Create a dynamic nested dictionary in python using for loop
<p>I am new to Python, and trying to learn it "on the job". And I am required to do this.</p>

<p>Is it possible to create a 'dictionary1' dynamically which takes another 'dictionary2' as value, where 'dictionary2' is also getting updated in every for loop. Basically in terms of code, I tried:</p>

<pre><code>fetch_value = range(5) #random list of values (not in sequence)

result = {} #dictionary2
ret = {} #dictionary1

list1 = [0, 1] #this list is actually variable length, ranging from 1 to 10 (assumed len(list1) = 2 for example purpose only)

for idx in list1:
    result[str(idx)] = float(fetch_value[1])
    ret['key1'] = (result if len(list1) &gt; 1 else float(fetch_value[1])) # key names like 'key1' are just for representations, actual names vary
    result[str(idx)] = float(fetch_value[2])
    ret['key2'] = (result if len(list1) &gt; 1 else float(fetch_value[2]))
    result[str(idx)] = float(fetch_value[3])
    ret['key3'] = (result if len(list1) &gt; 1 else float(fetch_value[3]))
    result[str(idx)] = float(fetch_value[4])
    ret['key4'] = (result if len(list1) &gt; 1 else float(fetch_value[4]))

print ret
</code></pre>

<p>This outputs to:</p>

<pre><code>{'key1': {'0': 4, '1', 4}, 'key2': {'0': 4, '1', 4}, 'key3': {'0': 4, '1', 4}, 'key4': {'0': 4, '1', 4}}
</code></pre>

<p>What I need:</p>

<pre><code>{'key1': {'0': 1, '1', 1}, 'key2': {'0': 2, '1', 2}, 'key3': {'0': 3, '1', 3}, 'key4': {'0': 4, '1', 4}}
</code></pre>

<p>anything obvious I am doing wrong here?</p>

<p>There are two problems:</p>

<ol>
<li>You needed to create a copy of the result dictionary when you set a key in <code>ret</code> to it. Otherwise, it will always hold a reference back to the same dictionary.</li>
<li>With that change, you would be keeping the last <code>ret</code> dictionary (containing <code>{'0': 4}</code>) at the beginning of your second loop, and that would get copied to all of the keys.</li>
</ol>

<p>A more concise way to do this would be a dictionary comprehension:</p>

<pre><code>fetch_value = range(5)
list1 = [0, 1]
print {
    'key{}'.format(i): {
        str(list_item): float(fetch_value[i]) for list_item in list1
    } if len(list1) &gt; 1 else float(fetch_value[i])
    for i in xrange(1, 5)
}
</code></pre>

<p>Output:</p>

<pre><code>{
    'key3': {'1': 3.0, '0': 3.0},
    'key2': {'1': 2.0, '0': 2.0},
    'key1': {'1': 1.0, '0': 1.0},
    'key4': {'1': 4.0, '0': 4.0}
}
</code></pre>

<p>And with <code>list1 = [0]</code>, where it seems you want a float value instead of a dictionary, the output would be:</p>

<pre><code>{'key3': 3.0, 'key2': 2.0, 'key1': 1.0, 'key4': 4.0}
</code></pre>

python for-loop dictionary nested dictionary-comprehension