why my code can not get the expected return?
<pre><code>grade=['Ben Anderson',95,90,100,-1,'Mary Johnson',75,78,79,-5,'Michael Walter',80,68,0]   

def convert_grades(lst):
    a = []
    b = []
    for i in lst:
        if isinstance(i,str):
            c = 0
            while lst[c] &lt; 0 or lst[c] == []:
                a = a + lst[c]
                c = c + 1
            b = b + a   
    return b
</code></pre>

<p>I want it return as </p>

<pre><code>[['Ben Anderson',95,90,100],['Mary Johnson',75,78,79],['Michael Walter',80,68,0]]
</code></pre>

<p>but it return as <code>[]</code></p>

<p>I do not know what's going on. needs help.</p>
