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POHow to extract from nested dictionaries using list comprehension
 

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  1. POHow to extract from nested dictionaries using list comprehension
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    <p>I'm trying to extract some data from XML. I'm using <a href="https://github.com/martinblech/xmltodict" rel="nofollow">xmltodict</a> to load the data into a dictionary, then using list comprehensions to pull out individual parts into separate lists. I will later be plotting these using matplotlib.</p> <p>XML:</p> <pre><code>&lt;?xml version="1.0" ?&gt; &lt;MYDATA&gt; &lt;SESSION ID="1234"&gt; &lt;INFO&gt; &lt;BEGIN LOAD="23"/&gt; &lt;/INFO&gt; &lt;TRANSACTION ID="2103645570"&gt; &lt;ANSWER&gt;Hello&lt;/ANSWER&gt; &lt;/TRANSACTION&gt; &lt;TRANSACTION ID="4315547431"&gt; &lt;ANSWER&gt;This is an answer&lt;/ANSWER&gt; &lt;/TRANSACTION&gt; &lt;/SESSION&gt; &lt;SESSION ID="5678"&gt; &lt;INFO&gt; &lt;BEGIN LOAD="28"/&gt; &lt;/INFO&gt; &lt;TRANSACTION ID="4099381642"&gt; &lt;ANSWER&gt;Hello&lt;/ANSWER&gt; &lt;/TRANSACTION&gt; &lt;TRANSACTION ID="1220404184"&gt; &lt;ANSWER&gt;A Different answer&lt;/ANSWER&gt; &lt;/TRANSACTION&gt; &lt;TRANSACTION ID="201506542"&gt; &lt;ANSWER&gt;Yet another one&lt;/ANSWER&gt; &lt;/TRANSACTION&gt; &lt;/SESSION&gt; &lt;/MYDATA&gt; </code></pre> <p>My code:</p> <pre><code>from collections import OrderedDict # doc contains the xml exactly as loaded by xmltodict doc = OrderedDict([(u'MYDATA', OrderedDict([(u'SESSION', [OrderedDict([(u'@ID', u'1234'), (u'INFO', OrderedDict([(u'BEGIN', OrderedDict([(u'@LOAD', u'23')]))])), (u'TRANSACTION', [OrderedDict([(u'@ID', u'2103645570'), (u'ANSWER', u'Hello')]), OrderedDict([(u'@ID', u'4315547431'), (u'ANSWER', u'This is an answer')])])]), OrderedDict([(u'@ID', u'5678'), (u'INFO', OrderedDict([(u'BEGIN', OrderedDict([(u'@LOAD', u'28')]))])), (u'TRANSACTION', [OrderedDict([(u'@ID', u'4099381642'), (u'ANSWER', u'Hello')]), OrderedDict([(u'@ID', u'1220404184'), (u'ANSWER', u'A Different answer')]), OrderedDict([(u'@ID', u'201506542'), (u'ANSWER', u'Yet another one')])])])])]))]) sess_ids = [i['@ID'] for i in doc['MYDATA']['SESSION']] print sess_ids sess_loads = [i['INFO']['BEGIN']['@LOAD'] for i in doc['MYDATA']['SESSION']] print sess_loads trans_ids = [[j['@ID'] for j in i['TRANSACTION']] for i in doc['MYDATA']['SESSION']] print trans_ids </code></pre> <p>Output:</p> <pre><code>sess_ids: [u'1234', u'5678'] sess_loads: [u'23', u'28'] trans_ids: [[u'2103645570', u'4315547431'], [u'4099381642', u'1220404184', u'201506542']] </code></pre> <p>You can see that I'm able to access the ID attributes from the SESSION elements and also the LOAD attributes from the BEGIN elements.</p> <p>I need to get the ID attributes from the TRANSACTION elements as a single list. Currently I'm getting a list of lists in variable <code>trans_ids</code>.</p> <p>How can I get just a flat list of the values?</p> <p>I have tried:</p> <pre><code>[j['@ID'] for j in i['TRANSACTION'] for i in doc['MYDATA']['SESSION']] </code></pre> <p>but that just repeats the second session twice, giving:</p> <pre><code>[u'4099381642', u'4099381642', u'1220404184', u'1220404184', u'201506542', u'201506542'] </code></pre>
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    1. USAndy Madge
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    1. COIs there any reason why you need to use a list comprehension? There’s nothing wrong with building the result list in more than a single line, maybe with a loop or something.
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    2. COActually, no that's just the best option I've come up with so far. I'm open to better suggestions.
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    3. COI’m not saying it’s better; but if you struggle to get a list comprehension working, it’s certainly an easy way to get you to the result. And it might be more readable than a long one-liner too.
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