解析具有相同标记和不同属性的xml

Question

我有一个使用Netconvert从osm创建的网络文件。根元素是具有不同属性的边缘。例如，在文件的第一部分，边缘的组织如下。

<edge id=":367367171_1" function="internal"> 
    <lane id=":367367171_1_0" index="0" disallow="tram rail_urban rail rail_electric ship" speed="5.56" length="15.86" shape="7413.68,8096.43 7409.39,8098.94 7406.50,8098.93 7405.03,8096.39 7404.96,8091.32"/> 
</edge> 
<edge id=":367367171_2" function="internal"> 
    <lane id=":367367171_2_0" index="0" disallow="tram rail_urban rail rail_electric ship" speed="5.56" length="9.40" shape="7413.68,8096.43 7412.34,8099.01 7410.83,8099.98 7409.14,8099.36 7407.28,8097.13"/> 
</edge> 
<edge id=":367367171_3" function="internal"> 
    <lane id=":367367171_3_0" index="0" disallow="tram rail_urban rail rail_electric ship" speed="5.56" length="5.56" shape="7408.25,8091.65 7407.28,8097.13"/> 
</edge> 
<edge id=":367367171_4" function="internal"> 
    <lane id=":367367171_4_0" index="0" disallow="tram rail_urban rail rail_electric ship" speed="5.56" length="5.69" shape="7408.25,8091.65 7408.69,8097.32"/> 
</edge> 

在第二部分的边缘，文件更改的属性，它看起来像下面

<edge id="102323265#13" from="1181188708" to="1181188720" priority="1" type="highway.cycleway"> 
    <lane id="102323265#13_0" index="0" allow="bicycle" speed="5.56" length="1.96" width="1.00" shape="14310.67,8986.24 14309.63,8984.59"/> 
</edge> 
<edge id="102323265#2" from="2577245263" to="1721713370" priority="1" type="highway.cycleway" shape="14903.54,9214.01 14891.64,9210.58 14796.11,9178.46 14789.16,9175.24"> 
    <lane id="102323265#2_0" index="0" allow="bicycle" speed="5.56" length="113.82" width="1.00" shape="14898.81,9213.21 14891.49,9211.10 14795.93,9178.98 14791.04,9176.72"/> 
</edge> 
<edge id="102323265#3" from="1721713370" to="1193980046" priority="1" type="highway.cycleway" shape="14789.16,9175.24 14783.34,9171.87 14779.91,9168.83 14776.75,9165.32"> 
    <lane id="102323265#3_0" index="0" allow="bicycle" speed="5.56" length="9.86" width="1.00" shape="14786.63,9174.41 14783.01,9172.31 14779.55,9169.24 14778.85,9168.47"/> 
</edge> 
<edge id="102323265#4" from="1193980046" to="1193980047" priority="1" type="highway.cycleway" shape="14776.75,9165.32 14764.89,9151.27 14762.54,9144.61"> 
    <lane id="102323265#4_0" index="0" allow="bicycle" speed="5.56" length="20.05" width="1.00" shape="14774.71,9163.77 14764.40,9151.55 14763.05,9147.72"/> 
</edge> 
<edge id="102323265#5" from="1193980047" to="1193980057" priority="1" type="highway.cycleway" shape="14762.54,9144.61 14760.31,9140.42 14753.93,9131.92 14749.20,9127.42 14743.90,9123.46 14738.81,9120.77 14731.67,9118.17 14707.61,9110.82"> 
    <lane id="102323265#5_0" index="0" allow="bicycle" speed="5.56" length="60.21" width="1.00" shape="14760.51,9141.98 14759.82,9140.67 14753.49,9132.25 14748.82,9127.82 14743.57,9123.90 14738.55,9121.26 14731.49,9118.68 14710.43,9112.25"/> 
</edge> 

正如你所看到的，有该元素的边缘不同的属性。当我尝试使用下面的代码来访问元素，

for elem in netFile.iter(tag='edge'): 
    print(elem.attrib['from']) 

我得到的'internal'一个KeyError:'from'

当我改变的关键在于'function'而不是'from'，代码打印我多行，当它接近第一部分结束时，再次把我

KeyError: 'function'。

我知道我必须有选择地遍历属性'from'存在的边缘，但不知道如何继续。有人可以帮忙吗？

感谢

Answer 1

Python对字典get()方法是在这些情况下非常有用，因为它在一个关键并不在dict发现返回None。

for elem in netFile.iter(tag='edge'): 
    if elem.attrib.get('from'): 
     # from stuff 
    else: 
     # other stuff 

Answer 2

你可以发现你是属性的存在等待处理的文件至极部分，例如：

# The !required! attributes for each part 
part1_attributes = ["id", "function"] 
part2_attributes = ["id", "from", "to", "priority", "type"] 

for elem in netFile.iter(tag='edge'): 
    if all([attr in elem.attrib for attr in part1_attributes]): 
     # part 1 
     print("function: " + elem.attrib["function"]) 
    elif all([attr in elem.attrib for attr in part2_attributes]): 
     # part 2 
     print("from: " + elem.attrib["from"]) 
    else: 
     print("Unknown part found while parsing xml") 
     # or raise Exception("message...") or exit program etc. 

如果其中一个边缘suddendly不包含的属性之一，这将对其进行排序并返回错误（或只是打印并继续），而不是返回None，如gr1zzly be4r's answer。

Answer 3

所以有更简单的方法来选择地在其中“从”属性存在边缘迭代，你可以使用下面的XPath来查找所有具有从边缘您已经标记了该LXML属性：

for e in root.xpath("//edge[@from]") 

如果您要检查有多种属性，你可以使用和：

.xpath("//edge[@from and @function]")