Question

In php, unpack() has the “*” flag which means “repeat this format until the end of input”. For example, this prints 97, 98, 99

$str = "abc";
$b = unpack("c*", $str);
print_r($b);

Is there something like this in python? Of course, I can do

str = "abc"
print struct.unpack("b" * len(str), str)

but I’m wondering if there is a better way.

Answer 1

There is no such facility built into struct.unpack, but it is possible to define such a function:

import struct

def unpack(fmt, astr):
    """
    Return struct.unpack(fmt, astr) with the optional single * in fmt replaced with
    the appropriate number, given the length of astr.
    """
    # http://stackoverflow.com/a/7867892/190597
    try:
        return struct.unpack(fmt, astr)
    except struct.error:
        flen = struct.calcsize(fmt.replace('*', ''))
        alen = len(astr)
        idx = fmt.find('*')
        before_char = fmt[idx-1]
        n = (alen-flen)/struct.calcsize(before_char)+1
        fmt = ''.join((fmt[:idx-1], str(n), before_char, fmt[idx+1:]))
        return struct.unpack(fmt, astr)

print(unpack('b*','abc'))
# (97, 98, 99)