Python if else exception over XML parsed with ElementTree

Question

I have the following XML which I have parsed from a webpage:

<!--
Parts from the iGEM Registry of Standard Biological Parts
-->
<rsbpml>
 <part_list>
  <part>
   <part_id>151</part_id>
   <part_name>BBa_B0034</part_name>
   <part_short_name>B0034</part_short_name>
   <part_short_desc>RBS (Elowitz 1999) -- defines RBS efficiency</part_short_desc>
   <part_type>RBS</part_type>
   <release_status>Released HQ 2013</release_status>
   <sample_status>In stock</sample_status>
   <part_results>Works</part_results>
   <part_nickname>SAMT</part_nickname>
   <part_rating/>
   <part_url>http://parts.igem.org/Part:BBa_J45001</part_url>
   <part_entered>2006-06-07</part_entered>
   <part_author>Kate Broadbent</part_author>
   <deep_subparts/>
   <specified_subparts/>
   <specified_subscars/>
   <sequences>...</sequences>
   <features>...</features>
   <parameters>
   <!--...-->
   <!--...-->
    <parameter>...</parameter>
    <parameter>
     <name>swisspro</name>
     <value>Q8H6N2</value>

I have some code to return the swisspro parameter value; Q8H6N2. However, I want the code to throw up an error if there is no swisspro parameter present.

So far I have tried the code below but it does not work:

def part_attrib(self,x):
        if x == 'uniprot_id':
            for parameter in self.root.iter(tag='parameter'):
                name = parameter.find('name')
                if name is not None and name.text == 'swisspro':
                    return parameter.find('value').text
                else:
                    return "No UniProt ID present."

With that else statement present, the code always outputs the error statement whether or not there is a swisspro parameter present. If I omit the else argument, the code works but does not throw up an error if there is no swisspro parameter present.

What am I doing wrong?

I should highlight that there are several of these <paramter> sections in the XML.

Answer 1

You can set a value before the loop:

swisspro_value = None

for parameter in self.root.iter(tag='parameter'):
    name = parameter.find('name')
    if name is not None and name.text == 'swisspro':
        swisspro_value = parameter.find('value').text
        break

return swisspro_value or "No UniProt ID present."

Answer 2

Since you're returning in your for loop, you could also just return your error message if you make it to the end of the function,

def part_attrib(self,x):
        if x == 'uniprot_id':
            for parameter in self.root.iter(tag='parameter'):
                name = parameter.find('name')
                if name is not None and name.text == 'swisspro':
                    return parameter.find('value').text
        return "No UniProt ID present."