Python Cookbook

Unzipping Simple List-Like Objects

Credit: gyro funch

Problem

You have a sequence and need to pull it apart into a number of pieces.

Solution

There’s no built-in unzip counterpart to zip, but it’s not hard to code our own:

def unzip(p, n):
    """ Split a sequence p into a list of n tuples, repeatedly taking the
    next unused element of p and adding it to the next tuple.  Each of the
    resulting tuples is of the same length; if p%n != 0, the shorter tuples
    are padded with None (closer to the behavior of map than to that of zip).
        Example:
        >>> unzip(['a','b','c','d','e'], 3)
        [('a', 'd'), ('b', 'e'), ('c', None)]
    """
    # First, find the length for the longest sublist
    mlen, lft = divmod(len(p), n)
    if lft != 0: mlen += 1

    # Then, initialize a list of lists with suitable lengths
    lst = [[None]*mlen for i in range(n)]

    # Loop over all items of the input sequence (index-wise), and
    # Copy a reference to each into the appropriate place
    for i in range(len(p)):
        j, k = divmod(i, n)    # Find sublist-index and index-within-sublist
        lst[k][j] = p[i]       # Copy a reference appropriately

    # Finally, turn each sublist into a tuple, since the unzip function
    # is specified to return a list of tuples, not a list of lists
    return map(tuple, lst)

Discussion

The function in this recipe takes a list and pulls it apart into a user-defined number of pieces. It acts like a sort of reverse zip function (although it deals with only the very simplest cases). This recipe was useful to me recently when I had to take a Python list and break ...