Substituting missing values in Python
<p>I want to substitute missing values (None) with the last previous known value. This is my code. But it doesn't work. Any suggestions for a better algorithm?</p>

<pre><code>t = [[1, 3, None, 5, None], [2, None, None, 3, 1], [4, None, 2, 1, None]]
def treat_missing_values(table):
    for line in table:
        for value in line:
            if value == None:
                value = line[line.index(value)-1]
    return table

print treat_missing_values(t)
</code></pre>
