Python Linked List

Here is some list functions based on Martin v. Löwis’s representation:

cons   = lambda el, lst: (el, lst)
mklist = lambda *args: reduce(lambda lst, el: cons(el, lst), reversed(args), None)
car = lambda lst: lst[0] if lst else lst
cdr = lambda lst: lst[1] if lst else lst
nth = lambda n, lst: nth(n-1, cdr(lst)) if n > 0 else car(lst)
length  = lambda lst, count=0: length(cdr(lst), count+1) if lst else count
begin   = lambda *args: args[-1]
display = lambda lst: begin(w("%s " % car(lst)), display(cdr(lst))) if lst else w("nil\n")

where w = sys.stdout.write

Although doubly linked lists are famously used in Raymond Hettinger’s ordered set recipe, singly linked lists have no practical value in Python.

I’ve never used a singly linked list in Python for any problem except educational.

Thomas Watnedal suggested a good educational resource How to Think Like a Computer Scientist, Chapter 17: Linked lists:

A linked list is either:

  • the empty list, represented by None, or
  • a node that contains a cargo object and a reference to a linked list. class Node: def __init__(self, cargo=None, next=None): = cargo self.cdr = next def __str__(self): return str( def display(lst): if lst: w("%s " % lst) display(lst.cdr) else: w("nil\n")

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