcgis特有的.
At first, I've tried this using list. Same output was also found for list when I've used 'append()' method. As tuple is immutable data structure, how can + overwrites all the previous elements of tuple. Although this code is written for arcpy, but I guess the problem isn't arcgis specific.
推荐答案
这意味着 row.getValue() 不断返回对同一个对象的引用，这它会一直更新到位.
What this suggests is that row.getValue() keeps returning references to the same object, which it keeps updating in place.
要验证，请尝试在第一个打印 id(feat)、id(feat.firstPoint) 和 id(feat.lastPoint)循环，并查看迭代之间是否有任何 id 保持相同.如果他们中的任何一个这样做，那就是你的问题.
To verify, try printing id(feat), id(feat.firstPoint) and id(feat.lastPoint) in the first loop, and see whether any of the ids remain the same between iterations. If any of them do, that's your problem.
由于元组是不可变的数据结构，如何+覆盖元组之前的所有元素.
As tuple is immutable data structure, how can + overwrites all the previous elements of tuple.
它没有.元组是不可变的，因为您不能在不创建新元组的情况下向其中添加或删除元素.您也不能更改元组元素的值.但是，如果该元素是对可变对象的引用，则您可以自由修改该对象本身.这就是我怀疑这里发生的事情:您对同一个对象有多个引用；当您修改其中一个时，它们似乎都发生了变化.
It doesn't. Tuple is immutable in the sense that you can't add or remove elements from it without creating a new tuple. You also can't change the value of a tuple element. However, if that element is a reference to a mutable object, you are free to modify the object itself. This is what I suspect is happening here: you have multiple references to the same object; when you modify one, they all appear to change.
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