解析和操作 Python 的 XML 数据

Question

So I have been struggling with this school project for some time and it is due in the next two days. I have tried everything from reaching out to other people and scouring online resources for understanding how I am to do this project, but I am thoroughly stumped on how I am to do this. We did not cover the material we are to use here in the readings, IE, we didn't read anything about parsing. Any assistance or guidance would be appreciated. I just need somewhere to look and something to use as I am on my last nerve.

Here is my screen if anyone is interested. I'll even copy down the instructions and the code for easy copy and pasting if that would be of aid. Please, I just need something.

An online plant distributor has recently experience a shortage in its supply of Anemone plants such that the price has increased by 20%. Their plant catalog is maintained in an XML file and they need a Python utility to find the plant by name, read the current price, change it by the specified percentage, and update the file. Writing this utility is your assignment.

Using Python’s ElementTree XML API, write a Python program to perform the following tasks below. Note that your program’s execution syntax must be as follows:

python xmlparse.py plant_catalog.xml plantName percentChange

1.  Using ElementTree, read in this assignments XML file plant_catalog.xml specified by a command line parameter as shown above.
2.  Find the plant by the name passed in as an argument on the command line (plantName above). 
3.  Once found, read the current price and adjust it by the command line argument percentChange. Note that this value could be anything in the range of -90 < percentChange < 100.

For example, if you run your script as follows:

python plant_catalog.xml "Greek Valerian" -20

with the original XML containing:

  <PLANT>
    <COMMON>Greek Valerian</COMMON>
    <BOTANICAL>Polemonium caeruleum</BOTANICAL>
    <ZONE>Annual</ZONE>
    <LIGHT>Shade</LIGHT>
    <PRICE>4.36</PRICE>
    <AVAILABILITY>071499</AVAILABILITY>
  </PLANT>

The resulting file should contain:

  <PLANT>
    <COMMON>Greek Valerian</COMMON>
    <BOTANICAL>Polemonium caeruleum</BOTANICAL>
    <ZONE>Annual</ZONE>
    <LIGHT>Shade</LIGHT>
    <PRICE>3.48</PRICE>
    <AVAILABILITY>071499</AVAILABILITY>
  </PLANT>

Note: You may reduce the precision of the calculation if you wish but it isn’t required.

Hints

Since XML is just a text file, you could write the code to read all the data and the decode the XML information. However, I certainly don’t recommend this approach. Instead, let Python do it for you! Using Python’s ElementTree module, parse the file into an “in-memory” representation of the XML data. Once parsed, the root (or starting place) is as simple as requesting it from the tree. Once you have the root, you can call methods to find what you are looking for and modify them appropriately. You'll want to "findall" the plants and, "for" each plant "in" the result, you'll want to compare the name with the name passed on the command line. If you find a match you'll apply the percentage change, save the result back to the tree.

When you are done with the search you will "write" the tree back to a file. I suggest using a different file name or you will be having to re-download the original with each run.

One note of caution, be sure to read about XML in the Distributed Systems text. From doing so and reviewing the data file you will not that there are no attributes in the XML file. Consequently, you do not need to use attribute methods when you attempt this assignment.

The following code snippet will give you a good starting point:

# Calling arguments: plant_catalog.xml plantName percentChange
import xml.etree.ElementTree as ET
import sys

# input parameters
searchName = sys.argv[2]
percent = float(sys.argv[3])

# parse XML data file
tree = ET.parse(sys.argv[1])
root = tree.getroot()

Here is the code I currently have:

import xml.etree.ElementTree as ET
import sys

searchName = sys.argv[2]
percent = float(sys.argv[3])

tree = ET.parse(sys.argv[1])
root = tree.getroot()


def main():
    file = ET.parse("plant_catalog.xml")


if __name__ == "__main__":
    main()

An edit to be made is this is what I have come up with while experimenting with the code one of the nice people here had given me. It's still not quite working so any help would be appreciated.

import xml.etree.ElementTree as ET
import sys

searchName = sys.argv[2]
percent = float(3.48)

tree = ET.parse(sys.argv[1])
root = tree.getroot()


def main():
    with open("plant_catalog.xml.xml", "r") as file:
        data = file.read()


for plant in root.findall("PLANT"):
    name = plant.find("COMMON").text

    if name == searchName:
        original_price = float(plant.find("PRICE").text)

with open("plant_catalog - output.xml", "wb") as file:
    file.write(percent)

if __name__ == "__main__":
    main()

Answer 1

I'm not real familiar with elementTree, but looking here it seems like the basic approach is:

for plant in root.findall('PLANT'):
    name = plant.find('COMMON').text
    if name == searchName:
        original_price = float(plant.find('PRICE').text)

Then you can manipulate the price as necessary.

Answer 2

Here is how you find the plant and change the price

import xml.etree.ElementTree as ET

xml = '''<r> <PLANT>
    <COMMON>Greek Valerian</COMMON>
    <BOTANICAL>Polemonium caeruleum</BOTANICAL>
    <ZONE>Annual</ZONE>
    <LIGHT>Shade</LIGHT>
    <PRICE>4.36</PRICE>
    <AVAILABILITY>071499</AVAILABILITY>
  </PLANT>  
  <PLANT>
    <COMMON>Other Valerian</COMMON>
    <BOTANICAL>Polemonium caeruleum</BOTANICAL>
    <ZONE>Annual</ZONE>
    <LIGHT>Shade</LIGHT>
    <PRICE>4.36</PRICE>
    <AVAILABILITY>071499</AVAILABILITY>
  </PLANT>  
  <PLANT>
    <COMMON>Another Valerian</COMMON>
    <BOTANICAL>Polemonium caeruleum</BOTANICAL>
    <ZONE>Annual</ZONE>
    <LIGHT>Shade</LIGHT>
    <PRICE>4.36</PRICE>
    <AVAILABILITY>071499</AVAILABILITY>
  </PLANT></r>'''


def change_plant_price(plant_name, new_price):
    root = ET.fromstring(xml)
    plant = root.find(".//*[COMMON='{}']".format(plant_name))
    plant.find('PRICE').text = str(new_price)
    ET.dump(root)


change_plant_price('Greek Valerian', 12.56)

output

<r> <PLANT>
    <COMMON>Greek Valerian</COMMON>
    <BOTANICAL>Polemonium caeruleum</BOTANICAL>
    <ZONE>Annual</ZONE>
    <LIGHT>Shade</LIGHT>
    <PRICE>12.56</PRICE>
    <AVAILABILITY>071499</AVAILABILITY>
  </PLANT>  
  <PLANT>
    <COMMON>Other Valerian</COMMON>
    <BOTANICAL>Polemonium caeruleum</BOTANICAL>
    <ZONE>Annual</ZONE>
    <LIGHT>Shade</LIGHT>
    <PRICE>4.36</PRICE>
    <AVAILABILITY>071499</AVAILABILITY>
  </PLANT>  
  <PLANT>
    <COMMON>Another Valerian</COMMON>
    <BOTANICAL>Polemonium caeruleum</BOTANICAL>
    <ZONE>Annual</ZONE>
    <LIGHT>Shade</LIGHT>
    <PRICE>4.36</PRICE>
    <AVAILABILITY>071499</AVAILABILITY>
  </PLANT></r>

Answer 3

Another method.

import sys
from simplified_scrapy import SimplifiedDoc, utils


# No carry
def float2str(f_str, n):
    f_str = str(f_str)
    a, _, c = f_str.partition('.')
    c = (c + "0" * n)[:n]
    return ".".join([a, c])


def main():
    # print(sys.argv)
    xmlFile = sys.argv[1]
    searchName = sys.argv[2]
    percent = float(sys.argv[3]) / 100
    xml = utils.getFileContent(xmlFile)

    doc = SimplifiedDoc(xml)
    plant = doc.getElementByText(searchName, tag='COMMON')
    if plant:
        plant = plant.parent
        price = plant.PRICE # Get price node
        oldPrice = float(price.text)
        newPrice = oldPrice * (1 + percent)
        price.setContent(float2str(newPrice, 2)) # Change price
        # price.setContent('%.2f' % newPrice)
        utils.saveFile(xmlFile, doc.html) # Save file


if __name__ == "__main__":
    main()

Run:

python xmlparse.py plant_catalog.xml "Greek Valerian" -20