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POPython3 — replacing a dynamically chosen namedtuple field
 

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    <p>I am modeling database records using <code>collections.namedtuple</code>. At times, I want the user to be able to replace the contents of an arbitrary field. The <code>_replace()</code> method permits replacing the contents of a specific field as long as we can specify its name as part of the argument: <code>somenamedtuple._replace(somefield=newcontent)</code>. But if the name itself is to be dynamically supplied by the user, I am unable to find a way to do it.</p> <p>Here is a minimal working example:</p> <pre><code>from collections import namedtuple fields = ['one', 'two', 'three'] Record = namedtuple('Record', fields) # Populate fields. record = Record(*tuple(['empty' for i in fields])) while True: # Show what we have already. print('0: quit') for i in range(len(fields)): print('{}: {}: {}'.format(i+1, fields[i], record[i])) to_change = int(input('Field to change: ')) if not to_change: break else: new_content = input('New content: ') field_to_change = {fields[to_change-1]:new_content} print('Setting', field_to_change) record._replace(**field_to_change) print('Finished.') print(record) </code></pre> <p>Output (Ipython 1.0.0, Python 3.3.1) follows. </p> <pre><code>In [1]: run test_setattr_namedtuple 0: quit 1: one: empty 2: two: empty 3: three: empty Field to set: 2 New content: asdf Setting {'two': 'asdf'} 0: quit 1: one: empty 2: two: empty 3: three: empty Field to set: 0 Finished. Record(one='empty', two='empty', three='empty') In [2]: </code></pre> <p>The <code>record._replace()</code> line is attempting to set 'two' to 'asdf', rather than <code>two</code> and so fails silently. I had thought of using <code>eval</code> inside <code>_replace()</code>, but <code>_replace()</code> does not accept expressions as arguments.</p> <p>I also tried the built-in function <code>setattr</code>, but it doesn't work with namedtuples, presumably because they are immutable. </p>
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