A basic algae L-system, as described on http://en.wikipedia.org/wiki/L-System.

lsystem.py

# lsystem.py -- simple algae L-System, as described on Wikipedia.
# Copyright (c) Philip Conrad 2013 -- MIT License

def IterSystem(inString):
    out = ""
    inLen = len(inString)
    for i in range(0, inLen):
        c = inString[i]
        if(c == "A"):
            out += "AB"
        elif(c == "B"):
            out += "A"
        else:
            print "Error, not in alphabet of this L-System."
    return out

if __name__ == '__main__':
    start = "A"
    inStr = start
    for x in range(0, 5):
        result = IterSystem(inStr)
        print result
        inStr = result
    print "Done!"

Verbatim text from the wikipedia article example:

---

Lindenmayer's original L-system for modelling the growth of algae.

• variables : A B
• constants : none
• start : A
• rules : (A → AB), (B → A)

which produces:

n = 0 : A
n = 1 : AB
n = 2 : ABA
n = 3 : ABAAB
n = 4 : ABAABABA
n = 5 : ABAABABAABAAB
n = 6 : ABAABABAABAABABAABABA
n = 7 : ABAABABAABAABABAABABAABAABABAABAAB