Storing values from list in matrix like form

I have the following list L for which I’m trying to store the values in "rows" logic locating each value
in the position given by dictionary HeadersPositions.

This is my current code, for which I tried while element[0] != "V": in order to change to a next row after get the last header that is V for each "row", but I got an infinite loop, then I changed to if element[0] != "V": in order to control when to switch to next row but is only storing in list row, the first position of header B

L = [
    ['B', 3], 
    ['T', 0], 
    ['N', 5], 
    ['V', 2], 
    ['N', 1], 
    ['V', 4],
    ['B', 7], 
    ['T', 2], 
    ['N', 9], 
    ['V', 1], 
    ['B', 1], 
    ['N', 8], 
    ['V', 3]
]

HeadersPositions = {'B': 0, 'T': 1, 'N': 2, 'V': 3}

rows = []
row=[None]*len(HeadersPositions)
for element in L:
    if element[0] != "V":
        row.insert(HeadersPositions[element[0]], element[1])
    rows.append(row)

My desired output is

Out = [
        [3,0,5,2], 
        [none,none,1,4], 
        [7,2,9,1], 
        [1,none,8,3],         
    ]      

Seen like table would be like this:

B   T   N   V
3   0   5   2
        1   4
7   2   9   1  
1       8   3

>Solution :

Having renamed L to data (since L is a capital letter and l would also not be a recommended variable name):

data = [
    ['B', 3], 
    ['T', 0], 
    ['N', 5], 
    ['V', 2], 
    ['N', 1], 
    ['V', 4],
    ['B', 7], 
    ['T', 2], 
    ['N', 9], 
    ['V', 1], 
    ['B', 1], 
    ['N', 8], 
    ['V', 3]
]

headers_positions= {'B': 0, 'T': 1, 'N': 2, 'V': 3}
rows = []
while data:
    rows.append([
        data.pop(0)[1] if data and data[0][0] == h else None
        for h in headers_positions.keys()
    ])

print(rows)

Output:

[[3, 0, 5, 2], [None, None, 1, 4], [7, 2, 9, 1], [1, None, 8, 3]]

Note: this is ignoring the values of the HeadersPositions, and you should probably use a list there if that’s OK.

Otherwise:

headers_positions = {'B': 0, 'T': 1, 'N': 2, 'V': 3}
headers = [k for k, v in sorted(headers_positions.items(), key=lambda h: h[1])]
rows = []
while data:
    rows.append([
        data.pop(0)[1] if data and data[0][0] == h else None
        for h in headers
    ])