Python：urllib.request.urlretrieve保存一个空文件。 在其中写道“提供的id参数为空。”

Question

This is the type of file I want to download and save:

http://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=nuccore&id=562868704,585641505&rettype=fasta&retmode=text

...and this is the test code:

import urllib.request
import xml.etree.ElementTree as ET
mystring = ' '
link = urllib.request.urlopen('http://eutils.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=protein&db=nuccore&linkname=protein_nuccore_mrna&id=13591999,149050462')
tree = ET.parse(link)
root = tree.getroot()
for branch in root.iter('Link'):
    for something in branch.iter('Id'):
        mystring += something.text + ','
mRNA = urllib.request.urlretrieve('http://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=nuccore&id=' + mystring + '&rettype=fasta&retmode=text', 'C:/Users/User/Documents/mRNA.fasta')

It creates the file but is then empty, i have no clue what the problem is. Thank you for the help.

Answer 1

Your resulting URL contains a space; you put it there yourself with:

mystring = ' '

If I replace that with an empty string instead your code appears to work:

mystring = ''

Rather than use string concatenation, you can use a list and use the str.join() method to build your value:

elements = []
for element in root.findall('.//Link/Id'):
    elements.append(element.text)
mystring = ','.join(elements)

I used the Element.findall() method with an XPath expression to list all matching Id nodes.