python mutating a list while for loop Error -

all want removing duplicate elements in list l.c(l) working, d(l) shows typeerror: argument of type 'nonetype' not iterable when run it. think there no differences between these 2 functions. can not figure out why comes out type error:

def c(l):     l = []     in range(len(l)):         if not(l[i] in l):             l = l + [l[i]]     return l def d(l):     l = []     print type(l)     in range(len(l)):         if not (l[i] in l):             l = l.append(l[i])     return l 

thanks help.

this:

l = l + [l[i]] 

is not equivalent this:

l = l.append(l[i]) 

the line above assigns l value return value of calling append function, none.

you can use instead:

l.append(l[i])