python-3.x - python3 datetime对象在列表中递增和使用时的行为问题
问题描述
我正在使用 PyEphem 计算卫星的轨道位置，但我遇到的问题与实际的 PyEphem 代码不同，因此我简化了示例，不包括任何 pyephem 代码。然而，为了项目上下文......在每个循环中，递增的 calcDT 用于计算该新 calcDT 的卫星通道详细信息，并且全部存储在列表中以供以后分析和绘图，检索我的值预计我实际上已附加到列表中。
涉及三个日期时间对象。上升时间、设定时间和计算时间分别是riseDT、setDT和calcDT。riseDT 和 setDT 根本不应该改变。他们不。calc_DT 在每个循环中步进。然后用 setDT 对 calcDT 进行相等性检查以查看传递是否结束。我已经使用 .id() 来查看（并显示）何时何地引用了哪些内存位置，显然表明“在循环中的列表中”calcDT 恢复为引用riseDT“对象（及其value)' 用于首先创建 calcDT，当我实际上希望它引用一个全新的 calcDT 递增“值”时。
这是演示该问题的完整代码。特别注意，在最后三个结果块中，列表中的riseDT 和calcDT 值是相同的（它们不应该是相同的），即使独立的var 版本适当地不同且正确。：
import datetime
riseDT = datetime.datetime(2018, 11, 13, 5, 30, 0, 0)
setDT = datetime.datetime(2018, 11, 13, 5, 30, 1, 500000)
calcDT = riseDT
stepS = 0.5
fullPassList = []
print('==============================================')
print('riseDT before loop :', riseDT)
print('riseDT id() :', id(riseDT))
print('calcDT before loop :', calcDT)
print('calcDT id() :', id(calcDT))
print('note that both are referencing the same memory')
print('==============================================')
print('riseDT before loop :', riseDT)
print('riseDT id() :', id(riseDT))
print('attempt to force an independent calcDT created')
print('with the value only of riseDT by using the')
print('.replace method with no change specified.')
calcDT = riseDT.replace()
print('calcDT before loop :', calcDT)
print('calcDT id() :', id(calcDT))
print('It worked before the loop, but issues inside it?')
print('==============================================')
print('Still alright after putting them into a list?')
fullPassList.append(riseDT) # index [0]
fullPassList.append(stepS) # index [1]
fullPassList.append(calcDT) # index [2]
print('riseDT before loop, in list :', fullPassList[0])
print('riseDT id() : ', id(fullPassList[0]))
print('stepS before loop, in list :', fullPassList[1])
print('stepS id() : ', id(fullPassList[1]))
print('calcDT before loop, in list :', fullPassList[2])
print('calcDT id() : ', id(fullPassList[2]))
print('==============================================')
print('==============================================')
while calcDT <= setDT: # should show 4 result sets
print('riseDT inside loop :', riseDT)
print('riseDT id() :', id(riseDT))
print('calcDT inside loop :', calcDT, ' alright as a var')
print('calcDT id() :', id(calcDT))
print('Looks alright here, but put them in a list ...')
print('- - - - - - - - - - - - - - - - - - - - - - - ')
# pyephem code start
# does stuff but irrelevant to issue
# pyephem code end
print('Still alright after putting them into a list?')
# No. calcDT goes back to referencing the memory location of riseDT
# when calcDT is put into a list and then accessed in the list.
fullPassList.append(riseDT) # index [0]
fullPassList.append(stepS) # index [1]
fullPassList.append(calcDT) # index [2]
print('riseDT inside loop, in a list :', fullPassList[0])
print('riseDT id() : ', id(fullPassList[0]))
print('stepS inside loop, in a list :', fullPassList[1])
print('stepS id() : ', id(fullPassList[1]))
print('calcDT inside loop, in a list :', fullPassList[2], ' NG in list?')
print('calcDT id() : ', id(fullPassList[2]))
print('----------------------------------------------')
calcDT += datetime.timedelta(seconds=stepS)
这是运行此代码的结果，显示了问题。我可以在 Toshiba L775 Ubuntu 18.04LTS（没有 venv）和 python 3.6.5 以及在 venv 中使用本地编译的 python 3.7.0 的几个 raspberry pi 3B+ Raspbian Stretch 上一致地复制该问题。可以看出结果一直是不受欢迎的，这里，在 rpi 3B+ 上运行：
==============================================
riseDT before loop : 2018-11-13 05:30:00
riseDT id() : 1990840856
calcDT before loop : 2018-11-13 05:30:00
calcDT id() : 1990840856
note that both are referencing the same memory
==============================================
riseDT before loop : 2018-11-13 05:30:00
riseDT id() : 1990840856
attempt to force an independent calcDT created
with the value only of riseDT by using the
.replace() method with no change specified.
calcDT before loop : 2018-11-13 05:30:00
calcDT id() : 1990671560
It worked before the loop, but issues inside it?
==============================================
Still alright after putting them into a list?
riseDT before loop, in list : 2018-11-13 05:30:00
riseDT id() : 1990840856
stepS before loop, in list : 0.5
stepS id() : 1990697152
calcDT before loop, in list : 2018-11-13 05:30:00
calcDT id() : 1990671560
==============================================
==============================================
riseDT inside loop : 2018-11-13 05:30:00
riseDT id() : 1990840856
calcDT inside loop : 2018-11-13 05:30:00 alright as a var
calcDT id() : 1990671560
Looks alright here, but put them in a list ...
- - - - - - - - - - - - - - - - - - - - - - -
Still alright after putting them into a list?
riseDT inside loop, in a list : 2018-11-13 05:30:00
riseDT id() : 1990840856
stepS inside loop, in a list : 0.5
stepS id() : 1990697152
calcDT inside loop, in a list : 2018-11-13 05:30:00 NG in list?
calcDT id() : 1990671560
----------------------------------------------
riseDT inside loop : 2018-11-13 05:30:00
riseDT id() : 1990840856
calcDT inside loop : 2018-11-13 05:30:00.500000 alright as a var
calcDT id() : 1990671608
Looks alright here, but put them in a list ...
- - - - - - - - - - - - - - - - - - - - - - -
Still alright after putting them into a list?
riseDT inside loop, in a list : 2018-11-13 05:30:00
riseDT id() : 1990840856
stepS inside loop, in a list : 0.5
stepS id() : 1990697152
calcDT inside loop, in a list : 2018-11-13 05:30:00 NG in list?
calcDT id() : 1990671560
----------------------------------------------
riseDT inside loop : 2018-11-13 05:30:00
riseDT id() : 1990840856
calcDT inside loop : 2018-11-13 05:30:01 alright as a var
calcDT id() : 1990669160
Looks alright here, but put them in a list ...
- - - - - - - - - - - - - - - - - - - - - - -
Still alright after putting them into a list?
riseDT inside loop, in a list : 2018-11-13 05:30:00
riseDT id() : 1990840856
stepS inside loop, in a list : 0.5
stepS id() : 1990697152
calcDT inside loop, in a list : 2018-11-13 05:30:00 NG in list?
calcDT id() : 1990671560
----------------------------------------------
riseDT inside loop : 2018-11-13 05:30:00
riseDT id() : 1990840856
calcDT inside loop : 2018-11-13 05:30:01.500000 alright as a var
calcDT id() : 1990479624
Looks alright here, but put them in a list ...
- - - - - - - - - - - - - - - - - - - - - - -
Still alright after putting them into a list?
riseDT inside loop, in a list : 2018-11-13 05:30:00
riseDT id() : 1990840856
stepS inside loop, in a list : 0.5
stepS id() : 1990697152
calcDT inside loop, in a list : 2018-11-13 05:30:00 NG in list?
calcDT id() : 1990671560
----------------------------------------------
我知道与其他一些语言相比，python 语言解释器有一些“奇怪的行为”，有些是隐藏的。我试图理解它们，同时也得到想要的结果。此处记录的其他奇怪行为作为示例。
https://github.com/satwikkansal/wtfpython
我没有权威认可该链接的经验和知识，但我遇到了其中一些问题。
一年来我一直在用许多成功的项目编写简单的 python3。我遇到了其中一些 python 怪事，并成功地学会了如何解决或使用它们。这让我一直很困惑。
我坦率地承认我遗漏了一些可能非常简单的东西，可能对我面前的东西变得“盲目”，所以我寻求帮助以查看一个简单的解决方案，该解决方案将使我的代码非常易于阅读并按预期工作，即使这需要纠正我的期望。
先感谢您。
更新：这已被回答并被标记为这样。令人尴尬的错误，但我把它留在这里作为一个例子，说明在花太多时间尝试所有可能的补救措施之前，另一组眼睛审查一个人的代码的重要性，同时仍然错过了简单的问题。有时人们可能会锁定问题肯定在哪里，从而对问题的实际位置视而不见，而且非常简单。
解决方案
简答
id()每次都吐出相同整数的原因是，您每次都在索引相同的索引。
您继续添加fullPassList而不清除它。在您的代码中只访问前三个元素。
随着您的列表随着 new的增长calcDT，索引它的代码跟不上。它只是停留在fullPassList[2]. 当然，那里的物体保持不变......
长答案
罪魁祸首
让我们看一下后面的代码部分calcDT = riseDT.replace()
print('calcDT before loop :', calcDT)
print('calcDT id() :', id(calcDT))
print('It worked before the loop, but issues inside it?')
print('==============================================')
print('Still alright after putting them into a list?')
fullPassList.append(riseDT) # index [0] ## OK
fullPassList.append(stepS) # index [1] ## OK
fullPassList.append(calcDT) # index [2] ## Yep
print('riseDT before loop, in list :', fullPassList[0]) ## All these are fine.
print('riseDT id() : ', id(fullPassList[0]))
print('stepS before loop, in list :', fullPassList[1])
print('stepS id() : ', id(fullPassList[1]))
print('calcDT before loop, in list :', fullPassList[2])
print('calcDT id() : ', id(fullPassList[2]))
print('==============================================')
print('==============================================')
while calcDT <= setDT: # should show 4 result sets
print('riseDT inside loop :', riseDT)
print('riseDT id() :', id(riseDT))
print('calcDT inside loop :', calcDT, ' alright as a var')
print('calcDT id() :', id(calcDT))
print('Looks alright here, but put them in a list ...')
print('- - - - - - - - - - - - - - - - - - - - - - - ')
# pyephem code start
# does stuff but irrelevant to issue
# pyephem code end
print('Still alright after putting them into a list?')
# No. calcDT goes back to referencing the memory location of riseDT
# when calcDT is put into a list and then accessed in the list.
然后是关键时刻
fullPassList.append(riseDT) # index [0] ## Nope
fullPassList.append(stepS) # index [1] ## Nope
fullPassList.append(calcDT) # index [2] ## Ditto
这些将在第一个循环中将日期时间推送到索引 3、4、5；6、7、8 次；等等。因此，
print('riseDT inside loop, in a list :', fullPassList[0]) ## oops?
print('riseDT id() : ', id(fullPassList[0]))
print('stepS inside loop, in a list :', fullPassList[1]) ## oops?
print('stepS id() : ', id(fullPassList[1]))
print('calcDT inside loop, in a list :', fullPassList[2], ' NG in list?') ## oops!
print('calcDT id() : ', id(fullPassList[2])) ## oops!!!
print('----------------------------------------------')
该代码反复查询fullPassList[2]. 这每次都会给出相同的结果。不是你想要的。:-)
好了，检测到错误。
建议的解决方案
有几种方法可以解决此问题。你只想要calcDT
list.clear()
fullPassList.clear()在循环中的某个地方抛出一个。也许# pyephem code end在你这样做之后和之前.append()？
这将使新添加的日期时间在 、 和 中fullPastList[0]正确fullPastList[1]引用fullPastList[2]。
list[-index]
您仍然可以保留旧的日期时间并访问新添加的日期时间，只需索引 的倒数第三个元素fullPastList[-3]、的riseDT倒数第二个元素和 的最后一个fullPastList[-2]元素。这可能更方便，因为它保留了以前的值。stepSfullPastList[-1]calcDT
但是你提到了这一点riseDT并且stepS不会改变......这让我推测
选项#3
在您的 while 循环中，就在代码结束之前，注释掉两行并...
# fullPassList.append(riseDT) ## You already appended these before the loop.
# fullPassList.append(stepS) ## Since they'll be constant, it'll be a waste of space
## to append these.
fullPassList.append(calcDT) ## You still want to append this since it will change
## on each loop.
print('riseDT inside loop, in a list :', fullPassList[0]) ## This is fine, this will
print('riseDT id() : ', id(fullPassList[0])) ## still access `riseDT`
print('stepS inside loop, in a list :', fullPassList[1]) ## This is also fine.
print('stepS id() : ', id(fullPassList[1]))
## Change these to access the LAST element, using list[-1]
print('calcDT inside loop, in a list :', fullPassList[-1], ' NG in list?')
print('calcDT id() : ', id(fullPassList[-1]))
print('----------------------------------------------')
因此，您需要添加到列表中的只是calcDT. 我确信您的代码比这复杂得多。但希望这个答案概述了您所面临的令人头疼和动荡的罪魁祸首。
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